3.14 __nearfunc Keyword

The IAR __nearfunc keyword places specified function in first 128 KB of program memory. When used with the definition of a function pointer, the pointer is made 16 bits wide so that it can access functions any function in this range.

Suggested Replacement

The keyword can be removed.

Function pointers are always 16 bits wide, but there is no guarantee that the function will be placed in the first 128 KB of program memory. However, when using MPLAB XC8, the -mrelax option automatically handles placement and indirect calls for functions, regardless of where they are located in program memory. On devices with more than 128 KB of program memory, function addresses point into a trampoline table placed in near flash. The trampoline table entries then jump to the actual function using a jmp instruction.

Caveats

When function addresses are obtained without a function symbol, for example when casting an integer constant to a function pointer, this method might not worked as expected. For example:

int main(void)
{
    ((int(*)(void)) 0x20000)();    /* will not work */
}

might fail. Instead, do the following:

int main(void)
{
    extern int foo(void); /* Force creation of a symbol, with address set at link time */
    foo();
}

and associate an address with the symbol using the xc8-cc driver option -Wl,-defsym,foo=0x40000, for example.

Examples

Consider migrating IAR code such as:

__nearfunc void incMode(void);

to MPLAB XC8 code similar to:

void incMode(void);

Further Information

See the Function Pointers section in the MPLAB XC8 C Compiler User's Guide for AVR MCUs for more information on how functions are handled.