15.7 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.

Table 15-1. Registers Required
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.