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Using 6809 assembly within Millfork programs

WARNING! Motorola 6809 support is not yet complete.

There are two ways to include raw assembly code in your Millfork programs:

• inline assembly code blocks

• whole assembly functions

Assembly syntax

Millfork inline assembly uses the same three-letter opcodes as most other 6809 assemblers.

Labels have to be followed by a colon, and they can optionally be on a separate line. Indentation is not important:

first:  INC a
second: 
        INC b
INC c

Global label names have to start with a letter and can contain digits, underscores and letters. Local label names (available since Millfork 0.3.22) start with a period and are visible only in the given function. Anonymous labels designated with + or - are also not supported.

Referring to a global label with an offset requires wrapping it in label(…):

STA .local_opcode              // ok
STA label(.local_opcode)       // ok
STA .local_opcode + 1          // ok
STA label(.local_opcode) + 1   // ok
STA global_opcode              // ok
STA label(global_opcode)       // ok
STA global_opcode + 1          // NOT OK
STA label(global_opcode) + 1   // ok

Assembly can refer to variables and constants defined in Millfork, but you need to be careful with using absolute vs immediate addressing:

const byte fiveConstant = 5
byte fiveVariable = 5

byte ten() {
    byte result
    asm {
        LDB fiveVariable  // not LDB #fiveVariable
        ADDB #fiveConstant
        STB result
    }
    return result
}

To use the direct addressing mode, prepend the argument with <:

CLR <$6f  // clears the byte $6f in the direct page
CLR $6f   // clears the byte $006f (absolute address!)

You can use > do signify the absolute addressing mode, but it is never necessary. This option exists only for compatibility with other assemblers.

Any assembly opcode can be prefixed with ?, which allows the optimizer change it or elide it if needed. Opcodes without that prefix will always be compiled as written.

The ‘!’ prefix marks the statement as volatile, which means it will be a subject to certain, but not all optimizations, in order to preserve its semantics.

You can insert macros into assembly, by prefixing them with + and using the same syntax as in Millfork:

macro void run(byte x) {
    output = x
}

byte output @$c000

void main () {
    byte a
    a = 7
    asm {
        + run(a)
    }
} 

You can insert raw bytes into your assembly using the array syntax:

[ $00, $00 ]
"this is a string to print" bbc
["this is a string to print but this time it's zero-terminated so it will actually work" bbc, 0]
[for x,0,until,8 [x]]

Assembly functions

Assembly functions can be declared as macro or not.

A macro assembly function is inserted into the calling function like an inline assembly block, and therefore usually it shouldn’t end with RTS.

A non-macro assembly function should end with RTS, JMP, or BRA as appropriate, or it should be an external function.

For both macro and non-macro assembly functions, the return type can be any valid return type, like for Millfork functions.
If the size of the return type is one byte, then the result is passed via the B register. If the size of the return type is two bytes, then the result is passed via the D register.

Assembly function parameters

An assembly function can have parameters. They differ from what is used by Millfork functions.

Macro assembly functions can have the following parameter types:

• reference parameters: byte ref paramname: every occurrence of the parameter will be replaced with the variable given as an argument

• constant parameters: byte const paramname: every occurrence of the parameter will be replaced with the constant value given as an argument

For example, if you have:

macro asm void increase(byte ref v, byte const inc) {
    LDB v
    ADDB #inc
    STB v
}

and call increase(score, 10), the entire call will compile into:

LDB score
ADDB #10
STB score

Non-macro functions can only have their parameters passed via registers:

• byte a, byte b: a single byte passed via the given CPU register; any 1-byte type can be used

• word d, word x, word y: a 2-byte word byte passed via given 16-bit register; any 2-byte type can be used

• the above, but written more explicitly: byte register(a) paramname, byte register(b) paramname, word register(x) paramname etc.

Parameters passed via other registers (U, S etc.) or combinations of registers do not work yet.

Work in progress: Only the following combinations of register parameters work reliably:

• zero or one register parameters

Macro assembly functions cannot have any parameter passed via registers.

Safe assembly

TODO