How do you switch between ARM and Thumb modes in assembly and C languages?

Technical tip: starting secondary boot loader (sbl/MLO) on Cortex A15 (Sitara am57xx) architecture. I found that any attempt to load Secondary Bootloader via JTAG causes CPU crash at the first instruction. It appears that bootloader is compiled with" .code 32" directive, while primary ROM bootloader leaves CPU in thumb mode. Suggested solution manually reset T bit in CPSR to 0 in JTAG debugger before loading sbl.out

Switching between ARM and Thumb modes in embedded systems is crucial for optimizing code size and performance. In assembly, use instructions like BX or BLX to switch modes. In C, utilize compiler directives like __asm__("BX LR") for explicit control. Remember, Thumb mode is generally preferred for smaller code size, while ARM mode often offers better performance for computationally intensive tasks. Choose the appropriate mode based on your specific application requirements.

Inline assembly within C code allows for precise control over mode switching by directly embedding assembly instructions like BX & BLX. This method is particularly useful when you need to optimise critical sections of code or handle specific mode transitions without relying entirely on the compiler. In a performance-critical application where latency was a concern, I used inline assembly within a C function to manually switch between ARM & Thumb modes. By embedding the BX instruction directly in the C code, I ensured that the transition was handled exactly as needed, without any unnecessary overhead. This approach gave me fine-grained control over the mode switching process,optimising the function’s performance while maintaining reliability

This point made by the AI is confusing IMHO. First off, it's true what the AI has generated, but it's important to note that when the branch to 0x1001 takes place, the LSB (or last 1) is NOT part of the address. This is very important because it makes things easier when using (for example) `B r0`. We can decide to switch mode to Thumb just by setting the LSB in r0 to 1, the jump will still happen to the address we stored in r0 previously, however the CPU will also be set to Thumb mode. (the processor will clear the LSB just before using the full value as the jump's address). HTH

The .arm and .thumb directives in assembly are helpful for indicating which mode (ARM or Thumb) -> .arm / .thumb: Switch between ARM and Thumb modes for a section of code. -> .thumb_func: Marks a function as Thumb-mode, when using .thumb_func, you don’t need to manually set the LSB of the address when calling func, as the directive takes care of it.