Inline Assembly: Passing pointers to a function and using it in that function in assembly

Question

Guys,

I'm using ARM/Cortex-A8 processor platform.

I have a simple function where I have to pass two pointers to a function. These pointers are later used in that function which has only my inline assembly code This plan is only to achieve performance.

function(unsigned char *input, unsigned char *output)
{
     // What are the assembly instructions to use these two pointers here?
     // I will inline the assembly instructions here
}

main()
{
    unsigned char input[1000], output[1000];

    function(input, output);
}

Thanks

Answer 1

Assuming you're using a normal ARM ABI, those two parameters will be passed in R0 and R1. Here is a quick example showing how to copy the bytes from the input buffer to the output buffer (gcc syntax):

.text
.globl _function

_function:
   mov  r2, #0        // initialize loop counter
loop:
   ldrb r3, [r0, r2]  // load r3 with input[r2]
   strb r3, [r1, r2]  // store r3 to output[r2]
   add  r2, r2, #1    // increment loop counter
   cmp  r2, #1000     // test loop counter
   bne  loop
   mov  pc, lr