进位标志比较，何时设置进位标志？

Question

The assembler has this code:

sub$0x22,%eax # %eax = %eax - 22
cmp$0x7,%eax  # %eax > 7 then jump *this is where I have trouble*
ja some address # jump if C = 1 or Z = 1

My goal is to not make the jump. I have tried the cases where %eax = 30, 14, 28, 16, 0 , 22

Question: I don't understand why c=0 and z=0 with all the cases I have tried.

Answer 1

0x22 , which is 34 decimal, is larger than all of those sample eax values (assuming they're decimal). Thus, the result of the subtraction is negative. But a "negative" integer is just an integer where the most significant bit is a 1, and can also be interpreted as a large unsigned number.

It's probably easier to think of ja in terms of what it means conceptually rather than the actual logic on the flags. ( see this answer ) You can think of ja as an unsigned comparison. So those negative numbers look like really big numbers that are larger than 7. If you used a jg instruction instead, it should behave more like what you're expecting. It can be thought of as a signed comparison.