The smallest game is a code-breaker. The computer picks a 4-digit number; you guess until you crack it.

# /usr/games/moo
MOO
new game
? 1234
0 bulls; 3 cows
? 3214
2 bulls; 1 cows
? 4123
4 bulls
 4 guesses

Bulls = right digit, right place. Cows = right digit, wrong place.

You might know this as Mastermind (1971), but Bulls and Cows predates the board game by decades. Bell Labs programmers were playing it on paper in the 1960s before someone wrote 624 bytes of C to automate it.

Full blackjack with proper casino rules—in under 2KB.

# /usr/games/bj
Black Jack
new game
Action $10
You have 5,6 = 11
Dealer has T
Double down? y
You have 5,6,T = 21
Dealer has T,8 = 18
You win $40

Features extracted from the binary:

• Insurance betting (when dealer shows Ace)
• Pair splitting
• Double down
• Proper shuffling

The strings in the executable reveal: A23456789TJQKHSDC — a complete deck representation in 17 characters.

This is the one that surprised me.

# /usr/games/ttt
Tic-Tac-Toe
Accumulated knowledge? y
268 'bits' of knowledge
new game
1
 X | O |
-----------
   |   |
-----------
   |   |

See that "Accumulated knowledge" prompt? The game learns from experience. It maintains ttt.k, a 268-byte file of game states that persists between sessions. Win against it, and it remembers. Play again, and it won't fall for the same trap.

This is machine learning in 1973. Donald Michie had built MENACE (Matchbox Educable Noughts and Crosses Engine) in 1961 using physical matchboxes and colored beads, but this Unix implementation brought the concept to software.

When I beat it:

I concede
134 'bits' returned

It updated its knowledge file. Next time, it'll be slightly smarter.

Not regular tic-tac-toe. This is 4×4×4 three-dimensional tic-tac-toe—64 positions with 76 possible winning lines.

# /usr/games/cubic
Type y to move first: y
  1111 2222 3333 4444
  1234 1234 1234 1234
1 ---- ---- ---- ----
2 ---- ---- ---- ----
3 ---- ---- ---- ----
4 ---- ---- ---- ----

Four layers of 4×4 boards. The display shows all four layers side by side. Your move is two digits: layer and position.

The impressive part is buried in the binary: I have a force win:

The AI can detect guaranteed winning sequences. In a game with 76 winning lines across three dimensions, that's non-trivial computation for 1973.

The legendary Hunt the Wumpus, created by Gregory Yob in 1973.

# /usr/games/wump
Welcome to 'Hunt the Wumpus.'
The Wumpus lives in a cave of 20 rooms.
Each room has 3 tunnels leading to other rooms.

You are in room 12
There are tunnels to 3,11,13

Move or shoot (m-s) m
which room? 3

You are in room 3
There are tunnels to 2,4,12
I smell a wumpus

Move or shoot (m-s) s
Give list of rooms terminated by 0
2
0
You slew the wumpus

This game invented the survival-horror genre before that was a phrase. You navigate a procedurally generated dodecahedral cave containing:

• One Wumpus (kills you if you enter its room)
• Two bottomless pits (instant death)
• Two super bat colonies (teleport you randomly)

Your only weapon: five arrows that can travel through multiple rooms. Your only intel: sensory warnings.

"When you are one room away from some other hazard: Bat - 'Bats nearby' Pit - 'I feel a draft'"

Wump directly influenced Adventure (1976) and the entire text adventure genre. Every modern survival game owes something to 5,386 bytes of C.

The crown jewel. A complete chess program with an opening book.

$ strings /usr/games/chess | head -10
Chess
/usr/lib/book       <- Opening book file!
kqrbnp PNBRQK       <- Piece notation
   a b c d e f g h  <- Board display
Resign
Illegal move

Features:

• Algebraic notation (e2e4)
• Opening book library at /usr/lib/book
• Clock/time tracking
• Draw by repetition detection
• The computer can resign when losing

This ran on a PDP-11/45 with 256KB of RAM (shared among all users). The entire program—UI, move generation, evaluation, search—fits in 14KB. Modern chess engines measure their neural networks in gigabytes.