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.
- DSRPAG/PSVPAG should be preserved if the
-mconst-in-code(auto_psv) memory model is being used.
| Data Type | Number of Working Registers Required |
|---|---|
char |
1 |
int |
1 |
short |
1 |
pointer |
1 (eds pointer requires 2) |
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 |
_Fract |
1 |
long _Fract |
2 (contiguous – aligned to even numbered register) |
_Accum |
3 (contiguous – aligned to quad numbered register) |
* 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.