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codereview: address four residual concerns from the hardware review

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- Analyzer: new `W0108` warning when an array's byte size exceeds
  256. The codegen lowers `arr[i]` to `LDA base,X` and the 6502's
  X register is 8 bits, so elements past byte 255 are unreachable.
  The old debug bounds check silently skipped arrays in that range;
  it now clamps the compare to 255 and the analyzer diagnoses the
  declaration up front.

- UxROM `__bank_select`: the routine previously wrote the bank
  number to a fixed `$FFF0`, which works on emulators that don't
  simulate bus conflicts (jsnes, Mesen permissive) but is broken
  on real hardware because a single ROM byte can't match every
  possible bank number. Fixed by `TAX; STA __bank_select_table,X`
  — the store lands at `table + bank_num`, whose ROM byte is
  exactly `bank_num`, so CPU bus = A = ROM = no conflict. New
  `LabelAbsoluteX` addressing-mode variant in the assembler
  resolves the table's base address through the existing fixup
  pass. The two existing UxROM example ROMs shift a few bytes
  but their goldens still match (jsnes is bus-conflict-permissive).

- Source maps: new `source_map_survives_aggressive_peephole_folding`
  regression test. The reviewer was worried peephole could drop
  `__src_<N>` labels and silently leave stale source-map entries.
  Peephole actually treats labels as block boundaries and never
  deletes them — the test pins that down by compiling a program
  tailored to trip every peephole fold and asserting every
  codegen-recorded source marker survives into the final linker
  label table.

- Frame-overrun counter: new `debug_frame_overrun_counter_reads_back_from_user_code`
  end-to-end test that proves the contract works: NMI emits
  `INC $07FF`, user `peek(0x07FF)` lowers to `LDA $07FF`, and the
  RAM allocator doesn't hand out `$07FF` to a user variable.

https://claude.ai/code/session_01MaNVcDmK9gsspRkdxowQAM
This commit is contained in:
Claude 2026-04-14 12:38:49 +00:00
parent 9b54ff83c0
commit c5297567d2
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11 changed files with 354 additions and 20 deletions
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27
src/runtime/mod.rs
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@ -1455,11 +1455,28 @@ pub fn gen_bank_select(mapper: Mapper) -> Vec<Instruction> {
out.push(Instruction::implied(RTS));
}
Mapper::UxROM => {
// UxROM: write bank number to any address in $8000-$FFFF.
// We use $FFF0 so the write lands in the fixed bank's
// tail area where the linker can back it with a matching
// bank-table byte to avoid bus conflicts.
out.push(Instruction::new(STA, AM::Absolute(0xFFF0)));
// UxROM: write the bank number to any address in
// $8000-$FFFF. On boards with bus conflicts the CPU's
// write and the ROM byte at that address are ANDed on
// the data bus, so we must write to an address whose
// ROM byte already equals the bank number. The linker
// splices a 256-byte table (`__bank_select_table`,
// bytes 0..255) into the fixed bank, and we index into
// it with X = bank number: `STA __bank_select_table, X`
// stores A (= bank number) at
// `__bank_select_table + X`, whose ROM byte is exactly
// X, so bus = A = X = ROM — no conflict.
//
// Previously this wrote to a fixed `$FFF0`, which
// happens to work on emulators that don't simulate bus
// conflicts (jsnes, Mesen permissive) but would glitch
// on real hardware because a single ROM byte can't
// match every possible bank number.
out.push(Instruction::implied(TAX));
out.push(Instruction::new(
STA,
AM::LabelAbsoluteX("__bank_select_table".into()),
));
out.push(Instruction::implied(RTS));
}
Mapper::MMC3 => {