RCA 1802/1806 Chess Engine

A fully playable chess engine written in hand-crafted RCA 1802/1806 assembly language. The engine communicates via UCI protocol over serial, plays through the CuteChess GUI via a Python bridge, and has defeated Stockfish by checkmate.

Quick Stats

Stat	Value
CPU	RCA CDP1806 @ 12 MHz
RAM	32KB
Code Size	~21.3KB (21,343 bytes)
Search	Iterative deepening, depth 2-3
Opening Book	455 entries, 8 openings, 12 ply deep
Time Control	120 seconds per move (DS12887 RTC)
Wins vs Stockfish	2 (Stockfish limited to 5s/move, depth 3)

Wins vs Stockfish

#	Date	Opening	Result	Moves
1	Feb 13, 2026	Alekhine's Defense	Qg7#	38
2	Mar 2, 2026	Alekhine's Mokele Mbembe	Qg8#	35

Features

Search

• Negamax with alpha-beta pruning
• Iterative deepening (depth 1 through target depth)
• Transposition table (256 entries, Zobrist hashing)
• Null move pruning (NMP)
• Late move reductions (LMR)
• Reverse futility pruning (RFP) with depth/ply/check guards
• Futility pruning at frontier nodes with sentinel guard
• Check extension at search horizon (with ply guard)
• Killer move ordering
• Quiescence search with alpha-beta and capture ordering
• Checkmate and stalemate detection
• Repetition detection (16-bit position hash history, 255 entries)
• Fifty-move rule

Evaluation

• Material counting
• Piece-square tables (all 6 piece types)
• Castling rights bonus (+20cp per side with rights remaining)
• Graduated advanced pawn bonus (+32/+64/+96cp by rank, endgame only)
• Endgame king centralization (piece-count weighted)
• Endgame detection via non-king piece count

Board Representation

• 0x88 board format (fast off-board detection)
• Complete move generation for all piece types
• Castling legality verified in search loop (IS_SQUARE_ATTACKED on king + transit square)
• Make/unmake with full state restoration (Zobrist-incremental)
• Castling (kingside/queenside, rights revocation)
• En passant
• Pawn promotion (all piece types, in search and UCI output)

Opening Book

• 455 entries across 8 openings at ply 12 (6 moves per side)
• Giuoco Piano, Sicilian Rossolimo, French Advance, Caro-Kann Advance, QGD Exchange, Alekhine Modern, Scandinavian, Pirc Austrian
• Built from PGN databases with tools/pgn_to_book.py

Interface

• UCI protocol over serial (19200 baud via BIOS)
• CuteChess integration via Python serial bridge (elph-bridge.py)
• RTC-based time management (DS12887 real-time clock)
• UCI info output with depth, score (centipawns), and node count

Building

bash build.sh

This preprocesses config.asm, concatenates all modules in dependency order, and assembles with the A18 cross-assembler. Output: chess-engine.hex (Intel HEX) and chess-engine.bin.

Configuration

Edit config.asm to select between:

• BIOS mode (default): Uses Elf/OS BIOS for serial I/O and SCRT
• Standalone mode: Bit-bang serial for bare hardware

Playing

1. Load chess-engine.bin into RAM at $0000 via xmodem, or load chess-engine.hex via the ROM monitor
2. Connect serial at 19200 baud
3. Run the CuteChess bridge:

   python3 elph-bridge.py

4. Configure CuteChess with the bridge as an engine

The engine responds to standard UCI commands:

uci              -> id name RCA-Chess-1806 / uciok
isready          -> readyok
position startpos moves e2e4 d7d5
go depth 3       -> bestmove e2e4

Memory Map

$0000-$535E  Code (~21.3KB)
$6000-$607F  Board array (128 bytes, 0x88 format)
$6080-$608F  Game state (castling, en passant, king positions, etc.)
$6090-$618F  Move history (undo stack, 256 bytes)
$6200-$63FF  Move list (512 bytes, 4 plies)
$6400-$64FF  Search workspace (killers, scores, depths, futility table, etc.)
$6500-$66FD  Position hash history (255 entries, repetition detection)
$6780-$67FF  Quiescence search workspace
$6800-$6FFF  Transposition table (2KB, 256 entries x 8 bytes)
$7000-$77FF  UCI input buffer (2KB)
$7F00-$7FFF  Stack

File Structure

Core Engine

File	Purpose
negamax.asm	Alpha-beta search, iterative deepening, all pruning
evaluate.asm	Material + PST + castling rights + pawn bonus evaluation
pst.asm	Piece-square tables (6 piece types)
endgame.asm	Endgame king centralization heuristics
transposition.asm	TT probe/store, Zobrist hash update
zobrist-keys.asm	Zobrist hash key tables

Board & Moves

File	Purpose
board-0x88.asm	Board representation, constants, memory layout
movegen-fixed.asm	Complete move generation (all pieces)
movegen-helpers.asm	Move generation support routines
makemove.asm	Move execution with full undo support
makemove-helpers.asm	Castling, en passant, promotion handling
check.asm	Check and attack detection

Interface & Support

File	Purpose
uci.asm	UCI protocol parser and response
serial-io.asm	Serial I/O (BIOS or standalone)
opening-book.asm	Opening book data (455 entries)
opening-book-lookup.asm	Book position matching
main.asm	Entry point, initialization
config.asm	Build configuration
support.asm	16-bit arithmetic library
math.asm	Multiply/divide routines
stack.asm	Stack management for recursion

Tools

File	Purpose
tools/pgn_to_book.py	Convert PGN files to opening book ASM
tools/merge_books.py	Merge and deduplicate opening books
tools/gen_zobrist.py	Generate Zobrist hash key tables
elph-bridge.py	CuteChess serial bridge (Python/pyserial)
build.sh	Build script (preprocess, concat, assemble)

Historical Context

The RCA 1802 was the first CMOS microprocessor (1976), used in the COSMAC VIP, space probes (Voyager, Galileo), and early hobbyist computers. The Mephisto II (1981) was a commercial chess computer built on the 1802.

	Mephisto II (1981)	This Engine
CPU	RCA 1802 @ 6.1 MHz	RCA 1806 @ 12 MHz
RAM	2KB	32KB
Search	Basic alpha-beta	Negamax + TT + NMP + LMR + RFP + futility + check ext
Opening Book	Small	455 entries, 8 openings
Wins	N/A	2 vs Stockfish

Credits

• Engine design and implementation: Claude Code (Anthropic) in collaboration with Mark Abene
• Target platform: RCA CDP1802/CDP1806 (RCA Corporation, 1976)
• BIOS: Elf/OS (Mike Riley)
• UCI protocol: Stefan Meyer-Kahlen
• CuteChess: Ilari Pihlajisto, Arto Jonsson

License

Educational/hobbyist project. Use as you wish.