Accessing scratch pad memory from C

Question

That might be a little bit an exotic problem, but I hope somebody can still help me out a bit ;). I would like to execute a standard C program, however, at some point during program execution I would like that a certain number of instructions, which are stored in a local scratch pad RAM, are executed. The scratchpad memory is accessable for all processes. Lets assume this local memory starts at address 0x80000000 and I would integrate this in the following C code fragement

int main {
int a=1;
int b=2;
int c=3;    

c = a + b;

%goto address 0x80000000 and execute three instructions before continuing
%program execution here 

return(0);

}

The program counter would go through the following stages, assuming main is loaded at 0x40000000

0x40000000    a=5; 
0x40000004    b=2; 
0x40000008    c=1;  
0x4000000C    c=a+b;
0x80000000    first instruction in the local scratch pad
0x80000004    second instruction in the local scratch pad
0x80000008    third instruction in the local scratch pad  
0x40000010    return(0);

Anyone an idea how to do this? Do I need to use assembler jump instructions or is there something more elegant.

Many thanks, Andi

Answer 1

Assuming that the instructions behave like a normal function, you can do:

#include <stdio.h>

void (*scratchpad_func)(void) = (void(*)(void))0x80000000;

int main()
{
    printf("before\n");
    scratchpad_func();
    printf("after\n");
    return 0;
}

Obviously, you'll have to be using a real mode operating system, or jump through whatever hoops your OS/processor combination requires to have direct access to that address space.

(On some architectures "behave like a normal function" is as simple as a "jump $ra" at the end if you don't touch callee-saved registers. E.g. MIPS.)