Page 121 - ARM 64 Bit Assembly Language

Data processing and other instructions 107

16
17 // return 0
18 mov w0, #0
19 ldp x29, x30, [sp], #16
20 ret
21 .size main, (. - main)

4.3.4 No operation

This instruction does nothing except waste execution time.

nop No Operation.

4.3.4.1 Syntax
nop

4.3.4.2 Operations

Name Effect Description
nop No effects No Operation.

4.3.4.3 Examples

nop’s can sometimes be inserted to optimize machine specific code. Other times they are used
in computer attacks. They can even be used just to experiment with a debugger. The following
example shows how one might make a counter to delay a short period using nop instructions
and a loop:

1 .text
2 .type main, %function
3 .global main
4 main:
5 // a programmed delay loop
6 mov x0, #0x3fffffff // load loop counter
7 loop: nop
8 nop
9 nop
10 nop
11 sub x0, x0, #1 // decrement counter
12 cmp w0, #0
13 bgt loop
14 // return 0