How to read the flags registers in x86 processor

Question

Are there any assembly instructions to let us directly read or write the overflow flag in Intel's 80386 16-bit Flags register ? If not, what pseudo code should we use?

One can use pushf and popf to save/restore them using stack. (popfd, popfq restore the 32-bit and 64-bit extensions of the EFLAGS / RFLAGS respectively) — Aki Suihkonen, Mar 13 '13 at 15:19

Michael · Answer 1 · 2013-03-13T16:07:48.213

8

To read only the overflow flag:

SETO AL
; AL now contains 1 if the overflow flag was set, and 0 if it wasn't

To invert the overflow flag:

PUSHF
POP    AX
XOR    AX,800h  ; The overflow flag is in bit 11
PUSH   AX
POPF

edited Mar 13 '13 at 16:07

answered Mar 13 '13 at 15:38

Michael

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The OP is asking for 16-bit FLAGS; your code is 32-bit. -1 for not reading the question. – Seva Alekseyev Mar 13 '13 at 15:57
3

That's a fairly minor detail to get hung up on considering that it should be obvious how to make the same code 16-bit. Edited the answer anyway. – Michael Mar 13 '13 at 16:07
3

That obviously wasn't the case since the question title only mentions reading the flags. I simply overlooked one detail when writing my initial answer, which I honestly don't think made the example "not useful". – Michael Mar 13 '13 at 16:23

score 1 · Answer 2 · answered Mar 13 '13 at 15:56

To read the flags into AX:

pushf
pop ax

To write the flags: if you need to set/clear specific bits, there are some commands like stc/clc (for Carry flag), std/cld (for Direction) and so forth; but not bits are exposed this way. To write the whole flags register, use

push ax
popf

score 0 · Answer 3 · answered Mar 13 '13 at 15:19

0

Use

Pushf
// modify or read ax register
Popf

answered Mar 13 '13 at 15:19

James

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`pushf` writes eflags to the stack, not to `ax`. – Stephen Canon Mar 13 '13 at 15:21

How to read the flags registers in x86 processor

3 Answers3