switching between ARM/THUMB state

arm

Solution

One explanation might be that ARM mode is the CPU's "native" operating mode, and that it's possible to do more operations in that mode than in the limited Thumb mode. The Thumb mode, as far as I've understood, is optimized for code size, which might mean it lacks certain instructions that perhaps are necessary in exception processing.

This page mentions that exception processing is always done in ARM mode. It doesn't provide any reasons why, so maybe it's just The Way It Is, by design. It does talk about ways to exit from exception processing back to the proper (ARM or Thumb) mode though, so as long as you're not writing the exception handler yourself, you might be able to ignore this issue. That, of course, assumes that your system is set up with a "default" exception handler that does retain the execution mode.

On the other hand, this page says this, about the interrupt vectors of the Cortex-M3 ARM implementation:

The LSB of each exception vector indicates whether the exception is to be executed in the Thumb state.

So it doesn't seem to be universally true, perhaps you can make your particular exception run in Thumb mode.

Problem

Original source