What gets compared to what in the cmovl opcode?

Question

In the assembly opcode cmovl, what gets compared? For example: EAX: 00000002 EBX: 00000001

cmovl eax,ebx

What is the result? Which one needs to be less so they can be moved?

Thank you!

Answer 1

cmov doesn't do a comparison, it uses the result of a previous comparison - if it is true, it will perform the mov. cmovl means "perform move if previous comparison resulted in "less than".

For example:

cmp ecx, 5
cmovl eax, ebx ; eax = ebx if ecx < 5

Answer 2

It should be preceded by another instruction that sets flags appropriately, like cmp .

cmp ebx, ecx   ; compare ebx to ecx and set flags.
cmovl ebx, eax ; if (ebx < ecx (comparison based on flags)) ebx = eax 

Answer 3

cmovl will perform the move if the flags register has the following: SF!=OF

Those flags would be set as the result of some previous operation (typically, but not necessarily, a compare of some sort).

The cmovl instruction does not perform a compare of its own.

Answer 4

In AT&T assembly the equivalent code would be:

cmp   %ebx, %eax
cmovl %ebx, %eax

which would copy the value of %ebx to %eax , if the value held in %eax was greater than the value held in %ebx at the time of the cmp call.

With your example values, the result would be that the conditional move would not copy the value from %ebx to %eax , as 0x02 is clearly greater than 0x01 .