Argument Passing

The same calling convention is used for a fixed as well as a variable number of arguments:

• Arguments which have primitive types are either passed in registers or pushed onto stack. In order to decide which approach to take, the compiler evaluates them from left to right. Depending on the size of the argument and whether a register is still available for that size, the argument will be passed in one of the following registers:
  • D0 (1 byte)
  • D2, D3 (2 bytes)
  • X (3 bytes)
  • D6 (4 bytes)

    or, alternatively, on the stack, if the size of the argument is greater than 4 bytes or there is no register left.

• All arguments with aggregate types are passed on stack.

The actual passing order is from right to left (that is, the first argument pushed is the rightmost one).

The following example demonstrates argument passing for a simple function with a fixed number of arguments:

void func(char a, char b, int c, long d, long e)
{

g1 = a;

g2 = b;

g3 = c;

g4 = d;

g5 = e;

}

...

void main(void)

{

...

func(0x12, 0x34, 0x5678, 0x1234ABCD, 0x5678ABCD);

...

}

Assuming inlining has been turned off, the compiler generates code as listed below:

29:   func(0x12, 0x34, 0x5678, 0x1234ABCD, 0x5678ABCD);
LD        D6,#1450748877

PSH       D6

LD        D0,#52

PSH       D0

LD        D6,#305441741

LD        D2,#22136

LD        D0,#18

JSR       func