YASM User Manual

2.2. Alternative Integration Steps

2.1. Integration Steps

2.3. Using VSYASM

The build system used in?Microsoft Visual Studio 2010 is based on?MSBUILD, Microsoft‘s dedicated build management tool, a change that requires that external tools are integrated into the development environment in a new way.?VSYASMhas been developed to facilitate Yasm integration with?Visual Studio 2010 in a robust and efficient manner. The main difference between VSYASM and other versions is that it is capable of assembling multiple source code files given on a single command line.

When assembling a single file VSYASM behaves in the same way as the normal?yasm?tool. The only change in this case is that VSYASM doesn‘t offer the pre-process only mode.

If however the VSYASM command line includes multiple source files, any output, list and map paths given on the command line are resolved to their directory components alone and each source code file is then assembled using these directories for the relevant outputs. Before assembly starts, any non-existent directories needed for VSYASM outputs are recursively created. The assembly process itself stops if any file being assembled generates errors.

The?-E?file?command line switch can be used to send error reports to a file, in which case this file will also include the command line used to invoke VSYASM. This provides a way to check that VSYASM is being called correctly from the controlling Visual Studio build process.

2.1.?Integration Steps

Firstly, the VSYASM executable file (vsyasm.exe) should be added to the Visual Studio directory holding the C tools. This is typically at:

C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin

Secondly, the three files--vsyasm.xml,?vsyasm.props?and?vsyasm.targets--should be added into the project directory of the project in which VSYASM is being used (an alternative will be explained later).

Thirdly, to add Yasm support to a project after the project has been opened in the IDE, right click on the project in the solution explorer and select?"Build Customisations…". If vsyasm is offered as an option in the resulting list you can then select it; if not, use the?"Find Existing…"?button and the resulting file dialogue to navigate to the?vsyasm.targets?that you put in the project directory, select it to add it to the list and then select it from the list.

Once you have done this, right clicking on the project in the solution explorer and selecting?"Properties"?will bring up a dialogue with a new item?"Yasm Assembler"?that will allow you to configure Yasm for building any assembler files added to the project.

2.2.?Alternative Integration Steps

If you have many projects that use VSYASM, you can put the three files mentioned above into MSBUILD‘s build customisation directory which is typically at:

C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\BuildCustomizations

VSYASM will then always be available in the Build Customisations dialogue. An alternative way of doing this is to put these files in a convenient location and then add the path to this location to the?"Build Customisations Search Path"?item under?"VC++ Project Settings"?in the Visual Studio 2010 Options dialogue.

2.3.?Using VSYASM

In a Visual Studio project with assembler source code files, Yasm settings are entered in the?"Yasm Assembler"?item in the projects Property Dialogue. The items available correspond with those available on Yasm‘s command line and are mostly self explanatory but one item--"Object Filename"--does need further explanation.

If the?"Object Filename"?item refers to a directory (the default), MSBUILD will collect all the assembler files in the project together as a batch and invoke VSYASM in multiple file mode. In order to assemble files one at a time it is necessary to change this to the name of an output?file?such as, for example,?"$(IntDir)%(Filename).obj".

Part?II.?NASM Syntax

The chapters in this part of the book document the NASM-compatible syntax accepted by the Yasm?"nasm"?parser and preprocessor.

3.1.?Layout of a NASM Source Line

Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see?Chapter?5) some combination of the four fields

label: instruction operands ; comment

As usual, most of these fields are optional; the presence or absence of any combination of a label, an instruction and a comment is allowed. Of course, the operand field is either required or forbidden by the presence and nature of the instruction field.

NASM uses backslash (\) as the line continuation character; if a line ends with backslash, the next line is considered to be a part of the backslash-ended line.

NASM places no restrictions on white space within a line: labels may have white space before them, or instructions may have no space before them, or anything. The?colon after a label is also optional. Note that this means that if you intend to code?lodsb?alone on a line, and type?lodab?by accident, then that‘s still a valid source line which does nothing but define a label. Running NASM with the command-line option?-w+orphan-labels?will cause it to warn you if you define a label alone on a line without a trailing colon.

Valid characters in labels are letters, numbers,?_,?$,?#,?@,?~,?., and??. The only characters which may be used as the?first?character of an identifier are letters,?.?(with special meaning: see?Section?3.9),?_?and??. An identifier may also be prefixed with a?$?to indicate that it is intended to be read as an identifier and not a reserved word; thus, if some other module you are linking with defines a symbol called?eax, you can refer to?$eax?in NASM code to distinguish the symbol from the register.

The instruction field may contain any machine instruction: Pentium and P6 instructions, FPU instructions, MMX instructions and even undocumented instructions are all supported. The instruction may be prefixed by?LOCK,?REP,?REPE/REPZ?orREPNE/REPNZ, in the usual way. Explicit address-size and?operand-size prefixes?A16,?A32,?O16?and?O32?are provided. You can also use the name of a segment register as an instruction prefix: coding?es mov [bx],ax?is equivalent to coding?mov [es:bx],ax. We recommend the latter syntax, since it is consistent with other syntactic features of the language, but for instructions such as?LODSB, which has no operands and yet can require a?segment override, there is no clean syntactic way to proceed apart from?es lodsb.

An instruction is not required to use a prefix: prefixes such as?CS,?A32,?LOCK?or?REPE?can appear on a line by themselves, and NASM will just generate the prefix bytes.

In addition to actual machine instructions, NASM also supports a number of pseudo-instructions, described in?Section?3.2.

Instruction?operands may take a number of forms: they can be registers, described simply by the register name (e.g.?AX,?BP,?EBX,?CR0): NASM does not use the?gas-style syntax in which register names must be prefixed by a?%?sign), or they can be?effective addresses (see?Section?3.3), constants (Section?3.5) or expressions (Section?3.6).

For?floating-point instructions, NASM accepts a wide range of syntaxes: you can use two-operand forms like MASM supports, or you can use NASM‘s native single-operand forms in most cases. For example, you can code:

fadd st1 ; this sets st0 := st0 + st1

fadd st0, st1 ; so does this

fadd st1, st0 ; this sets st1 := st1 + st0

fadd to st1 ; so does this

Almost any floating-point instruction that references memory must use one of the prefixes?DWORD,?QWORD,?TWORD,?DDQWORD, or?OWORD?to indicate what size of ((memory operand)) it refers to.

3.2.?Pseudo-Instructions

Pseudo-instructions are things which, though not real x86 machine instructions, are used in the instruction field anyway because that‘s the most convenient place to put them. The current?pseudo-instructions are?DB,?DW,?DD,?DQ,?DT,?DDQ,DO, their uninitialized counterparts?RESB,?RESW,?RESD,?RESQ,?REST,?RESDDQ, and?RESO, the?INCBIN?command, the?EQU?command, and the?TIMES?prefix.

3.2.1.?DB?and Friends: Declaring Initialized Data

DB,?DW,?DD,?DQ,?DT,?DDQ, and?DO?are used to declare initialized data in the output file. They can be invoked in a wide range of ways:

db 0x55 ; just the byte 0x55

db 0x55,0x56,0x57 ; three bytes in succession

db ‘a‘,0x55 ; character constants are OK

db ‘hello‘,13,10,‘$‘ ; so are string constants

dw 0x1234 ; 0x34 0x12

dw ‘a‘ ; 0x41 0x00 (it‘s just a number)

dw ‘ab‘ ; 0x41 0x42 (character constant)

dw ‘abc‘ ; 0x41 0x42 0x43 0x00 (string)

dd 0x12345678 ; 0x78 0x56 0x34 0x12

dq 0x1122334455667788 ; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11

ddq 0x112233445566778899aabbccddeeff00

; 0x00 0xff 0xee 0xdd 0xcc 0xbb 0xaa 0x99

; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11

do 0x112233445566778899aabbccddeeff00 ; same as previous

dd 1.234567e20 ; floating-point constant

dq 1.234567e20 ; double-precision float

dt 1.234567e20 ; extended-precision float

DT?does not accept?numeric constants as operands, and?DDQ?does not accept float constants as operands. Any size larger than?DD?does not accept strings as operands.

3.2.2.?RESB?and Friends: Declaring Uninitialized Data

RESB,?RESW,?RESD,?RESQ,?REST,?RESDQ, and?RESO?are designed to be used in the BSS section of a module: they declare?uninitialised?storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. NASM does not support the MASM/TASM syntax of reserving uninitialised space by writing?DW ??or similar things: this is what it does instead. The operand to a?RESB-type pseudo-instruction is a?critical expression: see Section?3.8.

For example:

buffer: resb 64 ; reserve 64 bytes

wordvar: resw 1 ; reserve a word

realarray resq 10 ; array of ten reals

3.2.3.?INCBIN: Including External Binary Files

INCBIN?includes a binary file verbatim into the output file. This can be handy for (for example) including?graphics and?sound data directly into a game executable file. However, it is recommended to use this for only?small?pieces of data. It can be called in one of these three ways:

incbin "file.dat" ; include the whole file

incbin "file.dat",1024 ; skip the first 1024 bytes

incbin "file.dat",1024,512 ; skip the first 1024, and

; actually include at most 512

3.2.4.?EQU: Defining Constants

EQU?defines a symbol to a given constant value: when?EQU?is used, the source line must contain a label. The action of?EQU?is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example,

message db ‘hello, world‘

msglen equ $-message

defines?msglen?to be the constant 12.?msglen?may not then be redefined later. This is not a?preprocessor definition either: the value of?msglen?is evaluated?once, using the value of?$?(see?Section?3.6?for an explanation of?$) at the point of definition, rather than being evaluated wherever it is referenced and using the value of?$?at the point of reference. Note that the operand to an?EQU?is also a?critical expression (Section?3.8).

3.2.5.?TIMES: Repeating Instructions or Data

The?TIMES?prefix causes the instruction to be assembled multiple times. This is partly present as NASM‘s equivalent of the?DUP?syntax supported by MASM-compatible assemblers, in that you can code

zerobuf: times 64 db 0

buffer: db ‘hello, world‘

times 64-$+buffer db ‘ ‘

which will store exactly enough spaces to make the total length of?buffer?up to 64. Finally,?TIMES?can be applied to ordinary instructions, so you can code trivial?unrolled loops in it:

times 100 movsb

Note that there is no effective difference between?times 100 resb 1?and?resb 100, except that the latter will be assembled about 100 times faster due to the internal structure of the assembler.

The operand to?TIMES, like that of?EQU?and those of?RESB?and friends, is a critical expression (Section?3.8).

Note also that?TIMES?can‘t be applied to?macros: the reason for this is that?TIMES?is processed after the macro phase, which allows the argument to?TIMES?to contain expressions such as?64-$+buffer?as above. To repeat more than one line of code, or a complex macro, use the preprocessor?%rep?directive.

3.3.?Effective Addresses

An?effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in?square brackets. For example:

wordvar dw 123

mov ax,[wordvar]

mov ax,[wordvar+1]

mov ax,[es:wordvar+bx]

Anything not conforming to this simple system is not a valid memory reference in NASM, for example?es:wordvar[bx].

More complicated effective addresses, such as those involving more than one register, work in exactly the same way:

mov eax,[ebx*2+ecx+offset]

mov ax,[bp+di+8]

NASM is capable of doing?algebra on these effective addresses, so that things which don‘t necessarily?look?legal are perfectly all right:

mov eax,[ebx*5] ; assembles as [ebx*4+ebx]

mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]

Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses?[eax*2+0]?and?[eax+eax], and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset.

NASM has a hinting mechanism which will cause?[eax+ebx]?and?[ebx+eax]?to generate different opcodes; this is occasionally useful because?[esi+ebp]?and?[ebp+esi]?have different default segment registers.

However, you can force NASM to generate an effective address in a particular form by the use of the keywords?BYTE,?WORD,?DWORD?and?NOSPLIT. If you need?[eax+3]?to be assembled using a double-word offset field instead of the one byte NASM will normally generate, you can code?[dword eax+3]. Similarly, you can force NASM to use a byte offset for a small value which it hasn‘t seen on the first pass (see?Section?3.8?for an example of such a code fragment) by using?[byte eax+offset]. As special cases,?[byte eax]?will code?[eax+0]?with a byte offset of zero, and?[dword eax]?will code it with a double-word offset of zero. The normal form,?[eax], will be coded with no offset field.

The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don‘t specify that it is a dword offset, NASM will cause the high word of the offset to be lost.

Similarly, NASM will split?[eax*2]?into?[eax+eax]?because that allows the offset field to be absent and space to be saved; in fact, it will also split?[eax*2+offset]?into?[eax+eax+offset]. You can combat this behaviour by the use of theNOSPLIT?keyword:?[nosplit eax*2]?will force?[eax*2+0]?to be generated literally.

3.3.1.?64-bit Displacements

In?BITS 64?mode, displacements, for the most part, remain 32 bits and are sign extended prior to use. The exception is one restricted form of the mov instruction: between an?AL,?AX,?EAX, or?RAX?register and a 64-bit absolute address (no registers are allowed in the effective address, and the address cannot be RIP-relative). In NASM syntax, use of the 64-bit absolute form requires?QWORD. Examples in NASM syntax:

mov eax, [1] ; 32 bit, with sign extension

mov al, [rax-1] ; 32 bit, with sign extension

mov al, [qword 0x1122334455667788] ; 64-bit absolute

mov al, [0x1122334455667788] ; truncated to 32-bit (warning)

3.3.2.?RIP?Relative Addressing

In 64-bit mode, a new form of effective addressing is available to make it easier to write position-independent code. Any memory reference may be made?RIP?relative (RIP?is the instruction pointer register, which contains the address of the location immediately following the current instruction).

In NASM syntax, there are two ways to specify RIP-relative addressing:

mov dword [rip+10], 1

stores the value 1 ten bytes after the end of the instruction.?10?can also be a symbolic constant, and will be treated the same way. On the other hand,

mov dword [symb wrt rip], 1

stores the value 1 into the address of symbol?symb. This is distinctly different than the behavior of:

mov dword [symb+rip], 1

which takes the address of the end of the instruction, adds the address of?symb?to it, then stores the value 1 there. If?symb?is a variable, this will?not?store the value 1 into the?symb?variable!

Yasm also supports the following syntax for RIP-relative addressing. The?REL?keyword makes it produce?RIP-relative addresses, while the?ABS?keyword makes it produce non-RIP-relative addresses:

mov [rel sym], rax ; RIP-relative

mov [abs sym], rax ; not RIP-relative

The behavior of?mov [sym], rax?depends on a mode set by the?DEFAULT?directive (see?Section?5.2), as follows. The default mode at Yasm start-up is always?ABS, and in?REL?mode, use of registers, a?FS?or?GS?segment override, or an explicit?ABSoverride will result in a non-RIP-relative effective address.

default rel

mov [sym], rbx ; RIP-relative

mov [abs sym], rbx ; not RIP-relative (explicit override)

mov [rbx+1], rbx ; not RIP-relative (register use)

mov [fs:sym], rbx ; not RIP-relative (fs or gs use)

mov [ds:sym], rbx ; RIP-relative (segment, but not fs or gs)

mov [rel sym], rbx ; RIP-relative (redundant override)

default abs

mov [sym], rbx ; not RIP-relative

mov [abs sym], rbx ; not RIP-relative

mov [rbx+1], rbx ; not RIP-relative

mov [fs:sym], rbx ; not RIP-relative

mov [ds:sym], rbx ; not RIP-relative

mov [rel sym], rbx ; RIP-relative (explicit override)

3.4.?Immediate Operands

Immediate operands in NASM may be 8 bits, 16 bits, 32 bits, and even 64 bits in size. The immediate size can be directly specified through the use of the?BYTE,?WORD, or?DWORD?keywords, respectively.

64 bit immediate operands are limited to direct 64-bit register move instructions in?BITS 64?mode. For all other instructions in 64-bit mode, immediate values remain 32 bits; their value is sign-extended into the upper 32 bits of the target register prior to being used. The exception is the mov instruction, which can take a 64-bit immediate when the destination is a 64-bit register.

All unsized immediate values in?BITS 64?in Yasm default to 32-bit size for consistency. In order to get a 64-bit immediate with a label, specify the size explicitly with the?QWORD?keyword. For ease of use, Yasm will also try to recognize 64-bit values and change the size to 64 bits automatically for these cases.

Examples in NASM syntax:

add rax, 1 ; optimized down to signed 8-bit

add rax, dword 1 ; force size to 32-bit

add rax, 0xffffffff ; sign-extended 32-bit

add rax, -1 ; same as above

add rax, 0xffffffffffffffff ; truncated to 32-bit (warning)

mov eax, 1 ; 5 byte

mov rax, 1 ; 5 byte (optimized to signed 32-bit)

mov rax, qword 1 ; 10 byte (forced 64-bit)

mov rbx, 0x1234567890abcdef ; 10 byte

mov rcx, 0xffffffff ; 10 byte (does not fit in signed 32-bit)

mov ecx, -1 ; 5 byte, equivalent to above

mov rcx, sym ; 5 byte, 32-bit size default for symbols

mov rcx, qword sym ; 10 byte, override default size

A caution for users using both Yasm and NASM 2.x: the handling of mov reg64, unsized immediate is different between Yasm and NASM 2.x; YASM follows the above behavior, while NASM 2.x does the following:

add rax, 0xffffffff ; sign-extended 32-bit immediate

add rax, -1 ; same as above

add rax, 0xffffffffffffffff ; truncated 32-bit (warning)

add rax, sym ; sign-extended 32-bit immediate

mov eax, 1 ; 5 byte (32-bit immediate)

mov rax, 1 ; 10 byte (64-bit immediate)

mov rbx, 0x1234567890abcdef ; 10 byte instruction

mov rcx, 0xffffffff ; 10 byte instruction

mov ecx, -1 ; 5 byte, equivalent to above

mov ecx, sym ; 5 byte (32-bit immediate)

mov rcx, sym ; 10 byte (64-bit immediate)

mov rcx, qword sym ; 10 byte, same as above

3.5.?Constants

NASM understands four different types of constant: numeric, character, string and floating-point.

3.5.1.?Numeric Constants

A numeric constant is simply a number. NASM allows you to specify numbers in a variety of number bases, in a variety of ways: you can suffix?H,?Q?or?O, and?B?for?hex,?octal, and?binary, or you can prefix?0x?for hex in the style of C, or you can prefix?$?for hex in the style of Borland Pascal. Note, though, that the?$?prefix does double duty as a prefix on identifiers (see?Section?3.1), so a hex number prefixed with a?$?sign must have a digit after the?$?rather than a letter.

Some examples:

mov ax,100 ; decimal

mov ax,0a2h ; hex

mov ax,$0a2 ; hex again: the 0 is required

mov ax,0xa2 ; hex yet again

mov ax,777q ; octal

mov ax,777o ; octal again

mov ax,10010011b ; binary

3.5.2.?Character Constants

A character constant consists of up to four characters enclosed in either single or double quotes. The type of quote makes no difference to NASM, except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa.

A character constant with more than one character will be arranged with?little-endian order in mind: if you code

mov eax,‘abcd‘

then the constant generated is not?0x61626364, but?0x64636261, so that if you were then to store the value into memory, it would read?abcd?rather than?dcba. This is also the sense of character constants understood by the Pentium‘s?CPUIDinstruction.

3.5.3.?String Constants

String constants are only acceptable to some pseudo-instructions, namely the?DB?family and?INCBIN.

A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent:

db ‘hello‘ ; string constant

db ‘h‘,‘e‘,‘l‘,‘l‘,‘o‘ ; equivalent character constants

And the following are also equivalent:

dd ‘ninechars‘ ; doubleword string constant

dd ‘nine‘,‘char‘,‘s‘ ; becomes three doublewords

db ‘ninechars‘,0,0,0 ; and really looks like this

Note that when used as an operand to?db, a constant like?‘ab‘?is treated as a string constant despite being short enough to be a character constant, because otherwise?db ‘ab‘?would have the same effect as?db ‘a‘, which would be silly. Similarly, three-character or four-character constants are treated as strings when they are operands to?dw.

3.5.4.?Floating-Point Constants

Floating-point constants are acceptable only as arguments to?DW,?DD,?DQ?and?DT. They are expressed in the traditional form: digits, then a period, then optionally more digits, then optionally an?E?followed by an exponent. The period is mandatory, so that NASM can distinguish between?dd 1, which declares an integer constant, and?dd 1.0?which declares a floating-point constant.

Some examples:

dw -0.5 ; IEEE half precision

dd 1.2 ; an easy one

dq 1.e10 ; 10,000,000,000

dq 1.e+10 ; synonymous with 1.e10

dq 1.e-10 ; 0.000 000 000 1

dt 3.141592653589793238462 ; pi

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the?Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

3.6.?Expressions

Expressions in NASM are similar in syntax to those in C.

NASM does not guarantee the size of the integers used to evaluate expressions at compile time: since NASM can compile and run on 64-bit systems quite happily, don‘t assume that expressions are evaluated in 32-bit registers and so try to make deliberate use of ((integer overflow)). It might not always work. The only thing NASM will guarantee is what‘s guaranteed by ANSI C: you always have?at least?32 bits to work in.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the?$?and?$$?tokens.?$?evaluates to the assembly position at the beginning of the line containing the expression; so you can code an?infinite loop using?JMP $.?$$?evaluates to the beginning of the current section; so you can tell how far into the section you are by using?($-$$).

The arithmetic?operators provided by NASM are listed here, in increasing order of?precedence.

3.6.1.?|: Bitwise OR Operator

The?|?operator gives a?bitwise OR, exactly as performed by the?OR?machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.6.2.?^: Bitwise XOR Operator

^?provides the?bitwise XOR operation.

3.6.3.?&: Bitwise AND Operator

&?provides the?bitwise AND operation.

3.6.4.?<<?and?>>: Bit Shift Operators

<<?gives a bit-shift to the left, just as it does in C. So?5<<3?evaluates to 5 times 8, or 40.?>>?gives a bit-shift to the right; in NASM, such a shift is?always?unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.6.5.?+?and?-: Addition and Subtraction Operators

The?+?and?-?operators do perfectly ordinary?addition and?subtraction.

3.6.6.?*,?/,?//,?%?and?%%: Multiplication and Division

*?is the?multiplication operator.?/?and?//?are both?division operators:?/?is?unsigned division and?//?is?signed division. Similarly,?%?and?%%?provide unsigned and signed?modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the?%?character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.6.7.?Unary Operators:?+,?-,?~?and?SEG

The highest-priority operators in NASM‘s expression grammar are those which only apply to one argument.?-?negates its operand,?+?does nothing (it‘s provided for symmetry with?-),?~?computes the?one‘s complement of its operand, and SEG?provides the?segment address of its operand (explained in more detail in?Section?3.6.8).

3.6.8.?SEG?and?WRT

When writing large 16-bit programs, which must be split into multiple?segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the?SEG?operator to perform this function.

The?SEG?operator returns the?preferred?segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

mov ax, seg symbol

mov es, ax

mov bx, symbol

will load?es:bx?with a valid pointer to the symbol?symbol.

Things can be more complex than this: since 16-bit segments and?groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the?WRT(With Reference To) keyword. So you can do things like

mov ax, weird_seg ; weird_seg is a segment base

mov es, ax

mov bx, symbol wrt weird_seg

to load?es:bx?with a different, but functionally equivalent, pointer to the symbol?symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax?call segment:offset, where?segment?and?offset?both represent immediate values. So to call a far procedure, you could code either of

call (seg procedure):procedure

call weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax?call far procedure?as a synonym for the first of the above usages.?JMP?works identically to?CALL?in these examples.

To declare a?far pointer to a data item in a data segment, you must code

dw symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7.?STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher, NASM will use size specifiers (BYTE,?WORD,?DWORD,?QWORD, or?TWORD), but will give them the smallest possible size. The keyword?STRICT?can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in?BITS 16?mode,

push dword 33

is encoded in three bytes?66 6A 21, whereas

push strict dword 33

is encoded in six bytes, with a full dword immediate operand?66 68 21 00 00 00.

3.8.?Critical Expressions

A limitation of NASM is that it is a?two-pass assembler; unlike TASM and others, it will always do exactly two?assembly passes. Therefore it is unable to cope with source files that are complex enough to require three or more?passes.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can‘t handle is code whose size depends on the value of a symbol declared after the code in question. For example,

times (label-$) db 0

label: db ‘Where am I?‘

The argument to?TIMES?in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the?TIMES?line when it first sees it. It will just as firmly reject the slightlyparadoxical code

times (label-$+1) db 0

label: db ‘NOW where am I?‘

in which?any?value for the?TIMES?argument is by definition wrong!

NASM rejects these examples by means of a concept called a?critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the?TIMES?prefix is a critical expression; for the same reason, the arguments to the?RESB?family of pseudo-instructions are also critical expressions.

Critical expressions can crop up in other contexts as well: consider the following code.

mov ax, symbol1

symbol1 equ symbol2

symbol2:

On the first pass, NASM cannot determine the value of?symbol1, because?symbol1?is defined to be equal to?symbol2?which NASM hasn‘t seen yet. On the second pass, therefore, when it encounters the line?mov ax,symbol1, it is unable to generate the code for it because it still doesn‘t know the value of?symbol1. On the next line, it would see the?EQU?again and be able to determine the value of?symbol1, but by then it would be too late.

NASM avoids this problem by defining the right-hand side of an?EQU?statement to be a critical expression, so the definition of?symbol1?would be rejected in the first pass.

mov eax, [ebx+offset]

offset equ 10

NASM, on pass one, must calculate the size of the instruction?mov eax,[ebx+offset]?without knowing the value of?offset. It has no way of knowing that?offset?is small enough to fit into a one-byte offset field and that it could therefore get away with generating a shorter form of the?effective-address encoding; for all it knows, in pass one,?offset?could be a symbol in the code segment, and it might need the full four-byte form. So it is forced to compute the size of the instruction to accommodate a four-byte address part. In pass two, having made this decision, it is now forced to honour it and keep the instruction large, so the code generated in this case is not as small as it could have been. This problem can be solved by defining?offset?before using it, or by forcing byte size in the effective address by coding?[byte ebx+offset].

3.9.?Local Labels

NASM gives special treatment to symbols beginning with a?period. A label beginning with a single period is treated as a?local?label, which means that it is associated with the previous non-local label. So, for example:

label1 ; some code

.loop ; some more code

jne .loop

label2 ; some code

.loop ; some more code

jne .loop

In the above code fragment, each?JNE?instruction jumps to the line immediately before it, because the two definitions of?.loop?are kept separate by virtue of each being associated with the previous non-local label.

NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of?defining?a local label in terms of the previous non-local label: the first definition of?.loop?above is really defining a symbol called?label1.loop, and the second defines a symbol called?label2.loop. So, if you really needed to, you could write

label3 ; some more code

; and some more

jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn‘t interfere with the normal local-label mechanism. Such a label can‘t be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can‘t be local because the macro that defined it wouldn‘t know the label‘s full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix?..@, then it does nothing to the local label mechanism. So you could code

label1: ; a non-local label

.local: ; this is really label1.local

..@foo: ; this is a special symbol

label2: ; another non-local label

.local: ; this is really label2.local

jmp ..@foo ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example,?..start?is used to specify the entry point in the?obj?output format.

Chapter?4.?The NASM Preprocessor

4.1.1. The Normal Way:?%define

4.1. Single-Line Macros

4.1.2. Enhancing %define:?%xdefine

4.1.3. Concatenating Single Line Macro Tokens:?%+

4.1.4. Undefining macros:?%undef

4.1.5. Preprocessor Variables:?%assign

4.2. String Handling in Macros

4.2.1. String Length:?%strlen

4.2.2. Sub-strings:?%substr

4.3. Multi-Line Macros

4.3.1. Overloading Multi-Line Macros

4.3.2. Macro-Local Labels

4.3.3. Greedy Macro Parameters

4.3.4. Default Macro Parameters

4.3.5.?%0: Macro Parameter Counter

4.3.6.?%rotate: Rotating Macro Parameters

4.3.7. Concatenating Macro Parameters

4.3.8. Condition Codes as Macro Parameters

4.3.9. Disabling Listing Expansion

4.4. Conditional Assembly

4.4.1.?%ifdef: Testing Single-Line Macro Existence

4.4.2.?%ifmacro: Testing Multi-Line Macro Existence

4.4.3.?%ifctx: Testing the Context Stack

4.4.4.?%if: Testing Arbitrary Numeric Expressions

4.4.5.?%ifidn?and?%ifidni: Testing Exact Text Identity

4.4.6.?%ifid,?%ifnum,?%ifstr: Testing Token Types

4.4.7.?%error: Reporting User-Defined Errors

4.5. Preprocessor Loops

4.6. Including Other Files

4.7. The Context Stack

4.7.1.?%push?and?%pop: Creating and Removing Contexts

4.7.2. Context-Local Labels

4.7.3. Context-Local Single-Line Macros

4.7.4.?%repl: Renaming a Context

4.7.5. Example Use of the Context Stack: Block IFs

4.8. Standard Macros

4.8.1.?__YASM_MAJOR__, etc: Yasm Version

4.8.2.?__FILE__?and?__LINE__: File Name and Line Number

4.8.3.?__YASM_OBJFMT__?and?__OUTPUT_FORMAT__: Output Object Format Keyword

4.8.4.?STRUC?and?ENDSTRUC: Declaring Structure Data Types

4.8.5.?ISTRUC,?AT?and?IEND: Declaring Instances of Structures

4.8.6.?ALIGN?and?ALIGNB: Data Alignment

NASM contains a powerful?macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a?"context stack"?mechanism for extra macro power. Preprocessor directives all begin with a?%?sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO

THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1.?Single-Line Macros

4.1.1.?The Normal Way:?%define

Single-line macros are defined using the?%define?preprocessor directive. The definitions work in a similar way to C; so you can do things like

%define ctrl 0x1F &

%define param(a,b) ((a)+(a)*(b))

mov byte [param(2,ebx)], ctrl ‘D‘

which will expand to

mov byte [(2)+(2)*(ebx)], 0x1F & ‘D‘

When the expansion of a single-line macro contains tokens which invoke another macro, the expansion is performed at invocation time, not at definition time. Thus the code

%define a(x) 1+b(x)

%define b(x) 2*x

mov ax,a(8)

will evaluate in the expected way to?mov ax,1+2*8, even though the macro?b?wasn‘t defined at the time of definition of?a.

Macros defined with?%define?are?case sensitive: after?%define foo bar, only?foo?will expand to?bar:?Foo?or?FOO?will not. By using?%idefine?instead of?%define?(the?"i"?stands for?"insensitive") you can define all the case variants of a macro at once, so that?%idefine foo bar?would cause?foo,?Foo,?FOO,?fOO?and so on all to expand to?bar.

There is a mechanism which detects when a macro call has occurred as a result of a previous expansion of the same macro, to guard against?circular references and infinite loops. If this happens, the preprocessor will only expand the first occurrence of the macro. Hence, if you code

%define a(x) 1+a(x)

mov ax,a(3)

the macro?a(3)?will expand once, becoming?1+a(3), and will then expand no further. This behaviour can be useful.

You can overload single-line macros: if you write

%define foo(x) 1+x

%define foo(x,y) 1+x*y

the preprocessor will be able to handle both types of macro call, by counting the parameters you pass; so?foo(3)?will become?1+3?whereas?foo(ebx,2)?will become?1+ebx*2. However, if you define

%define foo bar

then no other definition of?foo?will be accepted: a macro with no parameters prohibits the definition of the same name as a macro?with?parameters, and vice versa.

This doesn‘t prevent single-line macros being?redefined: you can perfectly well define a macro with

%define foo bar

and then re-define it later in the same source file with

%define foo baz

Then everywhere the macro?foo?is invoked, it will be expanded according to the most recent definition. This is particularly useful when defining single-line macros with?%assign?(see?Section?4.1.5).

You can?pre-define single-line macros using the?"-D"?option on the Yasm command line: see?Section?1.3.3.1.

4.1.2.?Enhancing %define:?%xdefine

To have a reference to an embedded single-line macro resolved at the time that it is embedded, as opposed to when the calling macro is expanded, you need a different mechanism to the one offered by?%define. The solution is to use %xdefine, or its case-insensitive counterpart?%xidefine.

Suppose you have the following code:

%define isTrue 1

%define isFalse isTrue

In this case,?val1?is equal to 0, and?val2?is equal to 1. This is because, when a single-line macro is defined using?%define, it is expanded only when it is called. As?isFalse?expands to?isTrue, the expansion will be the current value ofisTrue. The first time it is called that is 0, and the second time it is 1.

If you wanted?isFalse?to expand to the value assigned to the embedded macro?isTrue?at the time that?isFalse?was defined, you need to change the above code to use?%xdefine.

%xdefine isTrue 1

%xdefine isFalse isTrue

Now, each time that?isFalse?is called, it expands to 1, as that is what the embedded macro?isTrue?expanded to at the time that?isFalse?was defined.

4.1.3.?Concatenating Single Line Macro Tokens:?%+

Individual tokens in single line macros can be concatenated, to produce longer tokens for later processing. This can be useful if there are several similar macros that perform similar functions.

As an example, consider the following:

%define BDASTART 400h ; Start of BIOS data area

struc tBIOSDA ; its structure

Now, if we need to access the elements of tBIOSDA in different places, we can end up with:

mov ax,BDASTART + tBIOSDA.COM1addr

mov bx,BDASTART + tBIOSDA.COM2addr

This will become pretty ugly (and tedious) if used in many places, and can be reduced in size significantly by using the following macro:

; Macro to access BIOS variables by their names (from tBDA):

%define BDA(x) BDASTART + tBIOSDA. %+ x

Now the above code can be written as:

mov ax,BDA(COM1addr)

mov bx,BDA(COM2addr)

Using this feature, we can simplify references to a lot of macros (and, in turn, reduce typing errors).

4.1.4.?Undefining macros:?%undef

Single-line macros can be removed with the?%undef?command. For example, the following sequence:

%define foo bar

%undef foo

mov eax, foo

will expand to the instruction?mov eax, foo, since after?%undef?the macro?foo?is no longer defined.

Macros that would otherwise be pre-defined can be undefined on the command-line using the?"-U"?option on the Yasm command line: see?Section?1.3.3.5.

4.1.5.?Preprocessor Variables:?%assign

An alternative way to define single-line macros is by means of the?%assign?command (and its case-insensitive counterpart?%iassign, which differs from?%assign?in exactly the same way that?%idefine?differs from?%define).

%assign?is used to define single-line macros which take no parameters and have a numeric value. This value can be specified in the form of an expression, and it will be evaluated once, when the?%assign?directive is processed.

Like?%define, macros defined using?%assign?can be re-defined later, so you can do things like

%assign i i+1

to increment the numeric value of a macro.

%assign?is useful for controlling the termination of?%rep?preprocessor loops: see?Section?4.5?for an example of this.

The expression passed to?%assign?is a?critical expression (see?Section?3.8), and must also evaluate to a pure number (rather than a relocatable reference such as a code or data address, or anything involving a register).

4.2.?String Handling in Macros

It‘s often useful to be able to handle strings in macros. NASM supports two simple string handling macro operators from which more complex operations can be constructed.

4.2.1.?String Length:?%strlen

The?%strlen?macro is like?%assign?macro in that it creates (or redefines) a numeric value to a macro. The difference is that with?%strlen, the numeric value is the length of a string. An example of the use of this would be:

%strlen charcnt ‘my string‘

In this example,?charcnt?would receive the value 8, just as if an?%assign?had been used. In this example,?‘my string‘?was a literal string but it could also have been a single-line macro that expands to a string, as in the following example:

%define sometext ‘my string‘

%strlen charcnt sometext

As in the first case, this would result in?charcnt?being assigned the value of 8.

4.2.2.?Sub-strings:?%substr

Individual letters in strings can be extracted using?%substr. An example of its use is probably more useful than the description:

%substr mychar ‘xyz‘ 1 ; equivalent to %define mychar ‘x‘

%substr mychar ‘xyz‘ 2 ; equivalent to %define mychar ‘y‘

%substr mychar ‘xyz‘ 3 ; equivalent to %define mychar ‘z‘

In this example, mychar gets the value of?‘y‘. As with?%strlen?(see?Section?4.2.1), the first parameter is the single-line macro to be created and the second is the string. The third parameter specifies which character is to be selected. Note that the first index is 1, not 0 and the last index is equal to the value that?%strlen?would assign given the same string. Index values out of range result in an empty string.

4.3.?Multi-Line Macros

Multi-line macros are much more like the type of macro seen in MASM and TASM: a multi-line macro definition in NASM looks something like this.

This defines a C-like function prologue as a macro: so you would invoke the macro with a call such as

myfunc: prologue 12

which would expand to the three lines of code

myfunc: push ebp

mov ebp,esp

sub esp,12

The number?1?after the macro name in the?%macro?line defines the number of parameters the macro?prologue?expects to receive. The use of?%1?inside the macro definition refers to the first parameter to the macro call. With a macro taking more than one parameter, subsequent parameters would be referred to as?%2,?%3?and so on.

Multi-line macros, like single-line macros, are?case-sensitive, unless you define them using the alternative directive?%imacro.

If you need to pass a comma as?part?of a parameter to a multi-line macro, you can do that by enclosing the entire parameter in braces. So you could code things like

%macro silly 2

%2: db %1

silly ‘a‘, letter_a ; letter_a: db ‘a‘

silly ‘ab‘, string_ab ; string_ab: db ‘ab‘

silly {13,10}, crlf ; crlf: db 13,10

4.3.1.?Overloading Multi-Line Macros

As with single-line macros, multi-line macros can be overloaded by defining the same macro name several times with different numbers of parameters. This time, no exception is made for macros with no parameters at all. So you could define

to define an alternative form of the function prologue which allocates no local stack space.

Sometimes, however, you might want to?"overload"?a machine instruction; for example, you might want to define

so that you could code

push ebx ; this line is not a macro call

push eax,ecx ; but this one is

Ordinarily, NASM will give a warning for the first of the above two lines, since?push?is now defined to be a macro, and is being invoked with a number of parameters for which no definition has been given. The correct code will still be generated, but the assembler will give a warning. This warning can be disabled by the use of the?-wno-macro-params?command-line option (see?Section?1.3.2).

4.3.2.?Macro-Local Labels

NASM allows you to define labels within a multi-line macro definition in such a way as to make them local to the macro call: so calling the same macro multiple times will use a different label each time. You do this by prefixing?%%?to the label name. So you can invent an instruction which executes a?RET?if the?Z?flag is set by doing this:

You can call this macro as many times as you want, and every time you call it NASM will make up a different?"real"?name to substitute for the label?%%skip. The names NASM invents are of the form?..@2345.skip, where the number 2345 changes with every macro call. The?..@?prefix prevents macro-local labels from interfering with the local label mechanism, as described in?Section?3.9. You should avoid defining your own labels in this form (the?..@?prefix, then a number, then another period) in case they interfere with macro-local labels.

4.3.3.?Greedy Macro Parameters

Occasionally it is useful to define a macro which lumps its entire command line into one parameter definition, possibly after extracting one or two smaller parameters from the front. An example might be a macro to write a text string to a file in MS-DOS, where you might want to be able to write

writefile [filehandle],"hello, world",13,10

NASM allows you to define the last parameter of a macro to be?greedy, meaning that if you invoke the macro with more parameters than it expects, all the spare parameters get lumped into the last defined one along with the separating commas. So if you code:

mov cx,%%endstr-%%str

then the example call to?writefile?above will work as expected: the text before the first comma,?[filehandle], is used as the first macro parameter and expanded when?%1?is referred to, and all the subsequent text is lumped into?%2?and placed after the?db.

The greedy nature of the macro is indicated to NASM by the use of the?+?sign after the parameter count on the?%macro?line.

If you define a greedy macro, you are effectively telling NASM how it should expand the macro given?any?number of parameters from the actual number specified up to infinity; in this case, for example, NASM now knows what to do when it sees a call to?writefile?with 2, 3, 4 or more parameters. NASM will take this into account when overloading macros, and will not allow you to define another form of?writefile?taking 4 parameters (for example).

Of course, the above macro could have been implemented as a non-greedy macro, in which case the call to it would have had to look like

writefile [filehandle], {"hello, world",13,10}

NASM provides both mechanisms for putting ((commas in macro parameters)), and you choose which one you prefer for each macro definition.

See?Section?5.3.3?for a better way to write the above macro.

4.3.4.?Default Macro Parameters

NASM also allows you to define a multi-line macro with a?range?of allowable parameter counts. If you do this, you can specify defaults for?omitted parameters. So, for example:

%macro die 0-1 "Painful program death has occurred."

This macro (which makes use of the?writefile?macro defined in?Section?4.3.3) can be called with an explicit error message, which it will display on the error output stream before exiting, or it can be called with no parameters, in which case it will use the default error message supplied in the macro definition.

In general, you supply a minimum and maximum number of parameters for a macro of this type; the minimum number of parameters are then required in the macro call, and then you provide defaults for the optional ones. So if a macro definition began with the line

%macro foobar 1-3 eax,[ebx+2]

then it could be called with between one and three parameters, and?%1?would always be taken from the macro call.?%2, if not specified by the macro call, would default to?eax, and?%3?if not specified would default to?[ebx+2].

You may omit parameter defaults from the macro definition, in which case the parameter default is taken to be blank. This can be useful for macros which can take a variable number of parameters, since the?%0?token (see?Section?4.3.5) allows you to determine how many parameters were really passed to the macro call.

This defaulting mechanism can be combined with the greedy-parameter mechanism; so the?die?macro above could be made more powerful, and more useful, by changing the first line of the definition to

%macro die 0-1+ "Painful program death has occurred.",13,10

The maximum parameter count can be infinite, denoted by?*. In this case, of course, it is impossible to provide a?full?set of default parameters. Examples of this usage are shown in?Section?4.3.6.

4.3.5.?%0: Macro Parameter Counter

For a macro which can take a variable number of parameters, the parameter reference?%0?will return a numeric constant giving the number of parameters passed to the macro. This can be used as an argument to?%rep?(see?Section?4.5) in order to iterate through all the parameters of a macro. Examples are given in?Section?4.3.6.

4.3.6.?%rotate: Rotating Macro Parameters

Unix shell programmers will be familiar with the?shift?shell command, which allows the arguments passed to a shell script (referenced as?$1,?$2?and so on) to be moved left by one place, so that the argument previously referenced as?$2becomes available as?$1, and the argument previously referenced as?$1?is no longer available at all.

NASM provides a similar mechanism, in the form of?%rotate. As its name suggests, it differs from the Unix?shift?in that no parameters are lost: parameters rotated off the left end of the argument list reappear on the right, and vice versa.

%rotate?is invoked with a single numeric argument (which may be an expression). The macro parameters are rotated to the left by that many places. If the argument to?%rotate?is negative, the macro parameters are rotated to the right.

So a pair of macros to save and restore a set of registers might work as follows:

This macro invokes the?PUSH?instruction on each of its arguments in turn, from left to right. It begins by pushing its first argument,?%1, then invokes?%rotate?to move all the arguments one place to the left, so that the original second argument is now available as?%1. Repeating this procedure as many times as there were arguments (achieved by supplying?%0?as the argument to?%rep) causes each argument in turn to be pushed.

Note also the use of?*?as the maximum parameter count, indicating that there is no upper limit on the number of parameters you may supply to the?multipush?macro.

It would be convenient, when using this macro, to have a?POP?equivalent, which?didn‘t?require the arguments to be given in reverse order. Ideally, you would write the?multipush?macro call, then cut-and-paste the line to where the pop needed to be done, and change the name of the called macro to?multipop, and the macro would take care of popping the registers in the opposite order from the one in which they were pushed.

This can be done by the following definition:

This macro begins by rotating its arguments one place to the?right, so that the original?last?argument appears as?%1. This is then popped, and the arguments are rotated right again, so the second-to-last argument becomes?%1. Thus the arguments are iterated through in reverse order.

4.3.7.?Concatenating Macro Parameters

NASM can concatenate macro parameters on to other text surrounding them. This allows you to declare a family of symbols, for example, in a macro definition. If, for example, you wanted to generate a table of key codes along with offsets into the table, you could code something like

%macro keytab_entry 2

keypos%1 equ $-keytab

db %2

keyposReturn equ $-keytab

keytab:

keytab_entry F1,128+1

keytab_entry F2,128+2

keytab_entry Return,13

which would expand to

keytab:

keyposF1 equ $-keytab

db 128+1

keyposF2 equ $-keytab

db 128+2

db 13

You can just as easily concatenate text on to the other end of a macro parameter, by writing?%1foo.

If you need to append a?digit?to a macro parameter, for example defining labels?foo1?and?foo2?when passed the parameter?foo, you can‘t code?%11?because that would be taken as the eleventh macro parameter. Instead, you must code?%{1}1, which will separate the first?1?(giving the number of the macro parameter) from the second (literal text to be concatenated to the parameter).

This concatenation can also be applied to other preprocessor in-line objects, such as macro-local labels (Section?4.3.2) and context-local labels (Section?4.7.2). In all cases, ambiguities in syntax can be resolved by enclosing everything after the?%?sign and before the literal text in braces: so?%{%foo}bar?concatenates the text?bar?to the end of the real name of the macro-local label?%%foo. (This is unnecessary, since the form NASM uses for the real names of macro-local labels means that the two usages?%{%foo}bar?and?%%foobar?would both expand to the same thing anyway; nevertheless, the capability is there.)

4.3.8.?Condition Codes as Macro Parameters

NASM can give special treatment to a macro parameter which contains a condition code. For a start, you can refer to the macro parameter?%1?by means of the alternative syntax?%+1, which informs NASM that this macro parameter is supposed to contain a condition code, and will cause the preprocessor to report an error message if the macro is called with a parameter which is?not?a valid condition code.

Far more usefully, though, you can refer to the macro parameter by means of?%-1, which NASM will expand as the?inverse?condition code. So the?retz?macro defined in?Section?4.3.2?can be replaced by a general?conditional-return macro like this:

This macro can now be invoked using calls like?retc ne, which will cause the conditional-jump instruction in the macro expansion to come out as?JE, or?retc po?which will make the jump a?JPE.

The?%+1?macro-parameter reference is quite happy to interpret the arguments?CXZ?and?ECXZ?as valid condition codes; however,?%-1?will report an error if passed either of these, because no inverse condition code exists.

4.3.9.?Disabling Listing Expansion

When NASM is generating a listing file from your program, it will generally expand multi-line macros by means of writing the macro call and then listing each line of the expansion. This allows you to see which instructions in the macro expansion are generating what code; however, for some macros this clutters the listing up unnecessarily.

NASM therefore provides the?.nolist?qualifier, which you can include in a macro definition to inhibit the expansion of the macro in the listing file. The?.nolist?qualifier comes directly after the number of parameters, like this:

%macro foo 1.nolist

Or like this:

%macro bar 1-5+.nolist a,b,c,d,e,f,g,h

4.4.?Conditional Assembly

%if<condition>

; some code which only appears if <condition> is met

%elif<condition2>

; only appears if <condition> is not met but <condition2> is

%else

; this appears if neither <condition> nor <condition2> was met

The?%else?clause is optional, as is the?%elif?clause. You can have more than one?%elif?clause as well.

4.4.1.?%ifdef: Testing Single-Line Macro Existence

Beginning a conditional-assembly block with the line?%ifdef MACRO?will assemble the subsequent code if, and only if, a single-line macro called?MACRO?is defined. If not, then the?%elif?and?%else?blocks (if any) will be processed instead.

For example, when debugging a program, you might want to write code such as

; perform some function

%ifdef DEBUG

writefile 2,"Function performed successfully",13,10

; go and do something else

Then you could use the command-line option?-D DEBUG?to create a version of the program which produced debugging messages, and remove the option to generate the final release version of the program.

You can test for a macro?not?being defined by using?%ifndef?instead of?%ifdef. You can also test for macro definitions in?%elif?blocks by using?%elifdef?and?%elifndef.

4.4.2.?%ifmacro: Testing Multi-Line Macro Existence

The?%ifmacro?directive operates in the same way as the?%ifdef?directive, except that it checks for the existence of a multi-line macro.

For example, you may be working with a large project and not have control over the macros in a library. You may want to create a macro with one name if it doesn‘t already exist, and another name if one with that name does exist.

The?%ifmacro?is considered true if defining a macro with the given name and number of arguments would cause a definitions conflict. For example:

%ifmacro MyMacro 1-3

%error "MyMacro 1-3" causes a conflict with an existing macro.

%else

%macro MyMacro 1-3

; insert code to define the macro

This will create the macro?MyMacro 1-3?if no macro already exists which would conflict with it, and emits a warning if there would be a definition conflict.

You can test for the macro not existing by using the?%ifnmacro?instead of?%ifmacro. Additional tests can be performed in?%elif?blocks by using?%elifmacro?and?%elifnmacro.

4.4.3.?%ifctx: Testing the Context Stack

The conditional-assembly construct?%ifctx ctxname?will cause the subsequent code to be assembled if and only if the top context on the preprocessor‘s context stack has the name?ctxname. As with?%ifdef, the inverse and?%elif?forms?%ifnctx,%elifctx?and?%elifnctx?are also supported.

For more details of the context stack, see?Section?4.7. For a sample use of?%ifctx, see?Section?4.7.5.

4.4.4.?%if: Testing Arbitrary Numeric Expressions

The conditional-assembly construct?%if expr?will cause the subsequent code to be assembled if and only if the value of the numeric expression?expr?is non-zero. An example of the use of this feature is in deciding when to break out of a%rep?preprocessor loop: see?Section?4.5?for a detailed example.

The expression given to?%if, and its counterpart?%elif, is a critical expression (see?Section?3.8).

%if?extends the normal NASM expression syntax, by providing a set of?relational operators which are not normally available in expressions. The operators?=,?<,?>,?<=,?>=?and?<>?test equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively. The C-like forms?==?and?!=?are supported as alternative forms of?=?and?<>. In addition, low-priority logical operators?&&,?^^?and?||?are provided, supplying?logical AND,?logical XOR andlogical OR. These work like the C logical operators (although C has no logical XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that?^^, for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false.

4.4.5.?%ifidn?and?%ifidni: Testing Exact Text Identity

The construct?%ifidn text1,text2?will cause the subsequent code to be assembled if and only if?text1?and?text2, after expanding single-line macros, are identical pieces of text. Differences in white space are not counted.

For example, the following macro pushes a register or number on the stack, and allows you to treat?IP?as a real register:

Like most other?%if?constructs,?%ifidn?has a counterpart?%elifidn, and negative forms?%ifnidn?and?%elifnidn. Similarly,?%ifidni?has counterparts?%elifidni,?%ifnidni?and?%elifnidni.

4.4.6.?%ifid,?%ifnum,?%ifstr: Testing Token Types

Some macros will want to perform different tasks depending on whether they are passed a number, a string, or an identifier. For example, a string output macro might want to be able to cope with being passed either a string constant or a pointer to an existing string.

The conditional assembly construct?%ifid, taking one parameter (which may be blank), assembles the subsequent code if and only if the first token in the parameter exists and is an identifier.?%ifnum?works similarly, but tests for the token being a numeric constant;?%ifstr?tests for it being a string.

For example, the?writefile?macro defined in?Section?4.3.3?can be extended to take advantage of?%ifstr?in the following fashion:

%macro writefile 2-3+

%ifstr %2

jmp %%endstr

%if %0 = 3

%%str: db %2,%3

%else

%%str: db %2

%endif

%%endstr: mov dx,%%str

mov cx,%%endstr-%%str

%else

mov dx,%2

mov cx,%3

%endif

mov bx,%1

mov ah,0x40

int 0x21

%endmacro

Then the?writefile?macro can cope with being called in either of the following two ways:

writefile [file], strpointer, length

writefile [file], "hello", 13, 10

In the first,?strpointer?is used as the address of an already-declared string, and?length?is used as its length; in the second, a string is given to the macro, which therefore declares it itself and works out the address and length for itself.

Note the use of?%if?inside the?%ifstr: this is to detect whether the macro was passed two arguments (so the string would be a single string constant, and?db %2?would be adequate) or more (in which case, all but the first two would be lumped together into?%3, and?db %2,%3?would be required).

The usual?%elifXXX,?%ifnXXX?and?%elifnXXX?versions exist for each of?%ifid,?%ifnum?and?%ifstr.

4.4.7.?%error: Reporting User-Defined Errors

The preprocessor directive?%error?will cause NASM to report an error if it occurs in assembled code. So if other users are going to try to assemble your source files, you can ensure that they define the right macros by means of code like this:

%ifdef SOME_MACRO

; do some setup

%elifdef SOME_OTHER_MACRO

; do some different setup

%else

%error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.

Then any user who fails to understand the way your code is supposed to be assembled will be quickly warned of their mistake, rather than having to wait until the program crashes on being run and then not knowing what went wrong.

4.5.?Preprocessor Loops

NASM‘s?TIMES?prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level:?%rep.

The directives?%rep?and?%endrep?(%rep?takes a numeric argument, which can be an expression;?%endrep?takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor:

This will generate a sequence of 64?INC?instructions, incrementing every word of memory from?[table]?to?[table+126].

For more complex termination conditions, or to break out of a repeat loop part way along, you can use the?%exitrep?directive to terminate the loop, like this:

fib_number equ ($-fibonacci)/2

This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to?%rep. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi-user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing.

4.6.?Including Other Files

Using, once again, a very similar syntax to the C preprocessor, the NASM preprocessor lets you include other source files into your code. This is done by the use of the?%include?directive:

%include "macros.mac"

will include the contents of the file?macros.mac?into the source file containing the?%include?directive.

Include files are first searched for relative to the directory containing the source file that is performing the inclusion, and then relative to any directories specified on the Yasm command line using the?-I?option (see Section?1.3.3.3), in the order given on the command line (any relative paths on the Yasm command line are relative to the current working directory, e.g. where Yasm is being run from). While this search strategy does not match traditional NASM behavior, it does match the behavior of most C compilers and better handles relative pathnames.

The standard C idiom for preventing a file being included more than once is just as applicable in the NASM preprocessor: if the file?macros.mac?has the form

%ifndef MACROS_MAC

%define MACROS_MAC

; now define some macros

%endif

then including the file more than once will not cause errors, because the second time the file is included nothing will happen because the macro?MACROS_MAC?will already be defined.

You can force a file to be included even if there is no?%include?directive that explicitly includes it, by using the?-P?option on the Yasm command line (see?Section?1.3.3.4).

4.7.?The Context Stack

Having labels that are local to a macro definition is sometimes not quite powerful enough: sometimes you want to be able to share labels between several macro calls. An example might be a?REPEAT?…?UNTIL?loop, in which the expansion of the?REPEAT?macro would need to be able to refer to a label which the?UNTIL?macro had defined. However, for such a macro you would also want to be able to nest these loops.

The NASM preprocessor provides this level of power by means of a?context stack. The preprocessor maintains a stack of?contexts, each of which is characterised by a name. You add a new context to the stack using the?%push?directive, and remove one using?%pop. You can define labels that are local to a particular context on the stack.

4.7.1.?%push?and?%pop: Creating and Removing Contexts

The?%push?directive is used to create a new context and place it on the top of the context stack.?%push?requires one argument, which is the name of the context. For example:

%push foobar

This pushes a new context called?foobar?on the stack. You can have several contexts on the stack with the same name: they can still be distinguished.

The directive?%pop, requiring no arguments, removes the top context from the context stack and destroys it, along with any labels associated with it.

4.7.2.?Context-Local Labels

Just as the usage?%%foo?defines a label which is local to the particular macro call in which it is used, the usage?%$foo?is used to define a label which is local to the context on the top of the context stack. So the?REPEAT?and?UNTILexample given above could be implemented by means of:

and invoked by means of, for example,

which would scan every fourth byte of a string in search of the byte in?AL.

If you need to define, or access, labels local to the context?below?the top one on the stack, you can use?%$$foo, or?%$$$foo?for the context below that, and so on.

4.7.3.?Context-Local Single-Line Macros

The NASM preprocessor also allows you to define single-line macros which are local to a particular context, in just the same way:

%define %$localmac 3

will define the single-line macro?%$localmac?to be local to the top context on the stack. Of course, after a subsequent?%push, it can then still be accessed by the name?%$$localmac.

4.7.4.?%repl: Renaming a Context

If you need to change the name of the top context on the stack (in order, for example, to have it respond differently to?%ifctx), you can execute a?%pop?followed by a?%push; but this will have the side effect of destroying all context-local labels and macros associated with the context that was just popped.

The NASM preprocessor provides the directive?%repl, which?replaces?a context with a different name, without touching the associated macros and labels. So you could replace the destructive code

%pop

%push newname

with the non-destructive version?%repl newname.

4.7.5.?Example Use of the Context Stack: Block IFs

This example makes use of almost all the context-stack features, including the conditional-assembly construct?%ifctx, to implement a block IF statement as a set of macros.

%error "expected `if‘ before `else‘"

%error "expected `if‘ or `else‘ before `endif‘"

Yasm defines a set of standard macros in the NASM preprocessor which are already defined when it starts to process any source file. If you really need a program to be assembled with no pre-defined macros, you can use the?%cleardirective to empty the preprocessor of everything.

This code is more robust than the?REPEAT?and?UNTIL?macros given in?Section?4.7.2, because it uses conditional assembly to check that the macros are issued in the right order (for example, not calling?endif?before?if) and issues a?%errorif they‘re not.

In addition, the?endif?macro has to be able to cope with the two distinct cases of either directly following an?if, or following an?else. It achieves this, again, by using conditional assembly to do different things depending on whether the context on top of the stack is?if?or?else.

The?else?macro has to preserve the context on the stack, in order to have the?%$ifnot?referred to by the?if?macro be the same as the one defined by the?endif?macro, but has to change the context‘s name so that?endif?will know there was an intervening?else. It does this by the use of?%repl.

A sample usage of these macros might look like:

cmp ax,bx

if ae

cmp bx,cx

if ae

mov ax,cx

else

mov ax,bx

endif

else

cmp ax,cx

if ae

mov ax,cx

endif

The block-IF?macros handle nesting quite happily, by means of pushing another context, describing the inner?if, on top of the one describing the outer?if; thus?else?and?endif?always refer to the last unmatched?if?or?else.

4.8.?Standard Macros

Most user-level NASM syntax directives (see?Chapter?5) are implemented as macros which invoke primitive directives; these are described in?Chapter?5. The rest of the standard macro set is described here.

4.8.1.?__YASM_MAJOR__, etc: Yasm Version

The single-line macros?__YASM_MAJOR__,?__YASM_MINOR__, and?__YASM_SUBMINOR__?expand to the major, minor, and subminor parts of the?version number of Yasm being used. In addition,?__YASM_VER__?expands to a string representation of the Yasm version and?__YASM_VERSION_ID__?expands to a 32-bit BCD-encoded representation of the Yasm version, with the major version in the most significant 8 bits, followed by the 8-bit minor version and 8-bit subminor version, and 0 in the least significant 8 bits. For example, under Yasm 0.5.1,?__YASM_MAJOR__?would be defined to be 0,?__YASM_MINOR__?would be defined as 5,?__YASM_SUBMINOR__?would be defined as 1,?__YASM_VER__?would be defined as?"0.5.1", and?__YASM_VERSION_ID__?would be defined as?000050100h.

In addition, the single line macro?__YASM_BUILD__?expands to the Yasm?"build"?number, typically the Subversion changeset number. It should be seen as less significant than the subminor version, and is generally only useful in discriminating between Yasm nightly snapshots or pre-release (e.g. release candidate) Yasm versions.

4.8.2.?__FILE__?and?__LINE__: File Name and Line Number

Like the C preprocessor, the NASM preprocessor allows the user to find out the file name and line number containing the current instruction. The macro?__FILE__?expands to a string constant giving the name of the current input file (which may change through the course of assembly if?%include?directives are used), and?__LINE__?expands to a numeric constant giving the current line number in the input file.

These macros could be used, for example, to communicate debugging information to a macro, since invoking?__LINE__?inside a macro definition (either single-line or multi-line) will return the line number of the macro?call, rather thandefinition. So to determine where in a piece of code a crash is occurring, for example, one could write a routine?stillhere, which is passed a line number in?EAX?and outputs something like?"line 155: still here". You could then write a macro

and then pepper your code with calls to?notdeadyet?until you find the crash point.

4.8.3.?__YASM_OBJFMT__?and?__OUTPUT_FORMAT__: Output Object Format Keyword

__YASM_OBJFMT__, and its NASM-compatible alias?__OUTPUT_FORMAT__, expand to the object format?keyword?specified on the command line with?-f?keyword?(see?Section?1.3.1.2). For example, if?yasm?is invoked with?-f elf,?__YASM_OBJFMT__?expands toelf.

These expansions match the option given on the command line exactly, even when the object formats are equivalent. For example,?-f elf?and?-f elf32?are equivalent specifiers for the 32-bit ELF format, and?-f elf -m amd64?and?-f elf64?are equivalent specifiers for the 64-bit ELF format, but?__YASM_OBJFMT__?would expand to?elf?and?elf32?for the first two cases, and?elf?and?elf64?for the second two cases.

4.8.4.?STRUC?and?ENDSTRUC: Declaring Structure Data Types

The NASM preprocessor is sufficiently powerful that data structures can be implemented as a set of macros. The macros?STRUC?and?ENDSTRUC?are used to define a structure data type.

STRUC?takes one parameter, which is the name of the data type. This name is defined as a symbol with the value zero, and also has the suffix?_size?appended to it and is then defined as an?EQU?giving the size of the structure. Once?STRUChas been issued, you are defining the structure, and should define fields using the?RESB?family of pseudo-instructions, and then invoke?ENDSTRUC?to finish the definition.

For example, to define a structure called?mytype?containing a longword, a word, a byte and a string of bytes, you might code

The above code defines six symbols:?mt_long?as 0 (the offset from the beginning of a?mytype?structure to the longword field),?mt_word?as 4,?mt_byte?as 6,?mt_str?as 7,?mytype_size?as 39, and?mytype?itself as zero.

The reason why the structure type name is defined at zero is a side effect of allowing structures to work with the local label mechanism: if your structure members tend to have the same names in more than one structure, you can define the above structure like this:

This defines the offsets to the structure fields as?mytype.long,?mytype.word,?mytype.byte?and?mytype.str.

Since NASM syntax has no?intrinsic?structure support, does not support any form of period notation to refer to the elements of a structure once you have one (except the above local-label notation), so code such as?mov ax,[mystruc.mt_word]is not valid.?mt_word?is a constant just like any other constant, so the correct syntax is?mov ax,[mystruc+mt_word]?or?mov ax,[mystruc+mytype.word].

4.8.5.?ISTRUC,?AT?and?IEND: Declaring Instances of Structures

Having defined a structure type, the next thing you typically want to do is to declare instances of that structure in your data segment. The NASM preprocessor provides an easy way to do this in the?ISTRUC?mechanism. To declare a structure of type?mytype?in a program, you code something like this:

mystruc: istruc mytype

at mt_long, dd 123456

at mt_word, dw 1024

at mt_byte, db ‘x‘

at mt_str, db ‘hello, world‘, 13, 10, 0

iend

The function of the?AT?macro is to make use of the?TIMES?prefix to advance the assembly position to the correct point for the specified structure field, and then to declare the specified data. Therefore the structure fields must be declared in the same order as they were specified in the structure definition.

If the data to go in a structure field requires more than one source line to specify, the remaining source lines can easily come after the?AT?line. For example:

at mt_str, db 123,134,145,156,167,178,189

db 190,100,0

Depending on personal taste, you can also omit the code part of the?AT?line completely, and start the structure field on the next line:

at mt_str

db ‘hello, world‘

db 13,10,0

4.8.6.?ALIGN?and?ALIGNB: Data Alignment

The?ALIGN?and?ALIGNB?macros provide a convenient way to align code or data on a word, longword, paragraph or other boundary. The syntax of the?ALIGN?and?ALIGNB?macros is

align 4 ; align on 4-byte boundary

align 16 ; align on 16-byte boundary

align 16,nop ; equivalent to previous line

align 8,db 0 ; pad with 0s rather than NOPs

align 4,resb 1 ; align to 4 in the BSS

alignb 4 ; equivalent to previous line

Both macros require their first argument to be a power of two; they both compute the number of additional bytes required to bring the length of the current section up to a multiple of that power of two, and output either NOP fill or apply the?TIMES?prefix to their second argument to perform the alignment.

If the second argument is not specified, the default for?ALIGN?is?NOP, and the default for?ALIGNB?is?RESB 1.?ALIGN?treats a?NOP?argument specially by generating maximal NOP fill instructions (not necessarily NOP opcodes) for the currentBITS?setting, whereas?ALIGNB?takes its second argument literally. Otherwise, the two macros are equivalent when a second argument is specified. Normally, you can just use?ALIGN?in code and data sections and?ALIGNB?in BSS sections, and never need the second argument except for special purposes.

ALIGN?and?ALIGNB, being simple macros, perform no error checking: they cannot warn you if their first argument fails to be a power of two, or if their second argument generates more than one byte of code. In each of these cases they will silently do the wrong thing.

ALIGNB?(or?ALIGN?with a second argument of?RESB 1) can be used within structure definitions:

This will ensure that the structure members are sensibly aligned relative to the base of the structure.

A final caveat:?ALIGNB?works relative to the beginning of the?section, not the beginning of the address space in the final executable. Aligning to a 16-byte boundary when the section you‘re in is only guaranteed to be aligned to a 4-byte boundary, for example, is a waste of effort. Again, Yasm does not check that the section‘s alignment characteristics are sensible for the use of?ALIGNB.?ALIGN?is more intelligent and?does?adjust the section alignment to be the maximum specified alignment.

Chapter?5.?NASM Assembler Directives

5.1. Specifying Target Processor Mode

5.1.1.?BITS

5.1.2.?USE16,?USE32, and?USE64

5.2.?DEFAULT: Change the assembler defaults

5.3. Changing and Defining Sections

5.3.1.?SECTION?and?SEGMENT

5.3.2. Standardized Section Names

5.3.3. The?__SECT__?Macro

5.4.?ABSOLUTE: Defining Absolute Labels

5.5.?EXTERN: Importing Symbols

5.6.?GLOBAL: Exporting Symbols

5.7.?COMMON: Defining Common Data Areas

5.8.?CPU: Defining CPU Dependencies

NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, is nevertheless forced to support a?few?directives. These are described in this chapter.

NASM‘s directives come in two types:?user-level?directives and?primitive?directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms.

Primitive directives are enclosed in square brackets; user-level directives are not.

In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These?format-specific?directives are documented along with the formats that implement them, in?Part?IV.

5.1.?Specifying Target Processor Mode

5.1.1.?BITS

The?BITS?directive specifies whether Yasm should generate code designed to run on a processor operating in 16-bit mode, 32-bit mode, or 64-bit mode. The syntax is?BITS 16,?BITS 32, or?BITS 64.

In most cases, you should not need to use?BITS?explicitly. The?coff,?elf32,?macho32, and?win32?object formats, which are designed for use in 32-bit operating systems, all cause Yasm to select 32-bit mode by default. The?elf64,?macho64, andwin64?object formats, which are designed for use in 64-bit operating systems, both cause Yasm to select 64-bit mode by default. The?xdf?object format allows you to specify each segment you define as?USE16,?USE32, or?USE64, and Yasm will set its operating mode accordingly, so the use of the?BITS?directive is once again unnecessary.

The most likely reason for using the?BITS?directive is to write 32-bit or 64-bit code in a flat binary file; this is because the?bin?object format defaults to 16-bit mode in anticipation of it being used most frequently to write DOS?.COMprograms, DOS?.SYS?device drivers and boot loader software.

You do?not?need to specify?BITS 32?merely in order to use 32-bit instructions in a 16-bit DOS program; if you do, the assembler will generate incorrect code because it will be writing code targeted at a 32-bit platform, to be run on a 16-bit one. However, it?is?necessary to specify?BITS 64?to use 64-bit instructions and registers; this is done to allow use of those instruction and register names in 32-bit or 16-bit programs, although such use will generate a warning.

When Yasm is in?BITS 16?mode, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In?BITS 32?mode, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working in 16-bit addresses need an 0x67.

When Yasm is in?BITS 64?mode, 32-bit instructions usually require no prefixes, and most uses of 64-bit registers or data size requires a REX prefix. Yasm automatically inserts REX prefixes where necessary. There are also 8 more general and SSE registers, and 16-bit addressing is no longer supported. The default address size is 64 bits; 32-bit addressing can be selected with the 0x67 prefix. The default operand size is still 32 bits, however, and the 0x66 prefix selects 16-bit operand size. The REX prefix is used both to select 64-bit operand size, and to access the new registers. A few instructions have a default 64-bit operand size.

When the REX prefix is used, the processor does not know how to address the?AH,?BH,?CH?or?DH?(high 8-bit legacy) registers. Instead, it is possible to access the the low 8-bits of the?SP,?BP?SI, and?DI?registers as?SPL,?BPL,?SIL, and?DIL, respectively; but only when the REX prefix is used.

The?BITS?directive has an exactly equivalent primitive form,?[BITS 16],?[BITS 32], and?[BITS 64]. The user-level form is a macro which has no function other than to call the primitive form.

5.1.2.?USE16,?USE32, and?USE64

The?USE16,?USE32, and?USE64?directives can be used in place of?BITS 16,?BITS 32, and?BITS 64?respectively for compatibility with other assemblers.

5.2.?DEFAULT: Change the assembler defaults

The?DEFAULT?directive changes the assembler defaults. Normally, Yasm defaults to a mode where the programmer is expected to explicitly specify most features directly. However, sometimes this is not desirable if a certain behavior is very commmonly used.

Currently, the only?DEFAULT?that is settable is whether or not registerless effective addresses in 64-bit mode are?RIP-relative or not. By default, they are absolute unless overridden with the?REL?specifier (see?Section?3.3). However, ifDEFAULT REL?is specified,?REL?is default, unless overridden with the?ABS?specifier, a?FS?or?GS?segment override is used, or another register is part of the effective address.

The special handling of?FS?and?GS?overrides are due to the fact that these segments are the only segments which can have non-0 base addresses in 64-bit mode, and thus are generally used as thread pointers or other special functions. With a non-zero base address, generating?RIP-relative addresses for these forms would be extremely confusing. Other segment registers such as?DS?always have a base address of 0, so RIP-relative access still makes sense.

DEFAULT REL?is disabled with?DEFAULT ABS. The default mode of the assembler at start-up is?DEFAULT ABS.

5.3.?Changing and Defining Sections

5.3.1.?SECTION?and?SEGMENT

The?SECTION?directive (((SEGMENT)) is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence?SECTION?may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist.

5.3.2.?Standardized Section Names

The Unix object formats, and the?bin?object format, all support the?standardised section names?.text,?.data?and?.bss?for the code, data and uninitialised-data sections. The?obj?format, by contrast, does not recognise these section names as being special, and indeed will strip off the leading period of any section name that has one.

5.3.3.?The?__SECT__?Macro

The?SECTION?directive is unusual in that its user-level form functions differently from its primitive form. The primitive form,?[SECTION xyz], simply switches the current target section to the one given. The user-level form,?SECTION xyz, however, first defines the single-line macro?__SECT__?to be the primitive?[SECTION]?directive which it is about to issue, and then issues it. So the user-level directive

SECTION .text

expands to the two lines

%define __SECT__ [SECTION .text]

[SECTION .text]

Users may find it useful to make use of this in their own macros. For example, the?writefile?macro defined in the NASM Manual can be usefully rewritten in the following more sophisticated form:

mov cx,%%endstr-%%str

This form of the macro, once passed a string to output, first switches temporarily to the data section of the file, using the primitive form of the?SECTION?directive so as not to modify?__SECT__. It then declares its string in the data section, and then invokes?__SECT__?to switch back to?whichever?section the user was previously working in. It thus avoids the need, in the previous version of the macro, to include a?JMP?instruction to jump over the data, and also does not fail if, in a complicated?OBJ?format module, the user could potentially be assembling the code in any of several separate code sections.

5.4.?ABSOLUTE: Defining Absolute Labels

The?ABSOLUTE?directive can be thought of as an alternative form of?SECTION: it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the?RESB?family.

ABSOLUTE?is used as follows:

This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines?kbuf_chr?to be 0x1A,?kbuf_free?to be 0x1C, and?kbuf?to be 0x1E.

The user-level form of?ABSOLUTE, like that of?SECTION, redefines the?__SECT__?macro when it is invoked.

STRUC?and?ENDSTRUC?are defined as macros which use?ABSOLUTE?(and also?__SECT__).

ABSOLUTE?doesn‘t have to take an absolute constant as an argument: it can take an expression (actually, a?critical expression: see?Section?3.8) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this:

org 100h ; it‘s a .COM program

jmp setup ; setup code comes last

; the resident part of the TSR goes here

setup: ; now write the code that installs the TSR here

absolute setup

runtimevar1 resw 1

runtimevar2 resd 20

tsr_end:

This defines some variables?"on top of"?the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol?"tsr_end"?can be used to calculate the total size of the part of the TSR that needs to be made resident.

5.5.?EXTERN: Importing Symbols

The?EXTERN?directive takes as many arguments as you like. Each argument is the name of a symbol:

extern _printf

extern _sscanf, _fscanf

Some object-file formats provide extra features to the?EXTERN?directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the?obj?format allows you to declare that the default segment base of an external should be the group?dgroup?by means of the directive

extern _variable:wrt dgroup

The primitive form of?EXTERN?differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level.

You can declare the same variable as?EXTERN?more than once: NASM will quietly ignore the second and later redeclarations. You can‘t declare a variable as?EXTERN?as well as something else, though.

5.6.?GLOBAL: Exporting Symbols

GLOBAL?is the other end of?EXTERN: if one module declares a symbol as?EXTERN?and refers to it, then in order to prevent linker errors, some other module must actually?define?the symbol and declare it as?GLOBAL. Some assemblers use the name?PUBLIC?for this purpose.

The?GLOBAL?directive applying to a symbol must appear?before?the definition of the symbol.

GLOBAL?uses the same syntax as?EXTERN, except that it must refer to symbols which?are?defined in the same module as the?GLOBAL?directive. For example:

global _main

_main: ; some code

GLOBAL, like?EXTERN, allows object formats to define private extensions by means of a colon. The?elf?object format, for example, lets you specify whether global data items are functions or data:

global hashlookup:function, hashtable:data

Like?EXTERN, the primitive form of?GLOBAL?differs from the user-level form only in that it can take only one argument at a time.

5.7.?COMMON: Defining Common Data Areas

The?COMMON?directive is used to declare?common variables. A common variable is much like a global variable declared in the uninitialised data section, so that

common intvar 4

is similar in function to

global intvar

section .bss

intvar resd 1

The difference is that if more than one module defines the same common variable, then at link time those variables will be?merged, and references to?intvar?in all modules will point at the same piece of memory.

Like?GLOBAL?and?EXTERN,?COMMON?supports object-format specific extensions. For example, the?obj?format allows common variables to be NEAR or FAR, and the?elf?format allows you to specify the alignment requirements of a common variable:

common commvar 4:near ; works in OBJ

common intarray 100:4 ; works in ELF: 4 byte aligned

Once again, like?EXTERN?and?GLOBAL, the primitive form of?COMMON?differs from the user-level form only in that it can take only one argument at a time.

5.8.?CPU: Defining CPU Dependencies

The?CPU?directive restricts assembly to those instructions which are available on the specified CPU. See?Part?VI?for?CPU?options for various architectures.

All options are case insensitive. Instructions will be enabled only if they apply to the selected cpu or lower.

Part?III.?GAS Syntax

The chapters in this part of the book document the GNU AS-compatible syntax accepted by the Yasm?"gas"?parser.

The chapters in this part of the book document Yasm‘s support for various object file formats.

6. TBD

Chapter?6.?TBD

To be written.

Part?IV.?Object Formats

Chapter?7.?bin: Flat-Form Binary Output

The?bin?"object format"?does not produce object files: the output file produced contains only the section data; no headers or relocations are generated. The output can be considered?"plain binary", and is useful for operating system and boot loader development, generating MS-DOS?.COM?executables and?.SYS?device drivers, and creating images for embedded target environments (e.g.?Flash ROM).

The?bin?object format supports an unlimited number of named sections. See?Section?7.2?for details. The only restriction on these sections is that their storage locations in the output file cannot overlap.

When used with the x86 architecture, the?bin?object format starts Yasm in 16-bit mode. In order to write native 32-bit or 64-bit code, an explicit?BITS 32?or?BITS 64?directive is required respectively.

bin?produces an output file with no extension by default; it simply strips the extension from the input file name. Thus the default output filename for the input file?foo.asm?is simply?foo.

7.1.?ORG: Binary Origin

bin?provides the?ORG?directive in NASM syntax to allow setting of the memory address at which the output file is initially loaded. The?ORG?directive may only be used once (as the output file can only be initially loaded into a single location). If?ORG?is not specified,?ORG 0?is used by default.

This makes the operation of NASM-syntax?ORG?very different from the operation of?ORG?in other assemblers, which typically simply move the assembly location to the value given.?bin?provides a more powerful alternative in the form of extensions to the?SECTION?directive; see?Section?7.2?for details.

When combined with multiple sections,?ORG?also has the effect of defaulting the LMA of the first section to the?ORG?value to make the output file as small as possible. If this is not the desired behavior, explicitly specify a LMA for all sections via either?START?or?FOLLOWS?qualifiers in the?SECTION?directive.

7.2.?bin?Extensions to the?SECTION?Directive

The?bin?object format allows the use of multiple sections of arbitrary names. It also extends the?SECTION?(or?SEGMENT) directive to allow complex ordering of the segments both in the output file or initial load address (also known as LMA) and at the ultimate execution address (the virtual address or VMA).

The?VMA is the execution address. Yasm calculates absolute memory references within a section assuming that the program code is at the VMA while being executed. The LMA, on the other hand, specifies where a section is?initially?loaded, as well as its location in the output file.

Often, VMA will be the same as LMA. However, they may be different if the program or another piece of code copies (relocates) a section prior to execution. A typical example of this in an embedded system would be a piece of code stored in ROM, but is copied to faster RAM prior to execution. Another example would be overlays: sections loaded on demand from different file locations to the same execution location.

The?bin?extensions to the?SECTION?directive allow flexible specification of both VMA and LMA, including alignment constraints. As with other object formats, additional attributes may be added after the section name. The available attributes are listed in?Table?7.1.

Table?7.1.?bin?Section Attributes

Attribute	Indicates the section
progbits	is stored in the disk image, as opposed to allocated and initialized at load.
nobits	is allocated and initialized at load (the opposite of?progbits). Only one of?progbits?or?nobits?may be specified; they are mutually exclusive attributes.
start=address	has an LMA starting at?address. If a LMA alignment constraint is given, it is checked against the provided address and a warning is issued if?address?does not meet the alignment constraint.
follows=sectname	should follow the section named?sectname?in the output file (LMA). If a LMA alignment constraint is given, it is respected and a gap is inserted such that the section meets its alignment requirement. Note that as LMA overlap is not allowed, typically only one section may follow another.
align=n	requires a LMA alignment of?n?bytes. The value?n?must always be a power of 2. LMA alignment defaults to 4 if not specified.
vstart=address	has an VMA starting at?address. If a VMA alignment constraint is given, it is checked against the provided address and a warning is issued if?address?does not meet the alignment constraint.
vfollows=sectname	should follow the section named?sectname?in the output file (VMA). If a VMA alignment constraint is given, it is respected and a gap is inserted such that the section meets its alignment requirement. VMA overlap is allowed, so more than one section may follow another (possibly useful in the case of overlays).
valign=n	requires a VMA alignment of?n?bytes. The value?n?must always be a power of 2. VMA alignment defaults to the LMA alignment if not specified.

Only one of?start?or?follows?may be specified for a section; the same restriction applies to?vstart?and?vfollows.

Unless otherwise specified via the use of?follows?or?start, Yasm by default assumes the implicit ordering given by the order of the sections in the input file. A section named?.text?is always the first section. Any code which comes before an explicit?SECTION?directive goes into the?.text?section. The?.text?section attributes may be overridden by giving an explicit?SECTION .text?directive with attributes.

Also, unless otherwise specified, Yasm defaults to setting VMA=LMA. If just?"valign` is specified, Yasm just takes the LMA and aligns it to the required alignment. This may have the effect of pushing following sections"?VMAs to non-LMA addresses as well, to avoid VMA overlap.

Yasm treats?nobits?sections in a special way in order to minimize the size of the output file. As?nobits?sections can be 0-sized in the LMA realm, but cannot be if located between two other sections (due to the VMA=LMA default), Yasm moves all?nobits?sections with unspecified LMA to the end of the output file, where they can savely have 0 LMA size and thus not take up any space in the output file. If this behavior is not desired, a?nobits?section LMA (just like aprogbits?section) may be specified using either the?follows?or?start?section attribute.

7.3.?bin?Special Symbols

To facilitate writing code that copies itself from one location to another (e.g. from its LMA to its VMA during execution), the?bin?object format provides several special symbols for every defined section. Each special symbol begins with?section.?followed by the section name. The supported special?bin?symbols are:

section.sectname.start

Set to the LMA address of the section named?sectname.

section.sectname.vstart

Set to the VMA address of the section named?sectname.

section.sectname.length

Set to the length of the section named?sectname. The length is considered the runtime length, so?"nobits` sections"?length is their runtime length, not 0.

7.4.?Map Files

Map files may be generated in?bin?via the use of the?[MAP]?directive. The map filename may be specified either with a command line option (--mapfile=filename) or in the?[MAP]?directive. If a map is requested but no output filename is given, the map output goes to standard output by default.

If no?[MAP]?directive is given in the input file, no map output is generated. If?[MAP]?is given with no options, a brief map is generated. The?[MAP]?directive accepts the following options to control what is included in the map file. More than one option may be specified. Any option other than the ones below is interpreted as the output filename.

brief

Includes the input and output filenames, origin (ORG?value), and a brief section summary listing the VMA and LMA start and stop addresses and the section length of every section.

sections?,?segments

Includes a detailed list of sections, including the VMA and LMA alignment, any?"follows"?settings, as well as the VMA and LMA start addresses and the section length.

symbols

Includes a detailed list of all EQU values and VMA and LMA symbol locations, grouped by section.

all

All of the above.

Chapter?8.?coff: Common Object File Format

Chapter?9.?elf32: Executable and Linkable Format 32-bit Object Files

9.1. Debugging Format Support

9.2. ELF Sections

9.3. ELF Directives

9.3.1.?IDENT: Add file identification

9.3.2.?SIZE: Set symbol size

9.3.3.?TYPE: Set symbol type

9.3.4.?WEAK: Create weak symbol

9.4. ELF Extensions to the?GLOBAL?Directive

9.5. ELF Extensions to the?COMMON?Directive

9.6.?elf32?Special Symbols and?WRT

The Executable and Linkable Object Format is the primary object format for many operating systems including?FreeBSD or GNU/Linux. It appears in three forms:

Shared object files (.so)
Relocatable object files (.o)
Executable files (no convention)

Yasm only directly supports relocatable object files. Other tools, such as the GNU Linker?ld, help turn relocatable object files into the other formats. Yasm supports generation of both 32-bit and 64-bit ELF files, called?elf32?andelf64. An additional format, called?elfx32, is a 32-bit ELF file that supports 64-bit execution (instructions and registers) while limiting pointer sizes to 32-bit.

Yasm defaults to?BITS 32?mode when outputting to the?elf32?object format.

9.1.?Debugging Format Support

ELF supports two debugging formats:?stabs?(see?Chapter?20) and?dwarf2?(see?Chapter?19). Different debuggers understand these different formats; the newer debug format is?dwarf2, so try that first.

9.2.?ELF Sections

ELF‘s section-based output supports attributes on a per-section basis. These attributes include?alloc,?exec,?write,?progbits, and?align. Except for align, they can each be negated in NASM syntax by prepending?"no", e.g.,?"noexec". The attributes are later read by the operating system to select the proper behavior for each section, with the meanings shown in?Table?9.1.

Table?9.1.?ELF Section Attributes

Attribute	Indicates the section
alloc	is loaded into memory at runtime. This is true for code and data sections, and false for metadata sections.
exec	has permission to be run as executable code.
write	is writable at runtime.
progbits	is stored in the disk image, as opposed to allocated and initialized at load.
align=n	requires a memory alignment of?n?bytes. The value?n?must always be a power of 2.

In NASM syntax, the attribute?nobits?is provided as an alias for?noprogbits.

The standard primary sections have attribute defaults according their expected use, and any unknown section gets its own defaults, as shown in?Table?9.2.

Table?9.2.?ELF Standard Sections

Section	alloc	exec	write	progbits	align
.bss	alloc	?	write	?	4
.data	alloc	?	write	progbits	4
.rodata	alloc	?	?	progbits	4
.text	alloc	exec	?	progbits	16
.comment	?	?	?	progbits	0
unknown	alloc	?	?	progbits	1

9.3.?ELF Directives

ELF adds additional assembler directives to define weak symbols (WEAK), set symbol size (SIZE), and indicate whether a symbol is specifically a function or an object (TYPE). ELF also adds a directive to assist in identifying the source file or version,?IDENT.

9.3.1.?IDENT: Add file identification

The?IDENT?directive allows adding arbitrary string data to an ELF object file that will be saved in the object and executable file, but will not be loaded into memory like data in the?.data?section. It is often used for saving?version control keyword information from tools such as?cvs?or?svn?into files so that the source revision the object was created with can be read using the?ident?command found on most Unix systems.

The directive takes one or more string parameters. Each parameter is saved in sequence as a 0-terminated string in the?.comment?section of the object file. Multiple uses of the?IDENT?directive are legal, and the strings will be saved into the?.comment?section in the order given in the source file.

In NASM syntax, no wrapper macro is provided for?IDENT, so it must be wrapped in square brackets. Example use in NASM syntax:

[ident "$Id$"]

9.3.2.?SIZE: Set symbol size

ELF‘s symbol table has the capability of storing a size for a symbol. This is commonly used for functions or data objects. While the size can be specificed directly for?COMMON?symbols, the?SIZE?directive allows for specifying the size of any symbol, including local symbols.

The directive takes two parameters; the first parameter is the symbol name, and the second is the size. The size may be a constant or an expression. Example:

func:

ELF‘s symbol table has the capability of indicating whether a symbol is a function or data. While this can be specified directly in the?GLOBAL?directive (see?Section?9.4), the?TYPE?directive allows specifying the symbol type for any symbol, including local symbols.

.end:

size func func.end-func

9.3.3.?TYPE: Set symbol type

The directive takes two parameters; the first parameter is the symbol name, and the second is the symbol type. The symbol type must be either?function?or?object. An unrecognized type will cause a warning to be generated. Example of use:

9.3.4.?WEAK: Create weak symbol

ELF allows defining certain symbols as?"weak". Weak symbols are similar to global symbols, except during linking, weak symbols are only chosen after global and local symbols during symbol resolution. Unlike global symbols, multiple object files may declare the same weak symbol, and references to a symbol get resolved against a weak symbol only if no global or local symbols have the same name.

This functionality is primarily useful for libraries that want to provide common functions but not come into conflict with user programs. For example, libc has a syscall (function) called?"read". However, to implement a threaded process using POSIX threads in user-space, libpthread needs to supply a function also called?"read"?that provides a blocking interface to the programmer, but actually does non-blocking calls to the kernel. To allow an application to be linked to both libc and libpthread (to share common code), libc needs to have its version of the syscall with a non-weak name like?"_sys_read"?with a weak symbol called?"read". If an application is linked against libc only, the linker won‘t find a non-weak symbol for?"read", so it will use the weak one. If the same application is linked against libc?and?libpthread, then the linker will link?"read"?calls to the symbol in libpthread, ignoring the weak one in libc, regardless of library link order. If libc used a non-weak name, which?"read"?function the program ended up with might depend on a variety of factors; a weak symbol is a way to tell the linker that a symbol is less important resolution-wise.

The?WEAK?directive takes a single parameter, the symbol name to declare weak. Example:

9.4.?ELF Extensions to the?GLOBAL?Directive

ELF object files can contain more information about a global symbol than just its address: they can contain the size of the symbol and its type as well. These are not merely debugger conveniences, but are actually necessary when the program being written is a ((shared library)). Yasm therefore supports some extensions to the NASM syntax?GLOBAL?directive (see?Section?5.6), allowing you to specify these features. Yasm also provides the ELF-specific directives inSection?9.3?to allow specifying this information for non-global symbols.

You can specify whether a global variable is a function or a data object by suffixing the name with a colon and the word?function?or?data. (((object)) is a synonym for?data.) For example:

global hashlookup:function, hashtable:data

exports the global symbol?hashlookup?as a function and?hashtable?as a data object.

Optionally, you can control the ELF visibility of the symbol. Just add one of the visibility keywords:?default,?internal,?hidden, or?protected. The default is?default, of course. For example, to make?hashlookup?hidden:

global hashlookup:function hidden

You can also specify the size of the data associated with the symbol, as a numeric expression (which may involve labels, and even forward references) after the type specifier. Like this:

global hashtable:data (hashtable.end - hashtable)

hashtable:

db this,that,theother ; some data here

.end:

This makes Yasm automatically calculate the length of the table and place that information into the ELF symbol table. The same information can be given more verbosely using the?TYPE?(see?Section?9.3.3) and?SIZE?(see?Section?9.3.2) directives as follows:

global hashtable

type hashtable object

size hashtable hashtable.end - hashtable

hashtable:

db this,that,theother ; some data here

.end:

Declaring the type and size of global symbols is necessary when writing shared library code.

9.5.?ELF Extensions to the?COMMON?Directive

ELF also allows you to specify alignment requirements on common variables. This is done by putting a number (which must be a power of two) after the name and size of the common variable, separated (as usual) by a colon. For example, an array of doublewords would benefit from 4-byte alignment:

common dwordarray 128:4

This declares the total size of the array to be 128 bytes, and requires that it be aligned on a 4-byte boundary.

9.6.?elf32?Special Symbols and?WRT

The ELF specification contains enough features to allow position-independent code (PIC) to be written, which makes ELF shared libraries very flexible. However, it also means Yasm has to be able to generate a variety of strange relocation types in ELF object files, if it is to be an assembler which can write?PIC.

Since ELF does not support segment-base references, the?WRT?operator is not used for its normal purpose; therefore Yasm‘s?elf32?output format makes use of?WRT?for a different purpose, namely the PIC-specific relocation types.

elf32?defines five special symbols which you can use as the right-hand side of the?WRT?operator to obtain PIC relocation types. They are?..gotpc,?..gotoff,?..got,?..plt?and?..sym. Their functions are summarized here:

..gotpc

Referring to the symbol marking the?global offset table base using?wrt ..gotpc?will end up giving the distance from the beginning of the current section to the global offset table. (((_GLOBAL_OFFSET_TABLE_)) is the standard symbol name used to refer to the?GOT.) So you would then need to add?$$?to the result to get the real address of the GOT.

..gotoff

Referring to a location in one of your own sections using?wrt ..gotoff?will give the distance from the beginning of the GOT to the specified location, so that adding on the address of the GOT would give the real address of the location you wanted.

..got

Referring to an external or global symbol using?wrt ..got?causes the linker to build an entry?in?the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.

..plt

Referring to a procedure name using?wrt ..plt?causes the linker to build a?procedure linkage table entry for the symbol, and the reference gives the address of the?PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for?CALL?or?JMP), since ELF contains no relocation type to refer to PLT entries absolutely.

..sym

Referring to a symbol name using?wrt ..sym?causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.

Chapter?10.?elf64: Executable and Linkable Format 64-bit Object Files

10.1.?elf64?Special Symbols and?WRT

The?elf64?object format is the 64-bit version of the Executable and Linkable Object Format. As it shares many similarities with?elf32, only differences between?elf32?and?elf64?will be described in this chapter. For details on?elf32, see Chapter?9.

Yasm defaults to?BITS 64?mode when outputting to the?elf64?object format.

elf64?supports the same debug formats as?elf32, however, the?stabs?debug format is limited to 32-bit addresses, so?dwarf2?(see?Chapter?19) is the recommended debugging format.

elf64?also supports the exact same sections, section attributes, and directives as?elf32. See?Section?9.2?for more details on section attributes, and?Section?9.3?for details on the additional directives ELF provides.

10.1.?elf64?Special Symbols and?WRT

The primary difference between?elf32?and?elf64?(other than 64-bit support in general) is the differences in shared library handling and position-independent code. As?BITS 64?enables the use of?RIP-relative addressing, most variable accesses can be relative to RIP, allowing easy relocation of the shared library to a different memory address.

While RIP-relative addressing is available, it does not handle all possible variable access modes, so special symbols are still required, as in?elf32. And as with?elf32, the?elf64?output format makes use of?WRT?for utilizing the?PIC-specific relocation types.

elf64?defines four special symbols which you can use as the right-hand side of the?WRT?operator to obtain PIC relocation types. They are?..gotpcrel,?..got,?..plt?and?..sym. Their functions are summarized here:

..gotpcrel

While RIP-relative addressing allows you to encode an instruction pointer relative data reference to?foo?with?[rel foo], it‘s sometimes necessary to encode a RIP-relative reference to a linker-generated symbol pointer for symbol foo; this is done using?wrt ..gotpcrel, e.g.?[rel foo wrt ..gotpcrel]. Unlike in?elf32, this relocation, combined with RIP-relative addressing, makes it possible to load an address from the ((global offset table)) using a single instruction. Note that since RIP-relative references are limited to a signed 32-bit displacement, the?GOT size accessible through this method is limited to 2 GB.

..got

As in?elf32, referring to an external or global symbol using?wrt ..got?causes the linker to build an entry?in?the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.

..plt

As in?elf32, referring to a procedure name using?wrt ..plt?causes the linker to build a?procedure linkage table entry for the symbol, and the reference gives the address of the?PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for?CALL?or?JMP), since ELF contains no relocation type to refer to PLT entries absolutely.

..sym

As in?elf32, referring to a symbol name using?wrt ..sym?causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.

Chapter?11.?elfx32: ELF 32-bit Object Files for 64-bit Processors

11.1.?elfx32?Special Symbols and?WRT

The?elfx32?object format is the 32-bit version of the Executable and Linkable Object Format for 64-bit execution. Similar to?elf64, it allows for use of 64-bit registers and instructions, but like?elf32, limits pointers to 32 bits in size. As it shares many similarities with?elf32?and?elf64, only differences between these formats and?elfx32?will be described in this chapter. For details on?elf32, see?Chapter?9; for details on?elf64, see?Chapter?10. Operating system support for?elfx32?is currently less common than for?elf64.

Yasm defaults to?BITS 64?mode when outputting to the?elfx32?object format.

elfx32?supports the same debug formats, sections, section attributes, and directives as?elf32?and?elf64. See?Section?9.2?for more details on section attributes, and?Section?9.3?for details on the additional directives ELF provides.

11.1.?elfx32?Special Symbols and?WRT

Due to the availability of RIP-relative addressing,?elfx32?shared library handling and position-independent code is essentially identical to?elf64.

As in?elf64,?elfx32?defines four special symbols which you can use as the right-hand side of the?WRT?operator to obtain PIC relocation types. They are?..gotpcrel,?..got,?..plt?and?..sym?and have the same functionality as they do in?elf64. Their functions are summarized here:

..gotpcrel

While RIP-relative addressing allows you to encode an instruction pointer relative data reference to?foo?with?[rel foo], it‘s sometimes necessary to encode a RIP-relative reference to a linker-generated symbol pointer for symbol foo; this is done using?wrt ..gotpcrel, e.g.?[rel foo wrt ..gotpcrel]. As in?elf64, this relocation, combined with RIP-relative addressing, makes it possible to load an address from the ((global offset table)) using a single instruction. Note that since RIP-relative references are limited to a signed 32-bit displacement, the?GOT size accessible through this method is limited to 2 GB.

..got

As in?elf64, referring to an external or global symbol using?wrt ..got?causes the linker to build an entry?in?the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.

..plt

As in?elf64, referring to a procedure name using?wrt ..plt?causes the linker to build a?procedure linkage table entry for the symbol, and the reference gives the address of the?PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for?CALL?or?JMP), since ELF contains no relocation type to refer to PLT entries absolutely.

..sym

As in?elf64, referring to a symbol name using?wrt ..sym?causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.

Chapter?12.?macho32: Mach 32-bit Object File Format

Chapter?13.?macho64: Mach 64-bit Object File Format

Chapter?14.?rdf: Relocatable Dynamic Object File Format

Chapter?15.?win32: Microsoft Win32 Object Files

15.1.?win32?Extensions to the?SECTION?Directive

15.2.?win32: Safe Structured Exception Handling

The?win32?object format generates Microsoft?Win32 object files for use on the 32-bit native?Windows XP (and Vista) platforms. Object files produced using this object format may be linked with 32-bit Microsoft linkers such as?Visual Studio in order to produce 32-bit?PE executables.

The?win32?object format provides a default output filename extension of?.obj.

Note that although Microsoft say that Win32 object files follow the?COFF?(Common Object File Format) standard, the object files produced by Microsoft Win32 compilers are not compatible with COFF linkers such as DJGPP‘s, and vice versa. This is due to a difference of opinion over the precise semantics of PC-relative relocations. To produce COFF files suitable for DJGPP, use the?coff?output format; conversely, the?coff?format does not produce object files that Win32 linkers can generate correct output from.

15.1.?win32?Extensions to the?SECTION?Directive

The?win32?object format allows you to specify additional information on the?SECTION?directive line, to control the type and properties of sections you declare. Section types and properties are generated automatically by Yasm for thestandard section names?.text,?.data?and?.bss, but may still be overridden by these qualifiers.

The available qualifiers are:

code?or?text

Defines the section to be a code section. This marks the section as readable and executable, but not writable, and also indicates to the linker that the type of the section is code.

data?or?bss

Defines the section to be a data section, analogously to?code. Data sections are marked as readable and writable, but not executable.?data?declares an initialized data section, whereas?bss?declares an uninitialized data section.

rdata

Declares an initialized data section that is readable but not writable. Microsoft compilers use this section to place constants in it.

info

Defines the section to be an?informational section, which is not included in the executable file by the linker, but may (for example) pass information?to?the linker. For example, declaring an?info-type section called?.drectve?causes the linker to interpret the contents of the section as command-line options.

align=n

Specifies the alignment requirements of the section. The maximum you may specify is 8192: the Win32 object file format contains no means to request a greater section alignment. If alignment is not explicitly specified, the defaults are 16-byte alignment for code sections, 8-byte alignment for rdata sections and 4-byte alignment for data (and BSS) sections. Informational sections get a default alignment of 1 byte (no alignment), though the value does not matter. The alignment must be a power of 2.

Other qualifiers are supported which control specific section flags:?discard,?cache,?page,?share,?execute,?read,?write, and?base. Each of these sets the similarly-named section flag, while prefixing them with?no?clears the corresponding section flag; e.g.?nodiscard?clears the discard flag.

The defaults assumed by Yasm if you do not specify the above qualifiers are:

section .text code align=16

section .data data align=4

section .rdata rdata align=8

section .rodata rdata align=8

section .rdata$ rdata align=8

section .bss bss align=4

section .drectve info

section .comment info

Any other section name is treated by default like?.text.

15.2.?win32: Safe Structured Exception Handling

Among other improvements in Windows XP SP2 and Windows Server 2003 Microsoft introduced the concept of "safe structured exception handling." The general idea is to collect handlers‘ entry points in a designated read-only table and have each entry point verified against this table for exceptions prior to control being passed to the handler. In order for an executable to be created with a safe exception handler table, each object file on the linker command line must contain a special symbol named?@feat.00. If any object file passed to the linker does not have this symbol, then the exception handler table is omitted from the executable and thus the run-time checks will not be performed for the application. By default, the table is omitted from the executable silently if this happens and therefore can be easily overlooked. A user can instruct the linker to refuse to produce an executable without this table by passing the/safeseh?command line option.

As of version 1.1.0, Yasm adds this special symbol to?win32?object files so its output does not fail to link with?/safeseh.

Yasm also has directives to support registering custom exception handlers. The?safeseh?directive instructs the assembler to produce appropriately formatted input data for the safe exception handler table. A typical use case is given in Example?15.1.

Example?15.1.?Win32?safeseh?Example

section .text

extern _MessageBoxA@16

safeseh handler ; register handler as "safe handler"

handler:

push DWORD 1 ; MB_OKCANCEL

sub eax,1 ; incidentally suits as return value

; for exception handler

mov DWORD [fs:0],esp ; engage exception handler

xor eax,eax

mov eax,DWORD[eax] ; cause exception

pop DWORD [fs:0] ; disengage exception handler

add esp,4

text: db ‘OK to rethrow, CANCEL to generate core dump‘,0

caption:db ‘SEGV‘,0

section .drectve info

db ‘/defaultlib:user32.lib /defaultlib:msvcrt.lib ‘

If an application has a safe exception handler table, attempting to execute any unregistered exception handler will result in immediate program termination. Thus it is important to register each exception handler‘s entry point with the?safeseh?directive.

All mentions of linker in this section refer to the Microsoft linker version 7.x and later. The presence of the?@feat.00?symbol and the data for the safe exception handler table cause no backward incompatibilities and thus "safeseh" object files generated can still be linked by earlier linker versions or by non-Microsoft linkers.

Chapter?16.?win64: PE32+ (Microsoft Win64) Object Files

16.1.?win64?Extensions to the?SECTION?Directive

16.2.?win64?Structured Exception Handling

16.2.1. x64 Stack, Register and Function Parameter Conventions

16.2.2. Types of Functions

16.2.3. Frame Function Structure

16.2.4. Stack Frame Details

16.2.5. Yasm Primitives for Unwind Operations

16.2.6. Yasm Macros for Formal Stack Operations

The?win64?or?x64?object format generates Microsoft?Win64 object files for use on the 64-bit native?Windows XP x64 (and Vista x64) platforms. Object files produced using this object format may be linked with 64-bit Microsoft linkers such as that in?Visual Studio 2005 and 2008 in order to produce 64-bit?PE32+ executables.

win64?provides a default output filename extension of?.obj.

16.1.?win64?Extensions to the?SECTION?Directive

Like the?win32?format,?win64?allows you to specify additional information on the?SECTION?directive line, to control the type and properties of sections you declare.

16.2.?win64?Structured Exception Handling

Most functions that make use of the stack in 64-bit versions of Windows must support exception handling even if they make no internal use of such facilities. This is because these operating systems locate exception handlers by using a process called?"stack unwinding"?that depends on functions providing data that describes how they use the stack.

When an exception occurs the stack is?"unwound"?by working backwards through the chain of function calls prior to the exception event to determine whether functions have appropriate exception handlers or whether they have saved non-volatile registers whose value needs to be restored in order to reconstruct the execution context of the next higher function in the chain. This process depends on compilers and assemblers providing?"unwind data"?for functions.

The following sections give details of the mechanisms that are available in Yasm to meet these needs and thereby allow functions written in assembler to comply with the coding conventions used in 64-bit versions of Windows. These Yasm facilities follow those provided in MASM.

16.2.1.?x64 Stack, Register and Function Parameter Conventions

Figure?16.1?shows how the stack is typically used in function calls. When a function is called, an 8 byte return address is automatically pushed onto the stack and the function then saves any non-volatile registers that it will use. Additional space can also be allocated for local variables and a frame pointer register can be assigned if needed.

Figure?16.1.?x64 Calling Convention

The first four integer function parameters are passed (in left to right order) in the registers RCX, RDX, R8 and R9. Further integer parameters are passed on the stack by pushing them in right to left order (parameters to the left at lower addresses). Stack space is allocated for the four register parameters ("shadow space") but their values are not stored by the calling function so the called function must do this if necessary. The called function effectively owns this space and can use it for any purpose, so the calling function cannot rely on its contents on return. Register parameters occupy the least significant ends of registers and shadow space must be allocated for four register parameters even if the called function doesn‘t have this many parameters.

The first four floating point parameters are passed in XMM0 to XMM3. When integer and floating point parameters are mixed, the correspondence between parameters and registers is not changed. Hence an integer parameter after two floating point ones will be in R8 with RCX and RDX unused.

When they are passed by value, structures and unions whose sizes are 8, 16, 32 or 64 bits are passed as if they are integers of the same size. Arrays and larger structures and unions are passed as pointers to memory allocated and assigned by the calling function.

The registers RAX, RCX, RDX, R8, R9, R10, R11 are volatile and can be freely used by a called function without preserving their values (note, however, that some may be used to pass parameters). In consequence functions cannot expect these registers to be preserved across calls to other functions.

The registers RBX, RBP, RSI, RDI, R12, R13, R14, R15, and XMM6 to XMM15 are non-volatile and must be saved and restored by functions that use them.

Except for floating point values, which are returned in XMM0, function return values that fit in 64 bits are returned in RAX. Some 128-bit values are also passed in XMM0 but larger values are returned in memory assigned by the calling program and pointed to by an additional?"hidden"?function parameter that becomes the first parameter and pushes other parameters to the right. This pointer value must also be passed back to the calling program in RAX when the called program returns.

16.2.2.?Types of Functions

Functions that allocate stack space, call other functions, save non-volatile registers or use exception handling are called?"frame functions"; other functions are called?"leaf functions".

Frame functions use an area on the stack called a?"stack frame"?and have a defined prologue in which this is set up. Typically they save register parameters in their shadow locations (if needed), save any non-volatile registers that they use, allocate stack space for local variables, and establish a register as a stack frame pointer. They must also have one or more defined epilogues that free any allocated stack space and restore non-volatile registers before returning to the calling function.

Unless stack space is allocated dynamically, a frame function must maintain the 16 byte alignment of the stack pointer whilst outside its prologue and epilogue code (except during calls to other functions). A frame function that dynamically allocates stack space must first allocate any fixed stack space that it needs and then allocate and set up a register for indexed access to this area. The lower base address of this area must be 16 byte aligned and the register must be provided irrespective of whether the function itself makes explicit use of it. The function is then free to leave the stack unaligned during execution although it must re-establish the 16 byte alignment if or when it calls other functions.

Leaf functions do not require defined prologues or epilogues but they must not call other functions; nor can they change any non-volatile register or the stack pointer (which means that they do not maintain 16 byte stack alignment during execution). They can, however, exit with a jump to the entry point of another frame or leaf function provided that the respective stacked parameters are compatible.

These rules are summarized in?Table?16.1?(function code that is not part of a prologue or an epilogue are referred to in the table as the function‘s body).

Table?16.1.?Function Structured Exception Handling Rules

Function needs or can:	Frame Function with Frame Pointer Register	Frame Function without Frame Pointer Register	Leaf Function
prologue and epilogue(s)	yes	yes	no
use exception handling	yes	yes	no
allocate space on the stack	yes	yes	no
save or push registers onto the stack	yes	yes	no
use non-volatile registers (after saving)	yes	yes	no
use dynamic stack allocation	yes	no	no
change stack pointer in function body	yes?^[a]	no	no
unaligned stack pointer in function body	yes?^[a]	no	yes
make calls to other functions	yes	yes	no
make jumps to other functions	no	no	yes?^[b]
^[a]?but 16 byte stack alignment must be re-established when any functions are called. ^[b]?but the function parameters in registers and on the stack must be compatible.

16.2.3.?Frame Function Structure

As already indicated, frame functions must have a well defined structure including a prologue and one or more epilogues, each of a specific form. The code in a function that is not part of its prologue or its one or more epilogues will be referred to here as the function‘s body.

A typical function prologue has the form:

mov [rsp+8],rcx ; store parameter in shadow space if necessary

push r14 ; save any non-volatile registers to be used

push r13 ;

sub rsp,size ; allocate stack for local variables if needed

lea r13,[bias+rsp] ; use r13 as a frame pointer with an offset

When a frame pointer is needed the programmer can choose which register is used ("bias"?will be explained later). Although it does not have to be used for access to the allocated space, it must be assigned in the prologue and remain unchanged during the execution of the body of the function.

If a large amount of stack space is used it is also necessary to call?__chkstk?with size in RAX prior to allocating this stack space in order to add memory pages to the stack if needed (see the Microsoft Visual Studio 2005 documentation for further details).

The matching form of the epilogue is:

lea rsp,[r13-bias] ; this is not part of the official epilogue

add rsp,size ; the official epilogue starts here

pop r13

pop r14

The following can also be used provided that a frame pointer register has been established:

lea rsp,[r13+size-bias]

pop r13

pop r14

These are the only two forms of epilogue allowed. It must start either with an?add rsp,const?instruction or with?lea rsp,[const+fp_register]; the first form can be used either with or without a frame pointer register but the second form requires one. These instructions are then followed by zero or more 8 byte register pops and a return instruction (which can be replaced with a limited set of jump instructions as described in Microsoft documentation). Epilogue forms are highly restricted because this allows the exception dispatch code to locate them without the need for unwind data in addition to that provided for the prologue.

The data on the location and length of each function prologue, on any fixed stack allocation and on any saved non-volatile registers is recorded in special sections in the object code. Yasm provides macros to create this data that will now be described (with examples of the way they are used).

16.2.4.?Stack Frame Details

There are two types of stack frame that need to be considered in creating unwind data.

The first, shown at left in?Figure?16.2, involves only a fixed allocation of space on the stack and results in a stack pointer that remains fixed in value within the function‘s body except during calls to other functions. In this type of stack frame the stack pointer value at the end of the prologue is used as the base for the offsets in the unwind primitives and macros described later. It must be 16 byte aligned at this point.

Figure?16.2.?x64 Detailed Stack Frame

In the second type of frame, shown in?Figure?16.2, stack space is dynamically allocated with the result that the stack pointer value is statically unpredictable and cannot be used as a base for unwind offsets. In this situation a frame pointer register must be used to provide this base address. Here the base for unwind offsets is the lower end of the fixed allocation area on the stack, which is typically the value of the stack pointer when the frame register is assigned. It must be 16 byte aligned and must be assigned before any unwind macros with offsets are used.

In order to allow the maximum amount of data to be accessed with single byte offsets (-128 to \+127) from the frame pointer register, it is normal to offset its value towards the centre of the allocated area (the?"bias"?introduced earlier). The identity of the frame pointer register and this offset, which must be a multiple of 16 bytes, is recorded in the unwind data to allow the stack frame base address to be calculated from the value in the frame register.

16.2.5.?Yasm Primitives for Unwind Operations

Here are the low level facilities Yasm provides to create unwind data.

proc_frame?name

Generates a function table entry in?.pdata?and unwind information in?.xdata?for a function‘s structured exception handling data.

[pushreg?reg]

Generates unwind data for the specified non-volatile register. Use only for non-volatile integer registers; for volatile registers use an?[allocstack 8]?instead.

[setframe?reg,?offset]

Generates unwind data for a frame register and its stack offset. The offset must be a multiple of 16 and be less than or equal to 240.

[allocstack?size]

Generates unwind data for stack space. The size must be a multiple of 8.

[savereg?reg,?offset]

Generates unwind data for the specified register and offset; the offset must be positive multiple of 8 relative to the base of the procedure‘s frame.

[savexmm128?reg,?offset]

Generates unwind data for the specified XMM register and offset; the offset must be positive multiple of 16 relative to the base of the procedure‘s frame.

[pushframe?code]

Generates unwind data for a 40 or 48 byte (with an optional error code) frame used to store the result of a hardware exception or interrupt.

[endprolog]

Signals the end of the prologue; must be in the first 255 bytes of the function.

endproc_frame

Used at the end of functions started with?proc_frame.

Example?16.1?shows how these primitives are used (this is based on an example provided in Microsoft Visual Studio 2005 documentation).

Example?16.1.?Win64 Unwind Primitives

PROC_FRAME sample

db 0x48 ; emit a REX prefix to enable hot-patching

push rbp ; save prospective frame pointer

[pushreg rbp] ; create unwind data for this rbp register push

sub rsp,0x40 ; allocate stack space

[allocstack 0x40] ; create unwind data for this stack allocation

lea rbp,[rsp+0x20] ; assign the frame pointer with a bias of 32

[setframe rbp,0x20] ; create unwind data for a frame register in rbp

movdqa [rbp],xmm7 ; save a non-volatile XMM register

[savexmm128 xmm7, 0x20] ; create unwind data for an XMM register save

mov [rbp+0x18],rsi ; save rsi

[savereg rsi,0x38] ; create unwind data for a save of rsi

mov [rsp+0x10],rdi ; save rdi

[savereg rdi, 0x10] ; create unwind data for a save of rdi

[endprolog]

; We can change the stack pointer outside of the prologue because we

; have a frame pointer. If we didn‘t have one this would be illegal.

; A frame pointer is needed because of this stack pointer modification.

sub rsp,0x60 ; we are free to modify the stack pointer

mov rax,0 ; we can unwind this access violation

mov rax,[rax]

movdqa xmm7,[rbp] ; restore the registers that weren‘t saved

mov rsi,[rbp+0x18] ; with a push; this is not part of the

mov rdi,[rbp-0x10] ; official epilog

lea rsp,[rbp+0x20] ; This is the official epilog

pop rbp

16.2.6.?Yasm Macros for Formal Stack Operations

ENDPROC_FRAME

From the descriptions of the YASM primitives given earlier it can be seen that there is a close relationship between each normal stack operation and the related primitive needed to generate its unwind data. In consequence it is sensible to provide a set of macros that perform both operations in a single macro call. Yasm provides the following macros that combine the two operations.

proc_frame?name

Generates a function table entry in?.pdata?and unwind information in?.xdata.

alloc_stack?n

Allocates a stack area of?n?bytes.

save_reg?reg,?loc

Saves a non-volatile register?reg?at offset?loc?on the stack.

push_reg?reg

Pushes a non-volatile register?reg?on the stack.

rex_push_reg?reg

Pushes a non-volatile register?reg?on the stack using a 2 byte push instruction.

save_xmm128?reg,?loc

Saves a non-volatile XMM register?reg?at offset?loc?on the stack.

set_frame?reg,?loc

Sets the frame register?reg?to offset?loc?on the stack.

push_eflags

Pushes the eflags register

push_rex_eflags

Pushes the eflags register using a 2 byte push instruction (allows hot patching).

push_frame?code

Pushes a 40 byte frame and an optional 8 byte error code onto the stack.

end_prologue?,?end_prolog

Ends the function prologue (this is an alternative to?[endprolog]).

endproc_frame

Used at the end of funtions started with?proc_frame.

Example?16.2?is?Example?16.1?using these higher level macros.

Example?16.2.?Win64 Unwind Macros

PROC_FRAME sample ; start the prologue

rex_push_reg rbp ; push the prospective frame pointer

alloc_stack 0x40 ; allocate 64 bytes of local stack space

set_frame rbp, 0x20 ; set a frame register to [rsp+32]

save_xmm128 xmm7,0x20 ; save xmm7, rsi & rdi to the local stack space

save_reg rsi, 0x38 ; unwind base address: [rsp_after_entry - 72]

save_reg rdi, 0x10 ; frame register value: [rsp_after_entry - 40]

END_PROLOGUE

sub rsp,0x60 ; we can now change the stack pointer

mov rax,0 ; and unwind this access violation

mov rax,[rax] ; because we have a frame pointer

movdqa xmm7,[rbp] ; restore the registers that weren‘t saved with

mov rsi,[rbp+0x18] ; a push (not a part of the official epilog)

mov rdi,[rbp-0x10]

lea rsp,[rbp+0x20] ; the official epilogue

pop rbp