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134 lines
4.0 KiB
Markdown
134 lines
4.0 KiB
Markdown
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[< back to index](../doc_index.md)
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# Using 6809 assembly within Millfork programs
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**WARNING!** Motorola 6809 support is not yet complete.
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There are two ways to include raw assembly code in your Millfork programs:
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* inline assembly code blocks
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* whole assembly functions
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## Assembly syntax
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Millfork inline assembly uses the same three-letter opcodes as most other 6809 assemblers.
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Labels have to be followed by a colon and they can optionally be on a separate line.
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Indentation is not important:
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first: INC a
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second:
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INC b
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INC c
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Label names have to start with a letter and can contain digits, underscores and letters.
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This means than they cannot start with a period like in many other assemblers.
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Similarly, anonymous labels designated with `+` or `-` are also not supported.
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Assembly can refer to variables and constants defined in Millfork,
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but you need to be careful with using absolute vs immediate addressing:
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const byte fiveConstant = 5
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byte fiveVariable = 5
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byte ten() {
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byte result
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asm {
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LDB fiveVariable // not LDB #fiveVariable
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ADDB #fiveConstant
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STB result
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}
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return result
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}
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Any assembly opcode can be prefixed with `?`, which allows the optimizer change it or elide it if needed.
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Opcodes without that prefix will be always compiled as written.
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The '!' prefix marks the statement as volatile, which means it will be a subject to certain, but not all optimizations,
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in order to preserve its semantics.
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You can insert macros into assembly, by prefixing them with `+` and using the same syntax as in Millfork:
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macro void run(byte x) {
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output = x
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}
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byte output @$c000
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void main () {
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byte a
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a = 7
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asm {
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+ run(a)
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}
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}
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You can insert raw bytes into your assembly using the array syntax:
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[ $00, $00 ]
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"this is a string to print" bbc
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["this is a string to print but this time it's zero-terminated so it will actually work" bbc, 0]
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[for x,0,until,8 [x]]
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## Assembly functions
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Assembly functions can be declared as `macro` or not.
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A macro assembly function is inserted into the calling function like an inline assembly block,
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and therefore usually it shouldn't end with `RTS`.
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A non-macro assembly function should end with `RTS`, `JMP`, or `BRA` as appropriate,
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or it should be an external function.
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For both macro and non-macro assembly functions,
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the return type can be any valid return type, like for Millfork functions.
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If the size of the return type is one byte,
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then the result is passed via the B register.
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If the size of the return type is two bytes,
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then the result is passed via the D register.
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### Assembly function parameters
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An assembly function can have parameters.
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They differ from what is used by Millfork functions.
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Macro assembly functions can have the following parameter types:
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* reference parameters: `byte ref paramname`: every occurrence of the parameter will be replaced with the variable given as an argument
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* constant parameters: `byte const paramname`: every occurrence of the parameter will be replaced with the constant value given as an argument
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For example, if you have:
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macro asm void increase(byte ref v, byte const inc) {
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LDB v
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ADDB #inc
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STB v
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}
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and call `increase(score, 10)`, the entire call will compile into:
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LDB score
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ADDB #10
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STB score
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Non-macro functions can only have their parameters passed via registers:
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* `byte a`, `byte b`: a single byte passed via the given CPU register; any 1-byte type can be used
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* `word d`, `word x`, `word y`: a 2-byte word byte passed via given 16-bit register; any 2-byte type can be used
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Parameters passed via other registers (`U`, `S` etc.) or combinations of registers do not work yet.
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**Work in progress**:
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Only the following combinations of register parameters work reliably:
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* zero or one register parameters
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Macro assembly functions cannot have any parameter passed via registers.
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## Safe assembly
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**TODO**
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