Question

I tried to solve a question shown below;

By just using and, and jg or jle, how can one implement following code?

    if %eax > 4 
        jmp do
    else 
        jmp l1

of course without changing value of eax

This question is taken from a textbook, but it has no answer. Can you show me how to solve it?

EDIT:
I have tried ;

     subl $4, %eax
     andl %eax, %eax
     jg  do
     jmp l1
      .
      .
      .
     do :
     addl $4, %eax
      .
      .
      .
     l1:
     addl $4, %eax

Answer 1

The below sequence of and and jle solves the problem (NASM syntax is used):

; start execution here if eax is treated as an unsigned integer
unsigned_eax:
    and     dword [eighty - 3], eax
    and     dword [eighty], 80H
    jle     signed_eax ; jumps if eax <= 7FFFFFFFh, continues otherwise
    and     dword [eighty], 0
    jle     do ; jumps if eax > 7FFFFFFFh = always

; start execution here if eax is treated as a signed integer
signed_eax:
    and     dword [FFFFFFFC], eax
    jle     l1 ; jumps if eax < 4, continues if eax >= 4
    and     dword [FFFFFFFB], eax
    jle     l1 ; jumps if eax = 4, continues if eax > 4
    and     dword [zero], 0 ; this and the next instruction simulate "jmp do"
    jle     do
do:
    ; some code
l1:
    ; some code

FFFFFFFC    dd 0FFFFFFFCH
FFFFFFFB    dd 0FFFFFFFBH
zero        dd 0
eighty      dd 80H

The caveat here is that this is a use-once piece of code because it irreversibly modifies 2 (or 3) variables.