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Two slightly different syntaxes are support for ARM and THUMB instructions. The default, divided, uses the old style where ARM and THUMB instructions had their own, separate syntaxes. The new, unified syntax, which can be selected via the .syntax directive, and has the following main features:

Immediate operands do not require a # prefix.
The IT instruction may appear, and if it does it is validated against subsequent conditional affixes. In ARM mode it does not generate machine code, in THUMB mode it does.
For ARM instructions the conditional affixes always appear at the end of the instruction. For THUMB instructions conditional affixes can be used, but only inside the scope of an IT instruction.
All of the instructions new to the V6T2 architecture (and later) are available. (Only a few such instructions can be written in the divided syntax).
The .N and .W suffixes are recognized and honored.
All instructions set the flags if and only if they have an s affix.