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> SUB CL, 1 JNZ L1
The processor instruction set, however, includes a group of loop instructions for implementing iteration. The basic LOOP instruction has the following syntax:
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
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 the following result: