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

Within the loop body CL is decremented. JNZ checks to see if the DEC operation resulted in a non-zero result. If it resulted in a non-zero result we jump to the L1 label. If it is zero then we continue to the next line of operation.

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

The push and pop of ecx are required to ensure the counter isn’t lost. If you carry out an operation that uses ecx you need to push the counter value to the stack. You then need to pop that value back from the stack to ecx before you check for looping.

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