Accessing the MSP and PSP registers of the Cortex-M3 in C/C++ code using Keil's µVision

Question

I need to access the MSP and PSP registers (the main and process stack registers) of the Cortex-M3 processor.

I'm writing in C/C++.

The µVision and associated compiler doesn't let you do inline assembly for this Thumb-2 only core (and I'm not sure that'd be such a good idea anyway).

I need to do this so that I can extract the immediate value of an svc instruction regardless of whether it was executed while in thread or handler mode.

Thanks,

Answer 1

For something like this I usually use a tiny assembly call-out to do it.

You can use the special register access instruction (MRS / MSR) to access either SP. I think with uVision, the MSP is *SP_main*, and the PSP is *SP_process*.

My ARM assembly is a little rusty, but I think it'd be something like:

MRS   R0, SP_process   // R0 holds retval
BX    LR                // return

From your question, it seems that you already understand the distinction between the MSP & PSP, and know how to determine which one to read (via the LR).

Answer 2

I don't have access to a Keil compiler at the moment to verify this, but according to the docs you should be able to use the compiler's "named register variable" feature:

register uint32_t msp __asm("msp");
register uint32_t psp __asm("psp");

• http://www.keil.com/support/man/docs/armccref/armccref_cjahjdbg.htm
• http://www.keil.com/support/man/docs/armcc/armcc_CHDHCIEE.htm