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97 lines
4.3 KiB
Markdown
97 lines
4.3 KiB
Markdown
[< back to index](../doc_index.md)
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# Function definitions
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Syntax:
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`[segment (<segment>)] [<modifiers>] <return_type> <name> ( <params> ) [align ( <alignment> )] [@ <address>] { <body> }`
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`[segment (<segment>)] [<modifiers>] <return_type> <name> ( <params> ) [align ( <alignment> )] [@ <address>] = <expression>`
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`[segment (<segment>)] asm <return_type> <name> ( <params> ) @ <address> extern`
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Examples:
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void do_nothing() { }
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inline byte two() = 2
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asm void chkout(byte register(a) char) @ $FFD2 extern
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segment(prgrom0) void main_loop(word w, byte x) align(fast) { // body omitted
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* `<segment>`: segment name; if absent, then defaults to `default_code_segment` as defined for the platform (usually `default`)
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* `<modifiers>`: zero or more of the following:
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* `asm` – the function is written in assembly, not in Millfork (obligatory for `extern` functions),
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see [Using 6502 assembly within Millfork programs#Assembly functions](./assembly.md#assembly-functions)
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or [Using 8080/LR35902/Z80 assembly within Millfork programs#Assembly functions](./assemblyz80.md#assembly-functions);
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for 8086, see the [8086 support disclaimer](./x86disclaimer.md).
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* `macro` – the function is a macro,
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see [Macros_and inlining#Macros](../abi/inlining.md#macros)
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* `inline` – the function should preferably be inlined
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see [Macros_and inlining#Inlining](../abi/inlining.md#automatic-inlining)
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* `noinline` – the function should never be inlined
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* `interrupt` – the function is a hardware interrupt handler.
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You are not allowed to call such functions directly.
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The function cannot have parameters and the return type should be `void`.
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* `kernal_interrupt` – the function is an interrupt handler called from a generic vendor-provided hardware interrupt handler.
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The hardware instruction handler is assumed to have preserved the CPU registers,
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so this function only has to preserve the zeropage pseudoregisters.
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An example is the Commodore 64 interrupt handler that calls the function at an address read from $314/$315.
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Unlike hardware handlers with `interrupt`, you can treat functions with `kernal_interrupt` like normal functions.
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On non-6502-based targets, functions marked as `kernal_interrupt` don't differ from normal functions.
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* `const` – the function is pure and can be used in constant expressions. `const` functions are not allowed to:
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* use constants that have been declared after them
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* have local variables
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* call non-const functions
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* contain any other statements other than return statements and conditional statements
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* `<return_type>` is a valid return type, see [Types](./types.md)
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* `<params>` is a comma-separated list of parameters, in form `type name`. Allowed types are the same as for local variables.
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For assembly functions, certain parameter names are interpreted as CPU registers.
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* `<alignment>` is either a numeric literal that is a power of 2, or keyword `fast`.
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The function will be allocated at the address divisible by alignment.
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`fast` means different things depending on the target platform:
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* on 6502, it means that the function will not cross a page boundary if possible
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* on Z80, it is ignored
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* `<address>` is a constant expression that defines where in the memory the function is or will be located.
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* `extern` is a keyword than marks functions that are not defined in the current program,
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but are likely to be available at certain address in memory.
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Such functions should be marked as written in assembly and should have their parameters passed through registers.
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* `<body>` is a newline-separated list of either Millfork or assembly statements
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* `<expression>` is an expression. It is equivalent to a function body of form `{ return <expression> }`.
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The address of a non-macro function `f` is a constant `f.addr`.
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Non-macro, non-interrupt functions which have max one parameter of size max 2 bytes
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and return `void` or a value of size max 2 bytes,
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can be accessed via a pointer.
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void f() {}
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void g(byte x) {}
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function.void.to.void p = f.pointer
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function.byte.to.void p = g.pointer
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call(p)
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call(p, 13)
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The value of the pointer `f.pointer` may not be the same as the value of the function address `f.addr`.
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