disasm/_wip/Golf (JU)/copilot/OVERLAP_AUDIT.md

Pass 1 overlap-entry audit

The 19 held-back decoded entries reduce to nine intentional overlapping-instruction idioms plus one non-entry byte ($D9DA). These are valid 6502 size/speed tricks, not ambiguous code/data boundaries.

General idiom

BIT absolute consumes the following two bytes without changing A. Entering one byte later interprets the low/high operand bytes as a useful instruction stream. This permits conditional constant selection without a branch around the second load. Because ca65 cannot emit two overlapping instruction streams linearly, Pass 1 preserves each byte sequence in a localized .incbin span and documents all valid entry points.

Resolutions

• $C198: BIT $00A9; alternate entry $C199: LDA #$00 from branch $C194. Both entries valid. $C198 is fall-through constant-selection path.
• $D424: BIT $40A9; alternate entry $D425: LDA #$40 from $D49B. Both entries valid.
• $D9D8: BIT $0838; alternate entry $D9D9: SEC from $FE28. $D9DA (PHP) has no caller and is only the third byte of the BIT operand; it is not an independent entry point.
• $D9F9: BIT $FFA9; alternate entry $D9FA: LDA #$FF from $D9F4. Both entries valid.
• $DF3A: BIT $02A9; alternate entry $DF3B: LDA #$02 from $DF36. Both entries valid.
• $FB24: BIT $B8A9; alternate entry $FB25: LDA #$B8 from $FAE4/$FBAB. Both entries valid and implement the already traced terrain fallback selection.
• $FB83: BIT $FFA9; alternate entry $FB84: LDA #$FF from $FB5C. Both entries valid.
• $FC6B: BIT $18; alternate entry $FC6C: CLC from $FC68. Both entries valid. The BIT path preserves carry set by the preceding SEC; the alternate path explicitly clears carry before return.
• $FF00: BIT $00A9; alternate entry $FF01: LDA #$00 from $FEFC. Both entries valid.

Pass 1 source treatment

All nine physical byte sequences remain localized .incbin spans in prg.s. Named entry constants may point into these spans, but the overlapping bytes must not be emitted twice. Pass 2 may replace each span with explicit .byte values and entry aliases while retaining the dual interpretation in comments.