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.
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	Number and Type of Registers Required
char	1 W register
short	1 W register
int	1 W register
any C qualified pointer	1 W register
long	1 W register
float	1 F register
double (Note 1)	1 F register
long long int	2 W register, aligned to an even numbered register
long double	2 F registers, aligned to an even numbered register
_Fract	1 W register
_long _Fract	2 W registers, aligned to even numbered register
struct	1 register per 4 bytes in structure
Note 1: `double` is equivalent to `long double` if `-fno-short-double` is used.