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Differences in compiler support for inline and embedded assembly code

There are differences between the ways inline and embedded assembly are compiled.


  • Inline assembly code uses a high level of processor abstraction, and is integrated with the C and C++ code during code generation. Therefore, the compiler optimizes the C and C++ code and the assembly code together.

  • Unlike inline assembly code, embedded assembly code is assembled separately from the C and C++ code to produce a compiled object that is then combined with the object from the compilation of the C or C++ source.

  • Inline assembly code can be inlined by the compiler, but embedded assembly code cannot be inlined, either implicitly or explicitly.

The following table summarizes the main differences between inline assembler and embedded assembler.

Table 7-1 Differences between inline and embedded assembler

Feature Embedded assembler Inline assembler
Instruction set ARM and Thumb.

ARM on all processors.

Thumb on processors with Thumb-2 technology.

ARM assembler directives All supported. None supported.
ARMv6 instructions All supported.

Supports most instructions, with some exceptions, for example SETEND and some of the system extensions. The complete set of ARMv6 SIMD instructions is supported.

ARMv7 instructions All supported. Supports most instructions.
VFP and NEON instructions All supported. VFPv2 only.
C/C++ expressions Constant expressions only. Full C/C++ expressions.
Optimization of assembly code No optimization. Full optimization.
Inlining Never. Possible.
Register access Specified physical registers are used. You can also use PC, LR and SP.

Uses virtual registers. Using sp (r13), lr (r14), and pc (r15) gives an error.

Return instructions You must add them in your code. Generated automatically. (The BX, BXJ, and BLX instructions are not supported.)
BKPT instruction Supported directly. Not supported.