C64 Chess in action

     _____                              _                    __ _  _      _____ _
    / ____|                            | |                  / /| || |    / ____| |
   | |     ___  _ __ ___  _ __ ___   __| | ___  _ __ ___   / /_| || |_  | |    | |__   ___  ___ ___
   | |    / _ \| '_ ` _ \| '_ ` _ \ / _` |/ _ \| '__/ _ \ | '_ \__   _| | |    | '_ \ / _ \/ __/ __|
   | |___| (_) | | | | | | | | | | | (_| | (_) | | |  __/ | (_) | | |   | |____| | | |  __/\__ \__ \
    \_____\___/|_| |_| |_|_| |_| |_|\__,_|\___/|_|  \___|  \___/  |_|    \_____|_| |_|\___||___/___/

Because apparently I hate myself and have too much free time.

What Is This?

A chess game for the Commodore 64, written in 6510 assembly language. Yes, really. In 2026. For a computer from 1982. I'm fine. Everything is fine.

Why Would Anyone Do This?

Nostalgia is a hell of a drug. I grew up on Commodore machines - VIC-20, C64, C128, Amiga 500. Learned BASIC, dabbled in assembly, and apparently never fully recovered.

The constraint is the point. Modern development is drowning in dependencies. npm install downloads half the internet. Meanwhile, the C64 has 64KB of RAM and makes you fight for every byte. There's something beautifully brutal about that.

The tooling got good. Back in the day, writing C64 code meant no IDE, primitive debugging, and a lot of crying. Now we have KickAssembler, VICE emulator, and the edit-build-run cycle takes seconds. It's almost too easy. (It's not. I'm lying. Send help.)

Why Chess?

Chess is perfect for the C64's limitations:

8x8 board - fits the 8-bit aesthetic perfectly
Turn-based - no need for 60fps buttery smoothness
No sound required - good, because SID programming is its own circle of hell
32 pieces - wait, that's a problem...

The Sprite Problem

The C64 has 8 hardware sprites. Chess needs 32 pieces. Math doesn't math.

Solution: Sprite Multiplexing - a fancy term for "lying to the screen really fast." During raster interrupts, we reuse the same 8 sprites across different rows. The electron beam draws top-to-bottom, so by the time it reaches row 2, we've already repositioned the sprites. It's like a magic trick, except the magician is having a panic attack about cycle timing.

Quick Start

Prerequisites

Docker
Make
A tolerance for retro computing

Build

make build

Outputs C64Chess.d64 - a disk image you can load into VICE or a real C64 with a 1541 Ultimate II+.

Run (macOS with VICE)

make run

Builds and launches in VICE. Requires VICE installed at /Applications/VICE/x64sc.app/.

Project Structure

| File | What It Does | |------|--------------| | main.asm | Entry point - pulls everything together | | game.asm | Game flow, keyboard dispatch, player management | | input.asm | Coordinate input handling (A1-H8) | | display.asm | Screen updates, prompts, status display | | menus.asm | Menu system (main, player select, etc.) | | moves.asm | Move validation and piece movement | | board.asm | Board state and checkerboard pattern generation | | raster.asm | Sprite multiplexing via raster interrupts | | sprites.asm | Sprite rendering during interrupts | | clock.asm | Play clock (BCD arithmetic - shudder) | | routines.asm | Utility routines | | memory.asm | Memory operations (copy, fill, board flip) | | keyboard.asm | Keyboard scanning routine | | constants.asm | Constants, memory layout, zero page allocations | | storage.asm | Variable storage | | macros.asm | KickAssembler macros | | pseudocommands.asm | Custom pseudo-instructions (jne, stb, etc.) | | strings.asm | Text strings | | characters.asm | Custom character set | | layout.asm | Screen position calculations | | opening_moves.asm | Opening book (eventually) | | vic.asm | VIC-II register definitions |

Key Data Structures

BoardState (64 bytes)

The heart of the game. Each byte represents one square:

Bits 0-6: Sprite pointer (which piece)
Bit 7: Color (0=black, 1=white)

Position 0 = top-left (A8), position 63 = bottom-right (H1).

Zero Page ($02-$25)

Precious zero page real estate, carefully allocated:

$02-$0c: Memory copy/fill pointers
$0d-$12: Math operations
$13-$1a: Display pointers
$1b-$1e: Temp storage
$1f-$25: String printing

The keyboard routine also squats on $50-$5f. Don't touch those.

Sprite Multiplexing Tables

irqypos: Raster lines to trigger interrupts (4 lines before sprite Y)
spriteypos: Where sprites actually appear

Custom Pseudo-Commands

KickAssembler lets you create custom instructions. These appear throughout the code:

| Command | What It Does | |---------|--------------| | stb value:address | Store byte (like lda/sta combined) | | jne address:value:location | Jump if not equal | | jeq address:value:location | Jump if equal | | bfs flag:location | Branch if flag set (bit 7) | | bfc flag:location | Branch if flag clear | | sef flag | Set flag (bit 7) | | clf flag | Clear flag | | mult8 | Multiply accumulator by 8 |

Current Status

It's playable! You can:

Select pieces (they flash to confirm)
Enter moves via coordinates (e.g., E2 to E4)
Move pieces around the board
Track captured pieces
See play time for each player

Not yet implemented:

Move validation (currently you can make illegal moves - honor system!)
Check/checkmate detection
AI opponent
Castling, en passant, pawn promotion
Opening book
Endgame scenarios

Known Issues

The computer "thinks" but doesn't actually do anything yet. It's just vibes.
You can absolutely cheat. The game trusts you. Don't betray that trust.

Technical Highlights

~1000 bytes saved through recent refactoring (generated checkerboard pattern, in-place board flip, consolidated routines)
Cycle-accurate sprite multiplexing - 8 rows of sprites, 32 pieces total
BCD arithmetic for the play clock (because regular math wasn't painful enough)
RTS jump tables for keyboard dispatch (6502 doesn't have indirect indexed jumps, so we improvise)

Building Your Own C64 Software

Want to join this particular circle of masochism? Here's what I use:

KickAssembler - Genuinely excellent 6502/6510 assembler
VICE - The gold standard C64 emulator
Spritemate - Browser-based sprite editor
CharPad - Character set editor

Contributing

Found a bug? Want to add something? PRs welcome. Just remember: every byte counts, and I will judge your register usage.

License

Do whatever you want with this. It's a chess game for a 44-year-old computer. I'm not precious about it.

"640K ought to be enough for anybody." - Not Bill Gates, but whoever said it clearly never tried sprite multiplexing.

Chess

Install / Use

README