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

C.2 Pre-UAL pseudo-instruction NOP
In pre-UAL assembler code, NOP is a pseudo-instruction, equivalent to:
MOV R0,R0 in ARM code
MOV R8,R8 in Thumb code.
Legacy Instruction Mnemonics
Assembling the NOP mnemonic as UAL will not change the functionality of the code, but will change:
the instruction encoding selected
the architecture variants on which the resulting binary will execute successfully, because the NOP
instruction was introduced in ARMv6K and ARMv6T2.
To avoid these changes, replace NOP in the assembler source code with the appropriate one of MOV R0,R0 and
MOV R8,R8, before assembling as UAL.
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