Mechanic · movement · multidirectional shooter, shoot 'em up

Screen wrap

Objects that leave one edge of the playfield come back at the opposite edge.

stubAlso: wraparound, wrap-around, wrapping, toroidal playfield, looping worldUpdated 2026-10-22
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  2. Games
  3. References
  4. Sources

With screen wrap, the playfield has no edge to stop at. Asteroids wraps rocks, shots, and the ship both ways on one screen, but removes the saucer at a side edge1. Defender joins the ends of a world 2,048 pixels around2.

  • Both axesAsteroids (1979). Rocks, ship, and shots wrap both ways; the saucer is removed at a side edge.1
  • Looping worldDefender (1981). A world 2,048 pixels around, joined through 16-bit coordinates.2

See also

Mentioned in

Games

GameYearHowNote
Asteroids1979Both axesrocks, ship, and shots wrap both ways; the saucer is removed at a side edge
Defender1981Looping worlda world 2,048 pixels around, joined through 16-bit coordinates
Every claim about this mechanic, by game (2)
Asteroids 1979Each frame, Asteroids adds the velocity of every object that isn't exploding (rocks, ship, saucer, and shots) to its position, on a playfield of 32 by 24 units of 256 (8,192 by 6,144 position units, computed). An object that crosses the left or right edge comes back on the other side, except the saucer, which is removed instead; one that crosses the top or bottom comes back at the other edge, the saucer included.Code
Defender 1981Defender's world is a loop: objects keep 16-bit X world coordinates, 32 units to a pixel, so the world is 65,536 units, 2,048 pixels, around and joins itself through 16-bit overflow (computed). Objects off the screen also wrap from the bottom (row 240) to the top (row 42) and back.Code

References

  1. ^ abEach frame, Asteroids adds the velocity of every object that isn't exploding (rocks, ship, saucer, and shots) to its position, on a playfield of 32 by 24 units of 256 (8,192 by 6,144 position units, computed). An object that crosses the left or right edge comes back on the other side, except the saucer, which is removed instead; one that crosses the top or bottom comes back at the other edge, the saucer included.CodeComputer Archeology, Asteroids
    How we know
    JSR $6F57 ; {}
    content/Arcade/Asteroids/Code.md#L67 · upstream ↗
    LDX #$22 LDA $0200,X ; {ram.statusAsteroids} BNE $6F62 ; {} DEX BPL $6F59 ; {} RTS BPL $6FC7 ; {}
    content/Arcade/Asteroids/Code.md#L915-L921 · upstream ↗
    LDA $0223,X ; {ram.horzVelAsteroids} … ADC $0269,X ; {ram.hposhAsteroids} CMP #$20 BCC $6FEC ; {} AND #$1F CPX #$1C BNE $6FEC ; {} JSR $702D ; {} … ADC $028C,X ; {ram.vposhAsteroids} CMP #$18 …
    content/Arcade/Asteroids/Code.md#L968-L1016 · upstream ↗
    LDA #$00 STA $021C ; {ram.statusSaucer} Saucer Flag
    content/Arcade/Asteroids/Code.md#L1015-L1021 · upstream ↗
    99| 0200:021A | statusAsteroids    | Asteroids 27-1 |
⋮
101| 021C      | statusSaucer       | Saucer, 0 = No Saucer, 1 = Small Saucer, 2 = Large Saucer |
    content/Arcade/Asteroids/RAMUse.md lines 99–101 at 1adc2a7 · upstream ↗
    1290269 - 028B Horizontal Position High (0-31), 0 = Left, 31 = Right
⋮
141028C - 02AE Vertical Position High (0-23), 0 = Bottom, 23 = Top
    content/Arcade/Asteroids/RAMUse.md lines 129–141 at 1adc2a7 · upstream ↗

    Checked against the evidence by its author, an AI model, 2026-10-22.

    Checked by a second AI model, 2026-10-22.

    In the claim ledger

  2. ^ abDefender's world is a loop: objects keep 16-bit X world coordinates, 32 units to a pixel, so the world is 65,536 units, 2,048 pixels, around and joins itself through 16-bit overflow (computed). Objects off the screen also wrap from the bottom (row 240) to the top (row 42) and back.Codemwenge/defender
    How we know
    480OX16    EQU    $A               ;16 BIT X WORLD COORD
481OY16    EQU    $C               ;16 BIT Y WORLD COORD
    src/phr6.src lines 480–481 at e740063 · upstream ↗
    3333ISCAN   LDD    BGL
3334        SUBD   #100*32          ;100 PIXEL LEFT BUFFER
⋮
3345        ADDD   OY16,X
3346        CMPA   #YMIN            ;WRAP AROUND
3347        BHS    SCYV3
3348        LDA    #YMAX
3349SCYV3   CMPA   #YMAX
3350        BLS    SCYV4
3351        LDA    #YMIN
3352SCYV4   STD    OY16,X           ;ADD 8 TIMES VELOCITY
3353        LDD    OXV,X            ;UPDATE X POSITION
⋮
3360        ADDD   OX16,X
⋮
    src/defa7.src lines 3333–3363 at e740063 · upstream ↗
    20YMAX    EQU    240
21YMIN    EQU    42               ;MIN Y COORD
    src/phr6.src lines 20–21 at e740063 · upstream ↗

    Checked against the evidence by its author, an AI model, 2026-10-22.

    Checked by a second AI model, 2026-10-22.

    In the claim ledger

Sources

Code