How to check if a signed integer is neg or pos?

Question

I am new to x86 assembly language, I have a signed integer saved in register eax, and I want to check if the number is negative or positive. To do that, I used bt instruction to check the first bit.

Here is what I did:

bt eax,0
jnc isNegative

bt carries the first bit to carry flag, and I used jnc to check if carry flag is 0 or 1. If it's 1, it should be a negative number, and does negative instructions... however, the output is unpredictable, sometimes I have a positive and it recognize it as a negative number. Am I doing something wrong?

EDIT: I just realized it could have something to do with endianess. It is actually checking the last bit instead of the first bit. Let me try use bt, 7

ruslik · Accepted Answer · 2010-10-25T18:30:31.760

20

The value is negative if the MSB is set. It can be checked with

test eax, 0x80000000
jne is_signed

or, simplier:

test eax, eax
js signed

or for byte case:

test al, al
js is_signed
; or
test eax, 80h
jne is_signed

edited Oct 25 '10 at 18:30

answered Oct 25 '10 at 18:02

ruslik

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nmichaels · Answer 2 · 2010-10-25T19:08:09.757

5

You can use the jge and cmp instructions:

cmp eax, 0
jl isNegative

edited Oct 25 '10 at 19:08

answered Oct 25 '10 at 18:03

nmichaels

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Great, fixes the problem and simple! although, it should be cmp eax,0 – Yonk Oct 25 '10 at 18:07
Depends on which assembler you're using, but for Intel's, yes. I guess that's where the downvote came from. – nmichaels Oct 25 '10 at 19:01
cmp is longer than test so test is often preferred http://stackoverflow.com/a/147183/995714 – phuclv Sep 21 '14 at 06:55

Joe D · Answer 3 · 2010-10-25T18:08:40.727

Your current solution checks if the number is even or odd, instead test the 31st bit:

bt eax, 31
jc number_is_negative

Or you can compare the number to zero, and test whether it's greater than or equal to it.

main:
    ...
    cmp eax, 0 ; Compare the number to zero
    jge .number_is_positive ; If it's positive jump to .number_is_positive
    ; And only arrive here if a number is negative
    ...
.number_is_positive:
    ; Yay! number is positive
    ...

How to check if a signed integer is neg or pos?

3 Answers3