Assembly - Loops

The JMP instruction can be used for implementing loops. For example, the following code snippet can be used for executing the loop-body 10 times.

MOV CL, 10
L1:
<LOOP-BODY>
DEC CL
JNZ L1

The processor instruction set however includes a group of loop instructions for implementing iteration. The basic LOOP instruction has the following syntax:

LOOP  label

Where, label is the target label that identifies the target instruction as in the jump instructions. The LOOP instruction assumes that the ECX register contains the loop count. When the loop instruction is executed, the ECX register is decremented and the control jumps to the target label, until the ECX register value, i.e., the counter reaches the value zero.

The above code snippet could be written as:

mov ECX,10
l1:
<loop body>
loop l1

Example:

The following program prints the number 1 to 9 on the screen:

section .text
    global _start         ;must be declared for using gcc
_start:                 ;tell linker entry point
 mov ecx,10
 mov eax, '1'
 
l1:
 mov [num], eax
 mov eax, 4
 mov ebx, 1
 push ecx
 mov ecx, num        
        mov edx, 1        
        int 0x80
 mov eax, [num]
 sub eax, '0'
 inc eax
 add eax, '0'
 pop ecx
 loop l1
 mov eax,1       ;system call number (sys_exit)
 int 0x80        ;call kernel
section .bss
num resb 1

When the above code is compiled and executed, it produces following result:

123456789: