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236 lines
7.2 KiB
Markdown
236 lines
7.2 KiB
Markdown
[< back to index](../doc_index.md)
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# Using 6502 assembly within Millfork programs
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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 6502 assemblers.
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Indexing syntax is also the same. Only instructions available on the current CPU architecture are available.
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The short branching instructions and the `BRK` instruction support the immediate addressing mode,
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for more control over code generation.
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The `BBRn/BBSn/SMBn/RMBn` instructions cannot parameterize the tested bit. The syntax is as follows:
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BBR1 $10,label
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BBS3 zpvar,label
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SMB4 zpvar
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RMB0 $10
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The HuC6280 `TST` instruction has the following syntax:
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TST #1,$1000
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TST #2,$2000,X
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The HuC6280 `TAM` and `TMA` instruction syntactically use the immediate addressing mode (the `TAMn/TMAn` form is not supported):
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TAM #$10
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TMA #$20
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**Work in progress**:
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Currently, some extra 65CE02/65816 instructions are not supported yet.
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Undocumented instructions are supported using various opcodes.
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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 x
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second:
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INC y
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INC z
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Global label names have to start with a letter and can contain digits, underscores and letters.
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Local label names (available since Millfork 0.3.22) start with a period and are visible only in the given function.
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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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LDA #fiveConstant
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CLC
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ADC fiveVariable
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STA 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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[ $EA, $EA ]
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"this is a string to print" apple2
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["this is a string to print but this time it's zero-terminated so it will actually work" apple2, 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` or `RTI`.
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A non-macro assembly function should end with `RTS`, `JMP` or `RTI` 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 accumulator.
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If the size of the return type is two bytes,
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then the low byte of the result is passed via the accumulator
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and the high byte of the result is passed via the X 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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LDA v
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CLC
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ADC #inc
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STA v
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}
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and call `increase(score, 10)`, the entire call will compile into:
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LDA score
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CLC
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ADC #10
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STA score
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Non-macro functions can only have their parameters passed via registers:
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* `byte a`, `byte x`, `byte y`: a single byte passed via the given CPU register; any 1-byte type can be used
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* `word xa`, `word ax`, `word ay`, `word ya`, `word xy`, `word yx`: a 2-byte word byte passed via given two CPU registers,
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with the high byte passed through the first register and the low byte passed through the second register; any 2-byte type can be used
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* the above, but written more explicitly: `byte register(a) paramname`, `byte register(x) paramname`, `word register(ax) paramname` etc.
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For example, this piece of code:
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asm void f(word register(ax) value) @F_ADDR extern
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f(5)
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will compile to
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LDA #5
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LDX #0
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JSR F_ADDR
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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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* two register parameters where at least one of them is an 8-bit parameter passed via A
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Other combinations are guaranteed to work only with constant arguments.
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Macro assembly functions can have maximum one parameter passed via a register.
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### External functions
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An external function should be declared with a defined memory address
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and the `extern` keyword instead of the body:
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asm void putchar(byte register(a) char) @$FFD2 extern
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## Safe assembly
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Since assembly gives the programmer unlimited access to all machine features,
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certain assumptions about the code may be broken.
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In order to make assembly cooperate with the rest of the Millfork code,
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it should abide to the following rules:
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* don't leave the D flag set
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* don't jump between functions if either of functions has stack variables
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* don't do `RTS` or `RTI` if the function has stack variables
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* don't jump or call things that are not functions or labels
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* don't store data in locations other than variables or arrays
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* don't change the stack pointer
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* end non-inline assembly functions with `RTS`, `JMP` or `RTI` as appropriate
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* on NMOS 6502:
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* don't use `XAA`, `LXA`, `AHX`, `SHX`, `SHY`, `LAS` and `TAS` instructions
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* on 65816:
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* keep the direct page register set to $0000
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* keep the M and X flags set to 1 (8-bit registers by default, native mode)
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* if running in the native mode, be careful with the stack pointer (you should keep it between $000100 and $0001FF)
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* do not change the data page register (keep an eye at the `PLD`, `MVN`, `MVP` instructions)
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* explicitly use 16-bit immediate operands when appropriate; the assembler doesn't track flags and assumes 8-bit immediates by default (TODO: actually implement the 16-bit inline assembly correctly)
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* use far jumps unless you're sure that the called function returns with an `RTS`
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* on 65CE02:
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* keep the `B` register set to $00
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* don't change the `E` flag
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* on HuC6280
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* don't use the `SET` instruction
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The above list is not exhaustive.
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