Status Flags when comparing two values

Question

I am very confused when it comes to status flags. Can someone help me through an example and the steps I need to take to verify how the flags are behaving?

mov ah, 21
mov al, -21
cmp ah,al

Can someone help me determine the behavior of the CF, ZF,SF, and OF?

Thanks

See also [this question](http://stackoverflow.com/questions/26423548/68000-assembly-language-cmpi-b/26425658). While that is for 68k, the logic for the 4 flags (with `N`=`SF` and `V`=`OF`) is the same on x86 too except here the second operand is subtracted from the first. — Jester, Nov 16 '14 at 00:10
[`cmp` is just `sub`](http://stackoverflow.com/questions/7261535/why-does-cmp-0x84-0x30-trigger-the-overflow-flag), except that its operands are not affected, just the flags register. — , Nov 16 '14 at 00:21

Tony · Answer 1 · 2014-11-16T02:55:38.647

You can look at the eflags register in a debugger (like gdb). There is plenty of readily accessible documentation about eflags, but essentially bits in the register are 0 or 1 depending on whether particular status flags are set.

If it is not enough to just see the values in a debugger, and you need to do something with them:

you can push eflags onto the stack (pushfl works for me) and then pop the stack to a general purpose register.
You can use the jc, jz, jo, and js instructions which jump to a specified label if the carry, zero, overflow, or sign flags (respectively) are set.

For example:

    clc                     # clear carry flag (set CF = 0)
    addl eax, 0xffffffff    # some operation that might change status flags
    jc label1               # if the carry flag is set, jump to label

    ...                     # instructions to execute if carry not set (CF = 0)

    jmp label2

label1:

    ...                     # instructions to execute if carry set (CF = 0)

label2:

    ...                     # resume execution which does not depend on CF

Status Flags when comparing two values

1 Answers1