16.6 Function Parameters

MPLAB XC uses a fixed convention to pass arguments to a function. The method used to pass the arguments depends on the size and number of -arguments involved.

Note: The names “argument” and “parameter” are often used interchangeably, but typically an argument is the actual value that is passed to the function and a parameter is the variable defined by the function to store the argument.

The Stack Pointer is always aligned on an 8-byte boundary.

All integer types smaller than a 32-bit integer are first converted to a 32-bit value. The first four 32 bits of arguments are passed via registers a0-a3 (see the table below for how many registers are required for each data type).
Although some arguments may be passed in registers, space is still allocated on the stack for all arguments to be passed to a function (see the figure below). Application code should not assume that the current argument value is on the stack, even when space is allocated.
When calling a function:
- Registers a0-a3 are used for passing arguments to functions. Values in these registers are not preserved across function calls.
- Registers t0-t7 and t8-t9 are caller saved registers. The calling function must push these values onto the stack for the registers’ values to be saved.
- Registers s0-s7 are called saved registers. The function being called must save any of these registers it modifies.
- Register s8 is a saved register if the optimizer eliminates its use as the Frame Pointer. s8 is a reserved register otherwise.
- Register ra contains the return address of a function call.

Table 16-1. Registers Required
Data Type	Number of Registers Required
`char`	1
`short`	1
`int`	1
`long`	1
`long long`	2
`float`	1
`double`	1
`long double`	2
`structure`	Up to 4, depending on the size of the struct.