CODEWITHSUNDEEP
linuxassemblynasm
user902691
user902691

Reputation: 1159

Why cmp 2, 2 is't equal 0

extern putchar
extern exit
section .data

section .text
global main
main:

push 's'

mov eax, 2

cmp eax, 2

point:
call putchar
jz point

push 0
call exit

On the console I see only one 's' charcter.

Compile and run:

nasm -f elf ./prog.asm
gcc -m32 -o prog ./prog.o
./prog

Upvotes: 0

Views: 105

Answers (2)

Igor Skochinsky
Igor Skochinsky

Reputation: 25288

The cmp is "equal to 0" (that is, it does set the ZF flag). However, the call putchar in the next line is trashing the flags set by cmp, so your jz does not work (more or less by accident). If you want to save the flags for later comparison, you could use pushf and popf, however this won't really work in your case since putchar will expect the character on the stack, not the flags.

Now, to answer the actual problem which you didn't state. I'll assume you want to print 's' two times. Here's how to do it properly:

  mov eax, 2 ; init counter

print_loop:
  push eax; save the counter since it will be trashed by putchar
  push 's'
  call putchar
  add esp, 4 ; restore the stack pointer since putchar is cdecl
  pop eax ; restore the saved counter
  dec eax ; decrement it
  jnz print_loop ; if it's not yet zero, do another loop

add esp, 4 can be replaced by another pop eax for slightly shorter code.

Upvotes: 5

500 - Internal Server Error
500 - Internal Server Error

Reputation: 28839

The result of doing a cmp is that flags get set, zf for zero, and so on. You can then either branch on whether or not the flag was set or use one of the set? instructions to have a value, e.g. the al register, set based on whether or not the flag was set.

Upvotes: 2

Related Questions