How do I call assembly code from a C file within CodeWarrior?

Question

The professor would like us to program a part of the program using assembly code, and then he would like us to call this code from within a C program, and then jump back to the C program when it is completed. I can't seem to find any documentation on this. I am using the Dragon12 board that uses an 68HC12, if it matters. It doesn't sound like he wants us to use the in-line asm() function.

Answer 1

question long ago but answer might still help.

For Kinetis MCU and gcc, maybe the following snippet might illustrate a possibility. (It was part of my fault handler) Below, the "PE_ISR" interrupt uses inline assembler to call the FaultHandlerAsm function, which in turn is also inline-assembler. At the end, it branches to a c-function "faultHandlerC", which is not included here.

void FaultHandlerAsm(void)
{
  __asm volatile (
    " movs r0,#4       \n"
    " movs r1, lr      \n"
    " tst r0, r1       \n"
    " beq _MSP         \n"
    " mrs r0, psp      \n"
    " b _HALT          \n"
  "_MSP:               \n"
    " mrs r0, msp      \n"
  "_HALT:              \n"
    " ldr r1,[r0,#20]  \n"
    " b FaultHandlerC  \n"
    //" bkpt #0          \n"
  );
}

PE_ISR(Cpu_INT_Hard_FaultInterrupt)
{
   __asm(    " b FaultHandlerAsm \n");
}  

Answer 2

The easiest way to find out the proper way of calling your function is just to write a function in C with the same arguments and return, doing some minimal work inside (access each argument, fill in result value), and compile to assembler. Identify how each of the operations that interest you are done, and you are set.

Answer 3

You can call a function defined in assembly just as any normal function in C (provided you use the proper calling convention), just make sure the return type and arguments match. For example, if the assembly looks like:

my_func:
    ; assembly code here
    ; some more assembly code
    ; etc.
    xor ax, ax
    ret

Then you can call it from C as follows:

extern int my_func();

// ...
int zero = my_func();

Answer 4

The help menu documentation for CodeWarrior Development Studio 10.5 describes how to call pure assembly language functions from within C/C++ code as follows:

The labels defined in an assembly file have local scope. To access them from the other file (the .c file) they should be marked as global. For example, .global _my_asm_func.

To illustrate, here is an example code snippet:

.global _my_asm_func
.text

_my_asm_func:
    subq.l   #4,a7
    move.l   d1,(a7)
    add.l    (a7),d0
    addq.l   #4,a7
    rts

In your C/C++ code, first declare a prototype for the function. For example, int my_asm_func(int a, int b);. Then call that function like any other C/C++ function. For example, my_asm_func(5, 2);.

Notice that in the assembly code, the function is prefixed with an underscore, but in the C/C++ it is not. I am not sure if this is required, or just convention.

Parameters are passed into sequential data registers. In this case, you would expect to find 5 in D0 and 2 in D1.