binary - How to compare a signed value and an unsigned value in x86 assembly

Question

I am having trouble finding a way to compare a positive number and a negative number in x86 assembly code.

For example: when I compare -1 and 1 I always get -1 as greater. I know that it's because 2's complement format makes the -1 bigger than 1 in underlying binary.

But can anyone provide a snippet of x86 assembly to compare positive number with a negative one and get it mathematically correct? (e.g 1 > -1)

Thanks!

Answer 1

You're probably using one of the unsigned variants like:

cmp  eax, ebx
jb   lesser

There are equivalents for checking signed numbers against each other, such as:

cmp  eax, ebx
jl   lesser

This link gives a good run down on the jump variations, including their signed-ness and the flags they check, partially copied here for self-containment:

Instruction  Jump if ...           Signed?   Flags
-----------  -----------           --------  -----
JO           overflow                        OF=1
JNO          not overflow                    OF=0
JS           sign                            SF=1
JNS          not sign                        SF=0
JE/JZ        equal
             zero                            ZF=1
JNE/JNZ      not-equal
             not-zero                        ZF=0
JB/JNAE/JC   below
             not-above-or-equal
             carry                 unsigned  CF=1
JNB/JAE/JNC  not-below
             above-or-equal
             no-carry              unsigned  CF=0
JBE/JNA      below-or-equal
             not-above             unsigned  CF=1 or ZF=1
JA/JNBE      above
             not-below-or-equal    unsigned  CF=0 and ZF=0
JL/JNGE      less
             not-greater-or-equal  signed    SF<>OF
JGE/JNL      greater-or-equal
             not-less              signed    SF=OF
JLE/JNG      less-or-equal
             not-greater           signed    ZF=1 or SF<>OF
JG/JNLE      greater
             not-less-or-equal     signed    ZF=0 and SF=OF
JP/JPE       parity
             parity-even                     PF=1
JNP/JPO      not-parity
             parity-odd                      PF=0
JCXZ/JECXZ   CX register is zero
             ECX register is zero