CODEWITHSUNDEEP
assemblyx86
user091sof
user091sof

Reputation: 93

x86 assembly - compare SF and OF flags

How to compare the values of SF (sign flag) and OF (overflow flag) flags in the x86 assembly?

After cmp instruction, these flags are set, but I need to compare them in order to emulate the jl instruction.

Upvotes: 3

Views: 773

Answers (1)

zx485
zx485

Reputation: 29022

There are two major instructions testing the flags: the Jcc instruction and the SETcc instruction. Looking at both tables will give you a couple of instructions checking these two flags.

An example using the SETcc instruction would be:

seto al        ; Set byte to 1 if overflow (OF=1)
sets ah        ; Set byte to 1 if sign (SF=1)

Now EAX will contain the hexadecimal value 

0101 if both flags have been set
0100 if sign has been set
0001 if overflow has been set
0000 if both flags have been cleared

Now you can compare AL(overflow) to AH(sign) with cmp al, ah. The jl instruction checks for difference (SF ≠ OF) so a jne(no equal/zero) after this cmp has the same effect as the original jl.

Finally the whole code emulating a jl is:

seto al
sets ah
cmp al, ah
jne lesser

Upvotes: 5

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