14.7 Function Call Conventions

When calling a function:

Registers W0-W7 are caller saved. The calling function must preserve these values before the function call, if their value is required subsequently from the function call. The stack is a good place to preserve these values.
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.
Registers W0-W7 are used for argument transmission.
For dsPIC33C/E/F and dsPIC30F devices, DSRPAG/PSVPAG should be preserved if the -mconst-in-code (auto_psv) memory model is being used.
For dsPIC33A devices, the following conventions hold:
- Registers F0 - F7 are caller saved. The calling function must preserve these values before the function call if their value is required upon returning from the function. The stack is a good place to preserve these values.
- Registers F8 - F31 are callee saved. The function being called must save any of these registers it will modify.
- Registers F0 - F1 will be used to return float , double or long double values.
- Registers F0 - F7 will be used to transmit float , double or long double values.

Table 14-1. Registers Required
Data Type	Architecture	Number and Type of Registers Required
char	All	1 W register
short	All	1 W register
int	All	1 W register
any C qualified pointer	All	1 W register
any extended qualified pointer	dsPIC33C/E/F and dsPIC30F	2 W registers
long	dsPIC33C/E/F and dsPIC30F	2 W registers, aligned to even numbered register
long	dsPIC33A	1 W register
float	dsPIC33C/E/F and dsPIC30F	2 W registers, aligned to an even numbered register
float	dsPIC33A	1 F register
double (Note 1)	dsPIC33C/E/F and dsPIC30F	2 W registers, aligned to an even numbered register
double (Note 1)	dsPIC33A	1 F register
long long int	dsPIC33C/E/F and dsPIC30F	4 W registers, aligned to a quad numbered register
long long int	dsPIC33A	2 W register, aligned to an even numbered register
long double	dsPIC33C/E/F and dsPIC30F	4 W registers, aligned to a quad numbered register
long double	dsPIC33A	2 F registers, aligned to an even numbered register
_Fract	dsPIC33C/E/F and dsPIC30F	1 W register
_long _Fract	dsPIC33C/E/F and dsPIC30F	2 W registers, aligned to even numbered register
_Accum	dsPIC33C/E/F and dsPIC30F	3 W registers, aligned to a quad numbered register
struct	dsPIC33C/E/F and dsPIC30F	1 register per 2 bytes in structure
struct	dsPIC33A	1 register per 4 bytes in structure
Note 1: `double` is equivalent to `long double` if `-fno-short-double` is used.