User guide

8.7 Access from Thumb state

Because the Thumb instruction set does not contain coprocessor instructions, you cannot use the debug

communications channel while the core is in Thumb state.

There are three possible ways around this:

• You can write each polling routine in a SWI handler, which can then be executed while in either ARM or Thumb

state. Entering the SWI handler immediately puts the core into ARM state where the coprocessor instructions

are available. Refer to Chapter 5 Handling Processor Exceptions for more information on SWIs.

• Thumb code can make interworking calls to ARM subroutines which implement the polling. Refer to Chapter 3

Interworking ARM and Thumb for more information on mixing ARM and Thumb code.

• Use interrupt-driven communication rather than polled communication. The interrupt handler would be written in

ARM instructions, so the coprocessor instructions can be accessed directly.

Debug Communications Channel