Game

Tetris (NES, Tengen)

Tengen's unlicensed 1989 Tetris for the NES, read through zohassadar's disassembly.

draftAlso: Tengen Tetris, Tetris: The Soviet Mind GameUpdated 2026-10-21
On this page
  1. Dealing pieces
  2. Related
  3. Mechanics
  4. References
  5. Sources

The atlas reads Tengen’s Tetris through zohassadar’s TengenTetrisDisasm, which is only partly labeled. Nothing here has been measured in the ROM, and the atlas has no copy of it to check the build against.

#Dealing pieces

The piece randomizer has no memory. Each pick steps a 16-bit generator five times and keeps three bits, rerolling only a 0, with no check against the piece before1. Nintendo’s NES Tetris rerolls against repeats2.

Each player has a generator, and both start from the same seed, so in a two-player game both get the same pieces in the same order3.

See also

Mechanics

MechanicKindHow
Piece randomizerMemorylessthree bits of a 16-bit generator, rerolled only on 0; both players get the same pieces
Every claim about this game, by concept (2)
Piece randomizergetNextTetromino steps the player's 16-bit generator five times and keeps the low three bits, trying again only when they are 0; the result, 1 to 7, is the next piece, with no check against the piece before.Code
At the start of a game both players' generators are loaded with the same seed, and only getNextTetromino steps them, so in a two-player game both players get the same sequence of pieces. The shared seed itself is stepped on every pass of the main loop before the game.Code

References

  1. ^getNextTetromino steps the player's 16-bit generator five times and keeps the low three bits, trying again only when they are 0; the result, 1 to 7, is the next piece, with no check against the piece before.Codezohassadar/TengenTetrisDisasm
    How we know
    lda player1TetrominoNext,x … sta player1TetrominoCurrent,x … adc #player1RNGSeed … jsr genNextPseudoRandom5x … and #$07 … beq @loopUntilNotZero … sta player1TetrominoNext,x
    main.asm#L3693-L3705 · upstream ↗
    3805genNextPseudoRandom5x:
⋮
3819genNextPseudoRandom:
    main.asm lines 3805–3832 at df38b4f · upstream ↗

    Checked against the evidence by its author, an AI model, 2026-10-21.

    Checked by a second AI model, 2026-10-21.

    In the claim ledger

  2. ^pickRandomTetrimino adds a pick counter to the generator's high byte and keeps the low 3 bits; a result of 7, or the same piece as last time, makes it step the generator and take the high byte's low 3 bits plus the previous piece's orientation id, modulo 7, with no further check.CodeCelestialAmber/TetrisNESDisasm · Birken, "Applying AI to Nintendo Tetris"
    How we know
    2974        inc     spawnCount
2975        lda     rng_seed
2976        clc
2977        adc     spawnCount
2978        and     #$07
2979        cmp     #$07
2980        beq     @invalidIndex
2981        tax
2982        lda     spawnTable,x
2983        cmp     spawnID
2984        bne     useNewSpawnID
2985@invalidIndex:
⋮
    main.asm lines 2974–3002 at 94581c0 · upstream ↗
    3013spawnTable:
3014        .byte   tDown, jDown, zHoriz, oFixed, sHoriz, lDown, iHoriz
    main.asm lines 3013–3014 at 94581c0 · upstream ↗
    Otherwise, the next pseudorandom number is generated and a mask is applied to retain the lower 3 bits of the high byte before adding the prior spawn orientation ID.
    Picking Tetriminos · archived copy ↗

    Checked against the evidence by its author, an AI model, 2026-10-20.

    Checked by a second AI model, 2026-10-21.

    In the claim ledger

  3. ^At the start of a game both players' generators are loaded with the same seed, and only getNextTetromino steps them, so in a two-player game both players get the same sequence of pieces. The shared seed itself is stepped on every pass of the main loop before the game.Codezohassadar/TengenTetrisDisasm
    How we know
    lda rngSeed … sta player1RNGSeed … sta player2RNGSeed … lda rngSeed+1 … sta player1RNGSeed+1 … sta player2RNGSeed+1
    main.asm#L3319-L3326 · upstream ↗
    48mainLoop:
    main.asm lines 48–50 at df38b4f · upstream ↗

    Checked against the evidence by its author, an AI model, 2026-10-21.

    Checked by a second AI model, 2026-10-21.

    In the claim ledger

Sources

Code

Secondary