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millfork/docs/lang/interfacing.md
2019-07-15 14:21:50 +02:00

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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](./assembly.md#assembly-functions).
* for Z80: [Using Z80 assembly within Millfork programs#Assembly functions](./assemblyz80.md#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.