4.11 Unified Assembly Language Syntax Support

Unified assembly language (UAL) is the new assembly syntax introduced by ARM Ltd. to handle the ambiguities introduced by the original Thumb-2 assembly syntax and provide similar syntax for ARM, Thumb and Thumb-2. UAL is backwards compatible with old ARM assembly, but incompatible with the previous Thumb assembly syntax.

UAL syntax is the default assembly syntax beginning with ARMv7 architectures. When writing assembly code, the .arm and .thumb directives are used to specify ARM and Thumb UAL syntax, respectively. The .state32 and .state16 directives remain to specify non-UAL ARM and Thumb syntax. The .arm and .state32 directives are equivalent since UAL syntax is backwards compatible in ARM mode. Since non-UAL syntax is not supported for Thumb-2 instructions, Thumb-2 instructions cannot be used inside of a .state16 section. However, assembly code with .state16 sections that contain only non-UAL Thumb code can be assembled for ARMv7 architectures to allow easy porting of older code.

See Section 5.3 for more information about the .state16, .state32, .arm, and .thumb directives.

A full description of the UAL syntax can be found in the ARM Ltd. documentation, but there are a few key differences related to Thumb-2 syntax:

• The .W extension is used to indicate that an instruction should be encoded in a 32-bit form. A .N extension is used to indicate that an instruction should be encoded in a 16-bit form; the assembler reports an error if this is not possible. If no extension is used then the assembler uses a 16-bit encoding whenever possible.
• 16-bit Thumb ALU instructions that set status indicate this with a syntax that has a 'S' modifier. This is the same as how ARM ALU instructions that set status have always been handled.