进位/溢出和x86 中的减法 [英] carry/overflow & subtraction in x86

问题描述

我正在努力解决溢出问题 &在 x86 中携带标志.

I'm trying to wrap my head around overflow & carry flags in x86.

据我所知，对于有符号 2 的补码相加，标志只能以四种方式之一生成(我的例子是 4 位数字):

As I understand it, for addition of signed 2's complement numbers, the flags can only be generated in one of four ways (my examples are 4-bit numbers):

1. pos+pos = neg(溢出)
   • 0111 + 0001 = 1000 (7 + 1 = -8)

• 0011 + 1110 = 0001 (3 + -2 = 1)

• 1111 + 1111 = 1110 (-1 + -1 = -2)

• 1000 + 1001 = 0001 (-8 + -7 = 1)

那么，在 x86 程序集中，从 A 中减去 B 会生成与添加 A 和 -B 相同的标志吗?

So, in x86 assembly, does subracting B from A generate the same flags as adding A and -B?

推荐答案

这是一个可能有帮助的参考表.这显示了可以由 x86 上的 ADD 和 SUB 指令产生的 4 个算术标志的每种可能组合的示例.'h' 'ud' 和 'd' 代表每个值的十六进制、无符号十进制和有符号十进制表示.例如，SUB 的第一行表示 0xFF - 0xFE = 0x1 没有设置标志.

Here's a reference table that might help. This shows an example of every possible combination of the 4 arithmetic flags that can result from the ADD and SUB instructions on x86. 'h' 'ud' and 'd' stand for hex, unsigned decimal and signed decimal representations of each value. For example, the first row for SUB says 0xFF - 0xFE = 0x1 with no flags set.

但是，我认为小故事是亚历克斯的答案是正确的.

But, I think the short story is that Alex's answer is correct.

 ADD
       A                   B                   A + B              Flags  
 ---------------     ----------------    ---------------      -----------------
 h  |  ud  |   d   | h  |  ud  |   d   | h  |  ud  |   d   | OF | SF | ZF | CF
 ---+------+-------+----+------+-------+----+------+-------+----+----+----+---
 7F | 127  |  127  | 0  |  0   |   0   | 7F | 127  |  127  | 0  | 0  | 0  | 0
 FF | 255  |  -1   | 7F | 127  |  127  | 7E | 126  |  126  | 0  | 0  | 0  | 1
 0  |  0   |   0   | 0  |  0   |   0   | 0  |  0   |   0   | 0  | 0  | 1  | 0
 FF | 255  |  -1   | 1  |  1   |   1   | 0  |  0   |   0   | 0  | 0  | 1  | 1
 FF | 255  |  -1   | 0  |  0   |   0   | FF | 255  |  -1   | 0  | 1  | 0  | 0
 FF | 255  |  -1   | FF | 255  |  -1   | FE | 254  |  -2   | 0  | 1  | 0  | 1
 FF | 255  |  -1   | 80 | 128  | -128  | 7F | 127  |  127  | 1  | 0  | 0  | 1
 80 | 128  | -128  | 80 | 128  | -128  | 0  |  0   |   0   | 1  | 0  | 1  | 1
 7F | 127  |  127  | 7F | 127  |  127  | FE | 254  |  -2   | 1  | 1  | 0  | 0


 SUB
       A                   B                   A - B              Flags  
 ---------------     ----------------    ---------------      -----------------
 h  |  ud  |   d   | h  |  ud  |   d   | h  |  ud  |   d   || OF | SF | ZF | CF
----+------+-------+----+------+-------+----+------+-------++----+----+----+----
 FF | 255  |  -1   | FE | 254  |  -2   | 1  |  1   |   1   || 0  | 0  | 0  | 0
 7E | 126  |  126  | FF | 255  |  -1   | 7F | 127  |  127  || 0  | 0  | 0  | 1
 FF | 255  |  -1   | FF | 255  |  -1   | 0  |  0   |   0   || 0  | 0  | 1  | 0
 FF | 255  |  -1   | 7F | 127  |  127  | 80 | 128  | -128  || 0  | 1  | 0  | 0
 FE | 254  |  -2   | FF | 255  |  -1   | FF | 255  |  -1   || 0  | 1  | 0  | 1
 FE | 254  |  -2   | 7F | 127  |  127  | 7F | 127  |  127  || 1  | 0  | 0  | 0
 7F | 127  |  127  | FF | 255  |  -1   | 80 | 128  | -128  || 1  | 1  | 0  | 1

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