Question

I’m trying to save the context of the registers and store the stack pointer to a C variable from Assembly, it works but it throws many errors and crashes when it gets to the return statement.

Here is the code:

_saveContext:

    PUSH    SR                      
    PUSH    W0                      
    MOV     #32, W0
    MOV     W0, SR
    PUSH    W1                      
    PUSH.D  W2
    PUSH.D  W4
    PUSH.D  W6
    PUSH.D  W8
    PUSH.D  W10
    PUSH.D  W12
    PUSH    W14
    PUSH    RCOUNT
    PUSH    TBLPAG
    PUSH    ACCAL
    PUSH    ACCAH
    PUSH    ACCAU
    PUSH    ACCBL
    PUSH    ACCBH
    PUSH    ACCBU
    PUSH    DCOUNT
    PUSH    DOSTARTL
    PUSH    DOSTARTH
    PUSH    DOENDL
    PUSH    DOENDH                                                                                      
    PUSH    CORCON
    PUSH    PSVPAG
    MOV     W15, W0
    MOV     W0, _stackPointer  //Save to C Var
    RETURN

When simulating the code in MPLAB it works, the C variable gets assigned the value of the stack pointer but when it gets to RETURN it causes multiple errors:

“CORE-E0004: Trap due to unimplemented FLASH memory access, occurred from instruction at 0xXXXXXX” errors

I think it might have something to do with the way I am accessing the stackpointer (W15).

Anyone have any advice? Thanks

Answer 1

Well I found a work around, I will post it here maybe it will help someone else.

The solution was adding the code into my C file directly and getting rid of the assembly file completely. Instead of calling the assembly subroutine I just added a block of assembly into C:

asm volatile("PUSH      SR          \n"
            "PUSH.D     W0          \n"                                 
            "PUSH.D     W2          \n"
            "PUSH.D     W4          \n"
            "PUSH.D     W6          \n"
            "PUSH.D     W8          \n"
            "PUSH.D     W10         \n"
            "PUSH.D     W12         \n"
            "PUSH       W14         \n"
            "PUSH       RCOUNT      \n"
            "PUSH       TBLPAG      \n"
            "PUSH       ACCAL       \n"
            "PUSH       ACCAH       \n"
            "PUSH       ACCAU       \n"
            "PUSH       ACCBL       \n"
            "PUSH       ACCBH       \n"
            "PUSH       ACCBU       \n"
            "PUSH       DCOUNT      \n"
            "PUSH       DOSTARTL    \n"
            "PUSH       DOSTARTH    \n"
            "PUSH       DOENDL      \n"
            "PUSH       DOENDH      \n"                                                                                 
            "PUSH       CORCON      \n"
            "PUSH       PSVPAG      \n"
            "MOV        W15, _stackPointer  \n");   

This solved all the problems I was having