Coding Style

Review the ca65 Users Guide for syntax details.

Formatting

Spaces, not tabs
"Tab" stops for alignment are 8 characters (Emacs asm-mode default)
res/asmfmt.pl can be used for formatting
No trailing whitespace

Assembly

Use lowercase opcodes (lda, rts)
All A2D code is 6502 not 65C02

Comments

Comments are encouraged.
End-of-line comments: ; at a tab stop or aligned with nearby code
Indented stand-alone comments: ;; at first tab stop (8 chars)
Major comments: ;;; at start of line
Use ? for questions in comment text, ??? for questions about the code:

         lda     value
         cmp     #limit    ; less than the limit?
         bcc     less      ; yes, so go do that

         rol     $1234     ; what does this do ???

Naming

Prefer snake_case for procedures and labels

NOTE: MGTK uses TitleCase for procedures, so that is used in limited cases, e.g. HideCursor, HideCursorImpl, etc.

Equates from ROM (Applesoft, Monitor, Firmware, etc) and ProDOS are in UPPERCASE.

Flow control

Do make use of unnamed labels for local loops and forward branches to avoid pointless names. Only use one level.

        ;; Copy the thing
        ldy     #7
:       lda     (src),y
        sta     (dst),y
        dey
        bpl     :-

        lda     flag
        bne     :+
        inc     count
:       rts

Literals

Use binary %00110110 for bit patterns
Use decimal for numbers (counts, dimensions, etc)
Use hex for geeky values, e.g. $7F (bit mask), $80 (high bit), $FF (all bits set) when bits would be less readble.
Avoid magic numbers where possible:
- Define local variables (e.g. ptr := $06)
- Define offsets, constants, etc.
- Use .struct definitions to define offsets into structures
- Use math where necessary (e.g. ldy #offset2 - offset1)
- Use .sizeof() (or math if needed) rather than hardcoding sizes

Structure

Delimit code blocks with .proc:

.proc some_routine
        lda     $06
        rol
        rts
.endproc

Try to encapsulate locally used data as much as possible.

.proc some_routine
        ptr := $06
        lda     ptr
        sta     stash
        rts

stash:  .byte   0
.endproc

Use impl if the entry point is not at the start:

.proc some_routine_impl
stash:  .byte   0

        ptr := $06

start:  lda     ptr
        sta     stash
        rts

.endproc
        some_routine := some_routine_impl::start

Macros

Macro use is encouraged.
Use local macros to avoid repeating code.
Use macros.inc and extend as needed to capture patterns such as 16-bit operations
API calls such as ProDOS MLI calls should be done with macros

Param Blocks

Parameter blocks are used for ProDOS MLI calls and MGTK calls.

Wrap param data in .proc blocks:

.proc textwidth_params
textptr:        .addr   text_buffer
textlen:        .byte   0
result:         .word   0
.endproc

        ;; elsewhere...
        MGTK_CALL MGTK::TextWidth, textwidth_params

Namespaces

Currently, only MGTK constants are wrapped in a .scope to provide a namespace. We may want to do that for ProDOS and DeskTop stuff as well in the future.

Self-modifying code

Add a label for the value being modified (byte or address). Use cheap local labels via the @-prefix where possible so make self-modification references more visible.

        sta     @jump_addr
        stx     @jump_addr+1
        @jump_addr := *+1
        jmp     $0000

        sty     @count
        ldy     #0
:       sta     table,y
        iny
        @count := *+1
        cpy     #00
        bne     :-

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