标签：
有时间再写其它函数或者文件的:)
/* *Author : DavidLin *Date : 2014-11-11pm *Email : linpeng1577@163.com or linpeng1577@gmail.com *world : the city of SZ, in China *Ver : 000.000.001 *history : editor time do 1)LinPeng 2014-11-11 created this file! 2) */
下面是Linus的源代码：
/*
* Get physical address of first (actually last :-) free page, and mark it
* used. If no free pages left, return 0.
*/
unsigned long get_free_page(void)
{
register unsigned long __res asm("ax");
__asm__("std ; repne ; scasb\n\t"
"jne 1f\n\t"
"movb $1,1(%%edi)\n\t"
"sall $12,%%ecx\n\t"
"addl %2,%%ecx\n\t"
"movl %%ecx,%%edx\n\t"
"movl $1024,%%ecx\n\t"
"leal 4092(%%edx),%%edi\n\t"
"rep ; stosl\n\t"
"movl %%edx,%%eax\n"
"1:"
:"=a" (__res)
:"0" (0),"i" (LOW_MEM),"c" (PAGING_PAGES),
"D" (mem_map+PAGING_PAGES-1)
:"di","cx","dx");
return __res;
}
1.函数目的： al = 0; //假设mem_map[i] == 0,表示为空暇页，否则为已分配占用,al保存0值，用于比較
ecx = PAGING_PAGES; //主内存叶表个数
es:di = (mem_map+PAGING_PAGES-1); //内存管理数组最后一项 C语言实现例如以下:
......
index_ = 0;
for (i = PAGING_PAGES-1; i != 0; i--)
{
if(0 != mem_map[i]) {
continue; //继续循环
}
else {
index_ = i; //跳出循环
break;
}
}
if(0 == index_) {
goto Label_1;
}
Label_1:
return index_;
......
(3)"jne 1f\n\t" 不包含0-1M(0-1M,事实上是0-640K已经被内核占用)
#define LOW_MEM 0x100000
#define PAGING_MEMORY (15*1024*1024)
#define PAGING_PAGES (PAGING_MEMORY>>12)
#define MAP_NR(addr) (((addr)-LOW_MEM)>>12)
void mem_init(long start_mem, long end_mem)
{
int i;
HIGH_MEMORY = end_mem;
for (i=0 ; i<PAGING_PAGES ; i++){
mem_map[i] = USED;
}//全部主内存区初始化为被占用
i = MAP_NR(start_mem);
end_mem -= start_mem;
end_mem >>= 12;
while (end_mem-->0)
mem_map[i++]=0;
}
(8)"movl $1024,%%ecx\n\t"
将1024保存到ecx寄存器中，由于每一页占用4096字节(4K),
实际物理内存,每项占用4字节,有1024项。
(9)"leal 4092(%%edx),%%edi\n\t"
由于依照4字节对齐，所以每项占用4字节,
取当前物理页最后一项4096 = 4096-4 = 1023*4 = (1024-1)*4 。
将该物理页面的末端保存在edi寄存器中,
即ecx+4092处的地址保存在edi寄存器中。
(10)"rep ; stosl\n\t"
从ecx+4092处開始，反方向，步进4，反复1024次，
将该物理页1024项所有填入eax寄存器的值。
在例如以下代码定义中，eax初始化为0(al=0,eax =0,ax =0)
:"0" (0),"i" (LOW_MEM),"c" (PAGING_PAGES),
所以该物理页1024项所有清零。
(11)"movl %%edx,%%eax\n"
将该物理页面起始地址放入eax寄存器中。
Intel的EABI规则中，
eax寄存器用于保存函数返回值
(12)"1:"
标签1，用于"jne 1f\n\t"语句跳转返回0值。
注意：
eax寄存器仅仅在"movl %%edx,%%eax\n"中被赋值，
eax寄存器初始值是‘0‘，假设跳转到标签"1:"处，
返回值是0。表示没有空暇物理页。
(13):"=a" (__res)
输出寄存器列表。这里仅仅有一个，当中a表示eax寄存器
(14):"0" (0),"i" (LOW_MEM),"c" (PAGING_PAGES),
"0"表示与上面同个位置的输出同样的寄存器，即"0"等于输出寄存器eax。
即eax既是输出寄存器，同一时候也是输入寄存器，
当然，在时间颗粒度最小的情况小，eax不能同一时候作为输入或者输出寄存器，
仅仅能作为输入或者输出寄存器;
"i" (LOW_MEM)是%2，从输出寄存器到输入寄存器依次编号%0，%1，%2.....%N,
当中"i"表示马上数。不是edi的代号，edi的代号是"D";
"c" (PAGING_PAGES)表示将ecx寄存器存入PAGING_PAGES，
ecx寄存器代号"c"。
(15)"D" (mem_map+PAGING_PAGES-1)
"D"使用edi寄存器，即edi寄存器保存的值是(mem_map+PAGING_PAGES-1)
即%%edi = &mem_map[PAGING_PAGES-1]。
(16):"di","cx","dx");
保留寄存器，告诉编译器"di","cx","dx"三个寄存器已经被分配，
在编译器编译中，不会将这三个寄存器分配为输入或者输出寄存器。
(17)return __res;
返回__res保存的值，
相当于汇编的ret，隐含将eax寄存器返回。
C语言中是显式返回。
4.汇编指令及语法规则解析。參照Intel官方文档《Volume 2A Instruction Set Reference (A-M)》
《Volume 2B Instruction Set Reference (N-Z)》,GNU汇编规则
(1)std:
主要将ESI and/or EDI方向设置为递减。相应cld(用于方向设置为递增)
1)Operation
Sets the DF flag in the EFLAGS register. When the DF flag is set to 1, string operations
decrement the index registers (ESI and/or EDI).
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
2)Operation
DF -> 1;
(2)repne:
1)Description
Repeats a string instruction the number of times specified in the count register or
until the indicated condition of the ZF flag is no longer met. The REP (repeat), REPE
(repeat while equal), REPNE (repeat while not equal), REPZ (repeat while zero), and
REPNZ (repeat while not zero) mnemonics are prefixes that can be added to one of
the string instructions. The REP prefix can be added to the INS, OUTS, MOVS, LODS,
and STOS instructions, and the REPE, REPNE, REPZ, and REPNZ prefixes can be
added to the CMPS and SCAS instructions. (The REPZ and REPNZ prefixes are synonymous
forms of the REPE and REPNE prefixes, respectively.) The behavior of the REP
prefix is undefined when used with non-string instructions.
The REP prefixes apply only to one string instruction at a time. To repeat a block of
instructions, use the LOOP instruction or another looping construct. All of these
repeat prefixes cause the associated instruction to be repeated until the count in
register is decremented to 0. See Table 4-13.
2)Operation
IF AddressSize = 16
THEN
Use CX for CountReg;
ELSE IF AddressSize = 64 and REX.W used
THEN Use RCX for CountReg; FI;
ELSE
Use ECX for CountReg;
FI;
WHILE CountReg = 0
DO
Service pending interrupts (if any);
Execute associated string instruction;
CountReg <- (CountReg – 1);
IF CountReg = 0
THEN exit WHILE loop; FI;
IF (Repeat prefix is REPZ or REPE) and (ZF = 0)
or (Repeat prefix is REPNZ or REPNE) and (ZF = 1)
THEN exit WHILE loop; FI;
OD;
(3)scasb: ---------------------------------------------------------------------------------------
Code | Mnemonic | Description
---------------------------------------------------------------------------------------
AE | SCAS m8 | Compare AL with byte at ES:(E)DI and set status flags
---------------------------------------------------------------------------------------
AF | SCAS m16 | Compare AX with word at ES:(E)DI and set status flags
---------------------------------------------------------------------------------------
AF | SCAS m32 | Compare EAX with doubleword at ES(E)DI and set status flags
---------------------------------------------------------------------------------------
AE | SCASB | Compare AL with byte at ES:(E)DI and set status flags
---------------------------------------------------------------------------------------
AF | SCASW | Compare AX with word at ES:(E)DI and set status flags
---------------------------------------------------------------------------------------
AF | SCASD | Compare EAX with doubleword at ES:(E)DI and set status flags
---------------------------------------------------------------------------------------比如。REPNZ SCASB 语句表示当 寄存器ECX>0 且 标志寄存器ZF=0，则再运行一次SCASB指令。
比較寄存器AL的值不相等则反复查找的字
(4)sall
如sall $12, %ecx.
这个指令是算法左移,相当于c语言中的左移操作符<<.
intel汇编指令中的SAL,(Shit Arithmetic left).
依据AT&T的语法规则，
由于是一个长型的操作(ecx),
所以在intel汇编指令sal上加一个"l",
即转换成sall。
(5)stosl
STOSL指令相当于将EAX中的值保存到ES:EDI指向的地址中，
若设置了EFLAGS中的方向位置位(即在STOSL指令前使用STD指令)
则EDI自减4。否则(使用CLD指令)EDI自增4。
(6)eax,ax,ah,al
00000000 00000000 00000000 00000000
|===============EAX===============|--32个0,4个字节，2个字，1个双字
|======AX=======|--16个0,2个字节，1个字
|==AH===|-----------8个0,1个字节
|===AL==|---8个0,1个字节 EAX是32位的寄器，仅仅是在原有的8086CPU的寄存器AX上添加了一倍的数据位数。
Linux-0.11内核源代码分析系列：内存管理get_free_page()函数分析
标签：
原文地址：http://www.cnblogs.com/lcchuguo/p/5231735.html
