ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Instruction Details
A8.6.110 NOP
No Operation does nothing. This instruction can be used for code alignment purposes.
See Pre-UAL pseudo-instruction NOP on page AppxC-3 for details of NOP before the introduction of UAL
and the ARMv6K and ARMv6T2 architecture variants.
Note
The timing effects of including a NOP instruction in code are not guaranteed. It can increase execution time,
leave it unchanged, or even reduce it. NOP instructions are therefore not suitable for timing loops.
Encoding T1 ARMv6T2, ARMv7
NOP
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1 0 1 1 1 1 1 1 0 0 0 0 0 0 0 0
// No additional decoding required
Encoding T2 ARMv6T2, ARMv7
NOP.W
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1 1 1 1 0 0 1 1 1 0 1 0 (1)(1)(1) (1) 1 0 (0) 0 (0) 0 0 0 0 0 0 0 0 0 0 0
// No additional decoding required
Encoding A1 ARMv6K, ARMv6T2, ARMv7
NOP
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
cond 0 0 1 1 0 0 1 0 0 0 0 0 (1)(1)(1)(1)(0)(0)(0)(0) 0 0 0 0 0 0 0 0
// No additional decoding required
A8-222 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B