CODEWITHSUNDEEP
assemblyx86-64
emmy
emmy

Reputation: 21

How does "and eax,80000000h "in assembly work?

I have an assembly program: 

move eax, op1;
AND eax, 80000000h;
jz l1;

This program checks if op1 is positive or negative. How does comparing eax to 80000000h check is op1 is negative or positive?

Upvotes: 2

Views: 1342

Answers (1)

mbaitoff
mbaitoff

Reputation: 9071

Values in registers are stored in two's-complement format, and the highest bit corresponds to the negative sign. The constant 80000000h corresponds to highest bit set in binary value of 1000 0000 0000 0000 0000 0000 0000 0000. Applying and with it to the register eax results in non-zero if and only if the highest bit of eax is set, that is, the number stored in eax is negative. The conditional jump is not triggered in this case. It results in zero if the top bit is not set, that is, if the value is non-negative (0 or positive), and that triggers the jz conditional jump.

Upvotes: 4

Related Questions