mirror of
https://github.com/imjasonh/nescript
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688d9afcec
Adds `examples/platformer.ne`, a full side-scrolling game that
exercises nearly every subsystem of the compiler in one program:
custom CHR tileset, 32×30 background nametable with per-region
attribute palettes, 2×2 metasprite hero with gravity/jump physics,
wrap-around horizontal scrolling, moving enemies, coin pickups,
user-declared SFX + music, and a Title → Playing state machine
with autopilot so the headless jsnes harness captures real
gameplay at frame 180. Tile art + nametable are generated by
`scripts/gen_platformer_tiles.rs` (`cargo run --bin gen_platformer_tiles`).
Building this out uncovered three independent runtime bugs that
together made the example render as black-on-black smileys. All
three are fixed in this commit:
1. **`gen_init` enabled sprite rendering before the linker's
initial palette/background load runs.** The PPU's v-register
auto-increments on every `$2007` write *during active
rendering*, so the palette load (32 B) and nametable load
(1024 B) were scrambled past the first ~72 bytes — every
existing program with a `background Level { ... }` block was
silently rendering zero-filled VRAM. Fix: leave `PPU_MASK = 0`
at the end of `gen_init` and emit a new `gen_enable_rendering`
call *after* all initial VRAM writes complete.
2. **Audio tick corrupted `ZP_CURRENT_STATE`.** The audio
driver's period-table lookup reused `$02/$03` as a temporary
indirect pointer with a comment claiming the slots were free
because the tick doesn't call mul/div. But `$03` is also
`ZP_CURRENT_STATE` used by the state dispatch loop, so every
music note silently overwrote the state index with the high
byte of `__period_table` (`0xC5` in the platformer ROM),
wedging the state machine forever. Fix: `gen_nmi` now PHAs
`$02/$03` on entry and PLA-restores them on exit, and the
audio tick JSR moves inside that save/restore window (it used
to be spliced by the linker *before* the register saves, so
even A/X/Y were technically being trashed pre-save). Only
`audio_demo`'s audio hash shifts (its note timings move a few
cycles); every other golden is unchanged.
3. **Sub-palette mirroring footgun.** Writing a 32-byte palette
blob sequentially causes the sprite sub-palettes' "index 0"
slots at `$3F10/$3F14/$3F18/$3F1C` to clobber the background
universal colour at `$3F00/$3F04/$3F08/$3F0C` via NES hardware
mirroring. The example's palette sets all eight first bytes
to `$22` (sky blue) for this reason; `docs/future-work.md`
picks up a TODO to warn on inconsistent first-byte values in
the analyzer.
Also:
- `docs/platformer.gif` — 6-second recording of the example
running in jsnes, generated by the new
`tests/emulator/record_gif.mjs` puppeteer helper (encodes via
`gifenc`, committed as a dev-dependency under
`tests/emulator/package.json`).
- README / examples/README tables and the 497-test count are
updated to cover the new example.
https://claude.ai/code/session_01BcCcHi6FUmTh8jC7UgkA3A
187 lines
8.1 KiB
Markdown
187 lines
8.1 KiB
Markdown
# Future Work
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This document tracks the gaps between what NEScript currently compiles and
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what the spec describes. Items are grouped by area. Anything implemented and
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tested is omitted — `git log` is the authoritative record of what shipped.
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---
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## PNG-sourced palette and nametable assets
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**What ships today.** `palette Name { colors: [...] }` and
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`background Name { tiles: [...], attributes: [...] }` declarations with
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inline byte arrays. The first declared palette / background is loaded
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at reset time (before rendering enables), and both blocks get named
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data blobs in PRG ROM so `set_palette Name` / `load_background Name`
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can queue a vblank-safe swap via the NMI handler. See
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`examples/palette_and_background.ne`.
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**Still TODO.**
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- `@palette("file.png")` — analyze image colours and map to nearest
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NES master-palette indices. `nearest_nes_color()` already lives in
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`src/assets/palette.rs` but is not wired through the resolver.
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- `@nametable("file.png")` — convert a 256×240 image into a 960-byte
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nametable plus 64-byte attribute table with automatic tile
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deduplication (max 256 unique tiles per pattern table).
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- Per-state background rendering control — programs currently get a
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single fixed nametable at reset; per-state swaps work but are
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limited by the NMI-time write budget (~2273 cycles, enough for a
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palette but not a full 1024-byte nametable).
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- `--memory-map` should report palette and background PRG ROM usage
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alongside the variable layout.
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---
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## User code distribution across switchable banks
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**Status.** `mapper: MMC1 / UxROM / MMC3` plus top-level `bank Name { prg }`
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declarations are honored by the iNES header and by the linker, which reserves
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each declared bank as a 16 KB switchable slot. However, the IR codegen puts
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every user function and state handler into the fixed bank at `$C000-$FFFF` —
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the declared banks exist only as empty space. Programs outgrowing the fixed
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16 KB have nowhere to put their code.
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**What's needed.**
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- A bank-assignment step (analyzer or a new pass) that maps each user function
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/ state handler to a target bank, either via explicit `bank Foo { fun bar()
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... }` nesting or by greedy size-packing.
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- Codegen support for emitting into non-fixed banks and for generating
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cross-bank trampolines (the runtime helper scaffold already exists in
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`runtime/mod.rs::gen_bank_trampoline`; it just isn't invoked).
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- Linker changes so that functions in a switchable bank are found by the
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JSR fix-up logic when the call crosses bank boundaries.
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---
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## Language feature gaps (post-v0.1)
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From the spec's "Reserved for Future Versions" section:
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| Feature | Description |
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|--------------------|-----------------------------------------------------------------------|
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| **Fixed-point** | `fixed8.8` type for sub-pixel movement with operator support. |
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| **Text / HUD** | Font sheet declarations + layout system for scores, health, menus. |
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| **Metasprites** | Multi-tile sprite groups with relative positioning. |
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| **Tilemaps** | Declarative level data with built-in collision queries. |
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| **SRAM / saves** | Persistent storage declarations for battery-backed save data. |
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| **NES 2.0 header** | Extended iNES header format for additional metadata. |
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### Struct / array field widths
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Struct and array element types are currently restricted to the single-byte
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primitives (`u8`, `i8`, `bool`). `u16`, nested struct fields, and array fields
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are rejected with `E0201`. The analyzer's field-layout machinery needs to
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grow multi-byte offsets, and IR lowering needs to treat wide fields as the
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existing wide-var path already does for `u16` globals.
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---
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## Audio pipeline
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**What ships today.** Frame-walking pulse driver with `sfx Name { duty, pitch,
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volume }` and `music Name { duty, volume, repeat, notes }` blocks; builtin
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effects and tracks; a 60-entry period table; `__audio_used` marker that
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elides the whole subsystem when no program statement references it.
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**Still TODO for richer audio.**
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- Triangle / noise / DMC channels (today the driver only uses pulse 1 and
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pulse 2).
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- Multi-channel tracker playback (one `notes` list per channel).
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- `@sfx("file.nsf")` / `@music("file.ftm")` asset directives — neither the
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NSF nor the FamiTracker format is parsed yet.
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- Per-note pitch changes within a sfx (currently `pitch` latches once at
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trigger time).
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---
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## Debug instrumentation
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**What ships today.** `debug.log(...)` and `debug.assert(...)` lower to $4800
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writes when `--debug` is passed, and are stripped entirely in release builds.
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**Not yet implemented.**
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- Mesen-compatible symbol export (`.mlb` / `.sym` files) — the CLI does not
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emit them, and the previous `DebugSymbols` helper was removed as dead code
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during cleanup.
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- Source maps relating ROM addresses to source lines — the `SourceLoc`
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IR op exists but is not consumed by the linker or CLI.
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- Array bounds checking in debug mode.
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- Frame overrun detection (cycles-per-frame counting).
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- `debug.overlay(x, y, text)` — needs the text/HUD subsystem above.
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---
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## Code quality / tooling
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### Register allocator
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All IR temps currently spill to a recycled zero-page slot (`$80-$FF`). The
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peephole pass mops up the most obvious waste, but a real CFG-aware allocator
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that holds short-lived temps in `A`/`X`/`Y` would cut a noticeable number of
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LDA/STA pairs.
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### Cross-block temp live-range analysis
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The slot recycler is function-local per-block. Temps that flow across block
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boundaries get a dedicated slot for the entire function, even if a later
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block could reuse the slot.
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### `--no-opt` flag
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There is no way to disable the optimizer from the CLI. Adding one would make
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optimizer-introduced bugs easier to bisect.
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### Compilation benchmarks
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Compilation is fast (<100 ms for every example today) but has no `cargo
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bench` harness, so regressions would slip through.
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### WASM build target
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To build a browser IDE we would need to route file I/O through a trait so the
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core pipeline works on `&str → Vec<u8>` without touching `std::fs`. Today the
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parser's preprocess pass and the asset resolver read files directly.
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---
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## Error message polish
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### Unused error codes
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`ErrorCode` only defines codes that are actually emitted. Previously there
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were placeholder variants (`E0202` invalid cast, `E0403` unreachable state)
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marked `#[allow(dead_code)]`; those were removed during cleanup. If those
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semantics come back, add the codes at that point.
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### Missing diagnostics
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- No warning for implicit-drop of a function return value (`my_fun()` at
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statement position when `my_fun` returns non-void).
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- `W0102` ("loop without yield") is only emitted for bare `loop`, not for
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`while true` or `loop { if cond { continue } }`.
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- No warning for `fast` variables that never justify the zero-page slot
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(could cross-reference access counts).
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- No warning when a `palette` declaration has inconsistent "index 0" bytes
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across its eight sub-palettes. The NES hardware mirrors `$3F10/$3F14/
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$3F18/$3F1C` onto `$3F00/$3F04/$3F08/$3F0C`, so writing the full 32-byte
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blob sequentially causes the last four "sprite sub-palette 0" bytes to
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overwrite the background universal colour; the fix is a user-side
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convention (every sub-palette's first byte equals the chosen universal
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colour) but the analyzer doesn't warn when a declaration violates it.
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The mistake produced a solid-black screen in `examples/platformer.ne`
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until it was chased down by hand.
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---
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## Open design questions
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1. **Inline asm label syntax.** `.label:` (ca65 style) vs `label:` (generic)?
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Today the inline-asm parser accepts `label:` but not `.label`; migrating
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would be cheap but would invalidate any copy-pasted ca65 fragments.
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2. **Debug port address.** $4800 is conventional but not universal. Should
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we support multiple debug output methods?
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3. **OAM allocation strategy.** Sequential allocation vs priority-based with
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automatic sprite cycling for the 8-per-scanline limit?
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4. **Error recovery granularity.** How aggressively should the parser
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recover? More recovery means more errors per compile but also risks
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cascading false errors.
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