Live: nesy.netlify.app · Editor: nesy.netlify.app/editor · by delacannon
A Bitsy-style editor for making real NES games. Draw rooms, tiles,
sprites and dialogs in the browser , the app compiles your game to 6502 assembly, assembles it
in-browser into an iNES ROM (NROM, 32KB PRG + 8KB CHR), and lets you play-test it on an embedded
emulator. Download the .nes for any emulator or flash cart, and the generated .asm to study.
Same spirit as Bitsy: walk around, talk to people, pick up things, be somewhere. No physics , instant grid movement, one screen per room.
| World editor | Sprites |
|---|---|
 |
 |
| Music | Dialog |
|---|---|
 |
 |
pnpm install
pnpm dev # editor at http://localhost:5173
pnpm test # all unit + headless-emulator testsIn the editor: edit tab to draw, play tab to build + run the ROM (arrows walk, Z talks,
Enter starts), data tab to import/export the game as text (load sample for a demo game).
- Rooms: 16×15 grid of 16×16px cells , exactly one NES screen (256×240)
- Tiles: 16×16, 3 colors + backdrop, optional wall flag + 2-frame animation
- Avatar / sprites / items: 16×16 metasprites (2×2 hardware sprites), 2-frame animation
- Dialogs: paged text box (28 cols × 2 lines), uppercase 8×8 font
- Exits teleport between rooms; endings show a dialog and stop the game
- Palettes: real NES palettes , backdrop + 4 BG + 4 sprite sub-palettes per set, one BG sub-palette per cell (matches NES attribute granularity)
Budgets (enforced live in the status bar): ≤24 rooms, ≤62 tiles, ≤15 sprites+items per room, ≤128 item placements, 32KB PRG, 256+256 CHR tiles with automatic 8×8 deduplication.
| package | what |
|---|---|
packages/asm6502 |
two-pass 6502 assembler in TS (all 151 official opcodes, .org/.byte/.word/.res, expressions) |
packages/core |
game data model, text format parse/serialize, validation, NES master palette |
packages/compiler |
engine/engine.asm (the generic runtime), CHR generation, data table codegen, iNES builder , buildRom(game) |
apps/editor |
Vite + React editor with jsnes play-testing |
Bitsy-like, line-oriented (see the data tab):
GAME my cave adventure
VER 1
START 0 7,7
PAL 0
BKG 0F
BG0 21,11,30
SP0 30,27,16
...
TIL a
NAME rock
WALL true
16 rows of 16 pixels (0-3), '>' separates animation frames
ROOM 0
PAL 0
15 rows of 16 tile ids
PMAP (optional: per-cell BG sub-palette 0-3)
EXT 15,7 1 0,7 (exit at 15,7 -> room 1 at 0,7)
END e 8,3 (ending: dialog e)
ITM k 3,9 (item placement)
SPR b
DLG c
SPAL 1
POS 0 4,6
pixels...
DLG c
Meow. The cave is deeper than it looks.
NMI-driven 6502 runtime: title screen → walk state (d-pad, wall collision, sprite talk, item pickup with taken-flags, exits, endings) → paged dialog box with a typewriter effect → ending lock. Tile animation flips every 32 frames through a vram write buffer that never overruns vblank. Room loads write the full nametable + attributes + palettes with rendering off.
The downloadable .asm is the complete engine plus your game as commented data tables ,
assemble it yourself with the bundled assembler or ca65 (with minor directive tweaks).
- assembler: hand-typed fixture of all 151 opcodes → exact bytes
- compiler: golden ROM SHA-256 (
UPDATE_GOLDENS=1 pnpm testafter intentional engine changes) - engine: headless jsnes scenarios , boot, walk, collide, talk, pick up, exit, ending , asserting engine RAM via the assembler symbol table
NESty stands on the shoulders of two projects that made "anyone can make a game" real:
- Bitsy by Adam Le Doux , the whole spirit of NESty. Walk around, talk to people, pick up things, be somewhere. Tiny games, huge heart.
- GB Studio by Chris Maltby , proof that a friendly editor can target real retro hardware, and a north star for the editor's shape and polish.
- Uppercase-only ASCII font (saves CHR space for art)
- No audio, no scrolling, no dialog scripting (
VARis reserved in the format) - ≥4 sprites/items on one row will flicker on real hardware (editor warns)
- ROMs are Mesen/FCEUX/everdrive-friendly NROM; jsnes is forgiving