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44 lines
1.1 KiB
Markdown
44 lines
1.1 KiB
Markdown
[< back to index](../index.md)
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# Interfacing with external code
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## Calling external functions at a static address
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To call an external function, you need to declare it as `asm extern`. For example:
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```
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asm void putchar(byte a) @$FFD2 extern
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```
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The function parameter will be passed via the accumulator,
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the function itself is located in ROM at $FFD2. A call like this:
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```
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putchar(13)
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```
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will be compiled to something like this on 6502:
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```
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LDA #13
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JSR $FFD2
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```
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For more details about how to pass parameters to `asm` functions, see:
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* for 6502: [Using 6502 assembly within Millfork programs#Assembly functions](./assembly.md#assembly-functions).
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* for Z80: [Using Z80 assembly within Millfork programs#Assembly functions](./assemblyz80.md#assembly-functions).
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## Calling external functions at a dynamic address
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To call a function that has its address calculated dynamically,
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you just need to do the same as what you would do in assembly; 6502 example:
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```
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asm void call_function(byte a) {
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JMP (function_address)
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}
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```
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where `function_address` is a variable that contains the address of the function to call. |