ASM: Decrement content of specific address

Question

I need to decrement some bytes on/from the stack at a given address (indexed from esi).

pop esi ;

Now changing would not be a problem, I simply can do

mov [esi+13], al ;

to store the content from al in esi+13.

But how can I decrement what is in "esi+13".

E.g. value of esi+13 = 0xFF → New value of esi+13 = 0xFE.

I tried different things like

dec [esi+13] ;
sub [esi+13], 1; 
dec esi+13;

and so on, but I didn't find a solution.

I don't know which bytes will be in "esi+13" so I can't move the "result" to the address, I really have to decrement it.

What do you mean you can't move a result to a location for which you have a pointer in esi? I can understand why you don't *want* to do it in baby steps, but it should be *possible*. — Chris Stratton, Jun 25 '13 at 19:05

score 1 · Accepted Answer · answered Jun 25 '13 at 19:00

1

I think this should work:

dec byte ptr [esi+13]

answered Jun 25 '13 at 19:00

faffaffaff

Thanks for your idea. Using nasm it’s simply "dec byte [esi+13]" :) – CFP Jun 25 '13 at 19:13

1 Answers1