3 Games

Asteroids

Asteroids is a falling-object dodging game where the player steers a cursor left and right using keys 5 and 8 to avoid asterisks scrolling up the screen, with the step size doubling at score 104 and the marker changing to a block-graphic character at score 100.

This self-contained game occupies lines 50–570. An asterisk (A$="*") falls from the bottom of the screen as the display scrolls upward; the player steers a marker at column X (initially 12) using keys 5 (left) and 8 (right) to avoid it.

Structure of Asteroids

Lines	Purpose
50	REM label
100–135	Initialisation: A$, score counter N, position history A–D, step T=1, player column X=12
160–300	Main loop: random column R, print/scroll, update history ring buffer, collision test, key input
500–570	Crash handler: print score, CLEAR/PAUSE/CLS, restart with RUN

Notable Techniques in Asteroids

• Four-deep history ring buffer (A, B, C, D): each iteration shifts R back through the variables, giving a four-frame memory of where the falling object has been. Collision is tested against the oldest value E=D, providing a lookahead window X>=E-2 AND X<=E+T.
• Progressive difficulty: at N=100 the marker sprite changes to a two-cell block-graphic character (A$="\.'\. " — the zmakebas escape represents ▘▖); at N=104 the step size T doubles to 2, widening the collision zone and player speed simultaneously.
• SCROLL × 2 (lines 180–190): calling SCROLL twice per frame keeps the action near row 11 where the player bar is drawn.
• The erase strip at line 255 (PRINT AT 9,X-2;" ") clears six spaces to blank the previous player position before redrawing at line 260.
• No PAUSE loop — speed is controlled purely by the overhead of the display loop, typical of ZX81 SLOW mode programs.

London Bridge

London Bridge is a puzzle/strategy game played on a 5×11 grid stored in a 66-element array, where the player moves a token across numbered tiles using a numeric keypad direction input, scoring points for matches while avoiding falling into water.

Lines 1–930 implement a tile-hopping board game. A 66-element array A() stores tile values (1–5) for an 11×5 grid laid out on screen from row 5, columns 13–37. Element A(1) is repurposed as the running score accumulator. The player’s token starts at N=8 and must cross to beyond position 60.

Structure of London Bridge

Lines	Purpose
1–5	Label, FAST mode
10–135	Initialisation: title, WR display, grid drawing and random tile fill
140–440	Main game loop: pick random drop tile, get direction input, validate move, animate, score
500–510	Subroutine: translate 1-D index N to screen row/column via M=(N-1)/5
600–700	Fallen-in-water end
800–820	Successful crossing end (score bonus 1000)
900–930	Score update/display subroutine

Notable Techniques in London Bridge

• 1-D to 2-D mapping: M=(N-1)/5 maps flat index to row; INT M is the integer row, M-INT M is a fractional column indicator scaled by 5 at print time. This single formula drives the display subroutine at line 500.
• Numpad direction encoding (lines 200–230): I=(INT DIR-1)/3 converts a 1–9 keypad digit to a row offset and 3*(I-INT I) to a column offset, computing the destination index N1 arithmetically rather than with a lookup table.
• A(N)=-28 is used as a sentinel for the player’s current position (line 150, 240); adding 28 yields CHR$ 0, effectively a blank tile marker.
• Score formula (line 390): 10*(3-INT I)+10*(2-INT I)*(1-INT I) rewards moving forward (low INT I) and penalises backward moves; a tile-value match triples the score (line 400).
• RAND 0 at line 10 seeds the RNG to zero for reproducibility — unusual and potentially intentional for debugging.
• Potential anomaly: WR=1700 is displayed (line 50) but never used in any calculation; it appears to be a decorative or leftover variable.
• The \@@ escapes in PRINT strings are ZX81 inverse-space block characters used to draw the bridge pillars and deck.

Sketch Pad

Sketch Pad is a pixel-plotting utility that lets the user PLOT and UNPLOT a single point, toggle between three movement modes (continuous loop, reverse loop, or COPY screen), and move the point around the 64×44 ZX81 graphics canvas with keys 5–8.

Lines 10–600 provide an interactive pixel manipulation tool. The user positions a single pixel at (X,Y) (initially 32, 22) and switches between three drawing modes, plus a screen-dump trigger.

Structure of Sketch Pad

Lines	Mode / Purpose
40–70	Mode A: UNPLOT then PLOT — moves point (erase old, draw new)
80–90	Mode B: reverse — no draw, just calls movement subroutine
100–130	Mode C: PLOT then UNPLOT — draws a trail then erases current
500–590	Movement/mode-switch subroutine: keys 1/2/3/0 change mode, keys 5/6/7/8 move X/Y
600	COPY — sends screen to ZX printer

Notable Techniques in Sketch Pad

• Boundary guards at lines 555 (IF Y=43) and 565 (IF X=63) prevent the cursor from leaving the 64×44 graphics canvas before the corresponding move key is tested, avoiding an out-of-range PLOT error.
• Mode B (lines 80–90) loops calling the subroutine without ever plotting, making it effectively a “move without drawing” mode — useful for repositioning before switching back to a drawing mode.
• The three modes and the COPY command together form a rudimentary graphics editor; mode C’s PLOT-then-UNPLOT sequence in a tight loop would leave a brief visible trace at each visited coordinate.
• Key "0" jumps to line 100 (mode C) while keys "1", "2", "3" select modes A, B, and COPY respectively — a compact four-option menu embedded in the movement subroutine.