7.28 Effect of compiler ARM and Thumb states on embedded assembler

The initial state of the embedded assembler, ARM or Thumb state, is determined by the initial state of the compiler, as specified on the command line.

This means that:

  • If the compiler starts in ARM state, the embedded assembler uses --arm.

  • If the compiler starts in Thumb state, the embedded assembler uses --thumb.

The embedded assembler state at the start of each function is as set by the invocation of the compiler, as modified by #pragma arm and #pragma thumb pragmas.

You can change the state of the embedded assembler within a function by using explicit ARM, THUMB, or CODE16 directives in the embedded assembler function. Such a directive within an __asm function does not affect the ARM or Thumb state of subsequent __asm functions.

If you are compiling for a 32-bit Thumb capable processor, you can use both 32-bit encoded Thumb instructions and 16-bit encoded Thumb instructions when in Thumb state.

If you are compiling for a 16-bit Thumb capable processor, you can only use 16-bit encoded Thumb instructions when in Thumb state.

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