COMP-3

今天在hex中对9型和comp-3深入研究了下。

9型：

在普通情况下：F0 --> F9 表示 1 --> 9

在S9最后一位：正数：C1 --> C9   表示 1 --> 9

                          负数 D1 --> D9   表示 1 --> 9

comp-3型:

普通情况下：hex中一个数就代表comp-3中一个数

在有S的最后一位：正数：？C

                     负数：？D

无S的最后一位：？F  

Comp-3 fields in COBOL

Comp-3 fields are denoted in COBOL with the "usage is" clause after the PIC, like this:

PIC S9(5) usage is computational-3.

However, the "usage is" is not required and seldom used, and "computational-3" is usually abbreviated "comp-3", so you more commonly see:

PIC S9(5) comp-3.

The COBOL PIC, or picture, for a comp-3 packed field specifies the number of digits after unpacking. The actual number of bytes occupied in the file is about half that. To calculate the number of bytes from the PIC, add 1 (for the sign) to the total number of digits, divide by 2, and round up if necessary. For example:

PIC S9(7) COMP-3.     Byte size = (7 + 1) / 2 = 4

    PIC S9(5)V99 COMP-3.  Byte size = (5 + 2 + 1) / 2 = 4

    PIC S9(6) COMP-3.     Byte size = (6 + 1) / 2 = 3.5, rounded to 4

Comp-3 fields reserve a nybble for the sign, even for "unsigned" values, so the following fields are still 4 bytes:

PIC 9(7) COMP-3.     Byte size = (7 + 1) / 2 = 4

    PIC 9(6) COMP-3.     Byte size = (6 + 1) / 2 = 3.5, rounded to 4

Examples

Lets look at some examples of how comp-3 data is stored. The left column in the table below is the decimal value being stored, and the right column is the hexadecimal value you will see in the file:

     Value  Comp-3, hex
       +0           0C
       +1           1C
      +12        01 2C 
     +123        12 3C
    +1234     01 23 4C
       -1           1D
    -1234     01 23 4D