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The instruction width specifiers
.N control the size of Thumb instruction encodings for ARMv6T2 or later.
In Thumb code (ARMv6T2 or later) the
specifier forces the assembler to generate a 32-bit encoding, even
if a 16-bit encoding is available. The
has no effect when assembling to ARM code.
In Thumb code the
.N width specifier forces
the assembler to generate a 16-bit encoding. In this case, if the
instruction cannot be encoded in 16 bits or if
used in ARM code, the assembler generates an error.
If you use an instruction width specifier, you must place it immediately after the instruction mnemonic and any condition code, for example:
BCS.W label ; forces 32-bit instruction even for a short branch B.N label ; faults if label out of range for 16-bit instruction