问题描述
我正在努力把自己的头缠在满溢的& ;;在x86中进行标记.
I'm trying to wrap my head around overflow & carry flags in x86.
据我了解，要添加带符号的2的补码，只能以以下四种方式之一生成标志(我的示例是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):
- pos + pos = neg(溢出)
- 0111 + 0001 = 1000(7 +1 = -8)
- pos+pos = neg (overflow)
- 0111 + 0001 = 1000 (7 + 1 = -8)
- 0011 + 1110 = 0001(3 + -2 = 1)
- 1111 + 1111 = 1110(-1 + -1 = -2)
- 1000 + 1001 = 0001(-8 + -7 = 1)
因此，在x86汇编中，从A减去B会产生与添加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
以下是可能有用的参考表.这显示了x86上ADD和SUB指令可能导致的4种算术标志的每个可能组合的示例. '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形式.例如，SUB的第一行显示为0xFF-0xFE = 0x1，未设置任何标志.
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，我认为简短的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
这篇关于进位/溢出x86中的减法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！