Interworking

You can mix and match ARM and Thumb code within the same binary. This is called interworking and it lets you exploit the strengths of each instruction set.

For example:

• The bulk of your code could be written in Thumb, to minimise binary size.
• Performance critical parts could be written in ARM, for speed.

The ‘T’ bit in the CPSR register signals Thumb mode.

• Switch between modes with BX Rm.
  • Special form of branch instruction.
  • Bottom bit of Rm becomes the T bit.

ARMv5T and later architectures add:

• BLX <Rm|label>
• and other changes which make interworking much more pleasant.