MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

4.11 FUNCTION CALL CONVENTIONS
Run Time Environment
When calling a function:
• Registers W0-W7 are caller saved. The calling function must push these values
onto the stack for the register values to be preserved.
• Registers W8-W14 are callee saved. The function being called must save any of
these registers it will modify.
• Registers W0-W4 are used for function return values.
TABLE 4-2: REGISTERS REQUIRED
Data Type Number of Registers Required
char 1
int 1
short 1
pointer 1
long 2 (contiguous – aligned to even numbered register)
float 2 (contiguous – aligned to even numbered register)
double* 2 (contiguous – aligned to even numbered register)
long double 4 (contiguous – aligned to quad numbered register)
structure 1 register per 2 bytes in structure
* double is equivalent to long double if -fno-short-double is used.
Parameters are placed in the first aligned contiguous register(s) that are available. The
calling function must preserve the parameters, if required. Structures do not have any
alignment restrictions; a structure parameter will occupy registers if there are enough
registers to hold the entire structure. Function results are stored in consecutive
registers, beginning with W0.
4.11.1 Function Parameters
The first eight working registers (W0-W7) are used for function parameters.Parameters
are allocated to registers in left-to-right order, and a parameter is assigned to the first
available register that is suitably aligned.
In the following example, all parameters are passed in registers, although not in the
order that they appear in the declaration. This format allows the compiler to make the
most efficient use of the available parameter registers.
EXAMPLE 4-1: FUNCTION CALL MODEL
void
params0(short p0, long p1, int p2, char p3, float p4, void *p5)
{
/*
** W0 p0
** W1 p2
** W3:W2 p1
** W4 p3
** W5 p5
** W7:W6 p4
*/
...
}
The next example demonstrates how structures are passed to functions. If the
complete structure can fit in the available registers, then the structure is passed via
registers; otherwise the structure argument will be placed onto the stack.
© 2008 Microchip Technology Inc. DS51284H-page 73