Chess Through the Ages: 1973 vs 2025
Table of Contents
Introduction
A comparison between Unix V4's chess program (1973, 14KB) and Stockfish 17.1 (2025, 75MB)—52 years of chess engine evolution.
The Engines
| Feature | Unix V4 Chess (1973) | Stockfish 17.1 (2025) |
|---|---|---|
| Size | 14,310 bytes | ~75 MB |
| Platform | PDP-11/45 (0.4 MIPS) | Modern x86-64 (100,000+ MIPS) |
| Memory | ~64 KB total | Uses gigabytes |
| Evaluation | Material + position | NNUE neural network (133MB) |
| Opening Book | /usr/lib/book | Optional Polyglot books |
| Notation | e2e4 (coordinate) | UCI protocol standard |
| Search | Alpha-beta (limited) | NNUE + advanced pruning |
| ELO (estimated) | ~1200 | ~3600+ |
Sample Game: Scholar's Mate Defense
Both engines defending against the Scholar's Mate attempt:
Unix V4 Response
# /usr/games/chess Chess e2e4 1. e2e4 1. ... e7e5 <- King's pawn opening f1c4 2. f1c4 2. ... b8c6 <- Knight development (good) d1h5 3. d1h5 3. ... g7g6 <- Blocks the Scholar's Mate! (correct defense) h5f3 4. h5f3 4. ... g8f6 <- Knight development g1e2 5. g1e2 5. ... f8c5 <- Bishop development d2d3 6. d2d3 6. ... e8g8 <- Castles kingside! (excellent) e1g1 7. e1g1 7. ... d7d6 <- Solid center pawn time = 3 <- Thinking time in seconds
The 1973 engine:
- Correctly defended against Scholar's Mate with …g6
- Developed pieces harmoniously
- Castled early for king safety
- Played sound positional chess
Stockfish 17.1 Analysis of Same Position
$ stockfish << EOF position startpos moves e2e4 e7e5 f1c4 b8c6 d1h5 g7g6 h5f3 g8f6 g1e2 f8c5 d2d3 e8g8 e1g1 d7d6 go depth 15 EOF info depth 15 score cp 101 pv c1g5 c8g4 g5f6 d8d7 bestmove c1g5 ponder c8g4
Stockfish's assessment:
- White is slightly better: +1.01 pawns
- Best move: Bg5 (pinning the knight to the queen)
- The Unix V4 engine played reasonable moves
Performance Comparison
Thinking Time
| Position | Unix V4 (PDP-11) | Stockfish (Modern) |
|---|---|---|
| Opening moves | 0-2 seconds | Instant |
| Middlegame | 3-5 seconds | Instant |
| Complex tactic | 10-30 seconds? | Milliseconds |
Analysis Depth
| Engine | Typical Depth | Time |
|---|---|---|
| Unix V4 | ~4-6 ply | Seconds |
| Stockfish | 30+ ply | Seconds |
The Opening Book
Unix V4 chess uses an external book file:
$ strings /usr/games/chess | grep book /usr/lib/book
This was a space-saving technique—store common openings externally rather than computing them. Modern engines can optionally use opening books but typically compute opening moves too.
What Unix V4 Chess Got Right
Despite 50 years of advancement, the 1973 chess program shows:
- Sound defensive play - Correctly blocked Scholar's Mate
- Piece development - Knights and bishops came out
- King safety - Castled early
- Positional understanding - Solid pawn structure
For a 14KB program on a 0.4 MIPS machine, this is remarkable.
The Evolution
| Year | Engine | Size | ~ELO |
|---|---|---|---|
| 1973 | Unix V4 chess | 14 KB | ~1200 |
| 1974 | Chess 4.0 | - | ~1700 |
| 1983 | Belle | Hardware | ~2200 |
| 1997 | Deep Blue | Hardware | ~2850 |
| 2006 | Rybka | Software | ~3100 |
| 2017 | Stockfish 8 | Software | ~3400 |
| 2020 | Stockfish 12 NNUE | Software | ~3500 |
| 2025 | Stockfish 17 | 75 MB | ~3600 |
The gap between Unix V4 chess (~1200) and Stockfish (~3600) represents roughly 2400 ELO points—the difference between a club player and the strongest entity ever to play chess.
Installation
# Modern engine on FreeBSD sudo pkg install stockfish gnuchess # Compare with Unix V4 pdp11 << EOF set cpu 11/45 att rk0 disk.rk att tm0 unix_v4.tap d sr 1 boot -o tm EOF # Then: uboot k unix, login: root, /usr/games/chess
Conclusion
Unix V4's chess program, written when the world's best computer chess was still a research topic, played competent amateur chess. Stockfish 17.1, representing 52 years of accumulated knowledge, neural networks, and billions of CPU-hours of development, is essentially unbeatable.
Both engines, when shown the same position, "see" chess through completely different lenses—yet both arrived at playable positions. The 1973 engine represents the beginning of a journey that would eventually produce machines stronger than any human.
References
- Unix V4 Games - Full games analysis
- Stockfish Chess Engine
- Chess Programming Wiki: Unix