3.5 __farfunc Keyword

The IAR __farfunc keyword places the qualified function within program memory, and when used with the definition of a function pointer, the pointer is made 24 bits wide so that it can access functions anywhere in the range 0x0-FFFFFF.

Suggested Replacement

The keyword can be removed.

When using MPLAB XC8, the -mrelax option automatically handles placement and indirect calls for functions located anywhere in program memory. Function pointers are always 16-bit wide, but on devices with more than 128 KB of program memory, they 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 have 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:

__farfunc 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.