millfork/docs/lang/interfacing.md

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Interfacing with external code

Calling external functions at a static address

To call an external function, you need to declare it as asm extern. For example:

asm void putchar(byte a) @$FFD2 extern

The function parameter will be passed via the accumulator, the function itself is located in ROM at $FFD2. A call like this:

putchar(13)

will be compiled to something like this on 6502:

LDA #13
JSR $FFD2

For more details about how to pass parameters to asm functions, see:

• for 6502: Using 6502 assembly within Millfork programs#Assembly functions.

• for Z80: Using Z80 assembly within Millfork programs#Assembly functions.

Calling external functions at a dynamic address

To call a function that has its address calculated dynamically, you just need to do the same as what you would do in assembly; 6502 example:

asm void call_function(byte a) {
    JMP (function_address)
}

where function_address is a variable that contains the address of the function to call.