City Bomb

This is a city-bombing game in which the player controls a falling bomb across a procedurally generated cityscape, pressing NEWLINE to trigger an explosion before the bomb lands on a building. The city skyline is drawn using block graphics, with building heights randomised using RND across columns 0–31. Collision detection is performed by PEEKing the display file address, derived from system variables at addresses 16398 and 16399, to check whether the bomb character has landed on a solid cell (any non-NEWLINE, non-zero value). Scoring is calculated as H×31+X, rewarding the player for how far into the city they have progressed before crashing.

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Program Analysis

Program Structure

The program is divided into four logical sections:

1. Lines 10–30: Instructions prompt — checks if the player wants instructions by testing CODE C$=62 (the character >, meaning “skip instructions”).
2. Lines 40–230: Main game loop — draws the cityscape, animates the bomb across rows 1–19, and checks for a keypress or collision.
3. Lines 500–570: Bomb drop sequence — draws an explosion column downward, clears the bomb, then re-enters the scan loop at line 170 (a non-existent line, which falls through to 180).
4. Lines 700–730 / 800–890: Crash handler and instructions screen, plus a SAVE block at lines 870–890.

City Generation

The skyline is built in lines 50–90. For each column X from 0 to 31, a random height range starting somewhere in rows 11–21 up to row 19 is selected using INT(RND*11)+11. Each cell in that column is filled with the block graphic character \' (▘ followed by space), creating a ragged roofline. The city is drawn before the main loop begins and remains static throughout play.

Bomb Animation

The bomb character is defined in A$ at line 100 as ">=\.:\ '\:.", a string containing block graphic characters to visually represent a falling object. The blank string C$ (five spaces) defined at line 110 is used to erase the bomb at each position. The outer loop (H, step 2) iterates over even rows 1–19; the inner loop (X) sweeps each column left to right, printing and then erasing the bomb character.

Display-File Collision Detection

Lines 150–160 implement collision detection by directly reading the ZX81 display file. The expression PEEK 16398 + 1 + PEEK 16399 * 256 computes the start address of the display file from system variables (D_FILE is stored at addresses 16398–16399 on the ZX81), then adds 1 to skip the leading NEWLINE. The value fetched is checked: if it is not 118 (the ZX81 NEWLINE/halt character) and is greater than 0, the bomb has hit a building and execution jumps to the crash handler at line 700. Note that this peek address is fixed and does not track the current bomb position (H, X), so the collision check is reading a constant memory location rather than the cell under the bomb — this is a bug in the collision logic.

Keypress and Explosion

Line 180 polls INKEY$; any keypress triggers the explosion subroutine at line 500. The explosion loop (lines 500–540) fills each row from H+1 to 19 in column X first with \' (▘) and then immediately overwrites with \@@ — two @ characters — suggesting a flash or rubble effect. After the explosion, X is slightly randomised at line 560 (INT(RND*2)+X, shifting 0 or 1 columns), and the game jumps to line 170 (non-existent), which causes execution to resume at line 180, effectively resuming the inner scan loop mid-row.

Scoring

On a crash, the score is printed as H*31+X, approximating a linearised screen position. Higher values indicate the bomb travelled further across the city before impact.

Notable Techniques and Anomalies

• PRINT AT H,X,; at line 140 uses a trailing comma before the semicolon — on the ZX81 this is a syntax oddity but accepted; the semicolon suppresses the trailing newline.
• The GOTO 170 at line 570 targets a non-existent line, intentionally falling through to line 180 to re-enter the keypress-check mid-scan.
• The display-file PEEK at line 150 does not correctly offset by the current row and column of the bomb, making collision detection unreliable — it always reads near the start of the display file regardless of bomb position.
• Lines 870–890 contain a CLEAR, SAVE, and RUN sequence that is unreachable during normal play, intended for saving the program to tape.
• The instructions check at line 30 uses CODE C$=62 rather than comparing the string directly, testing for the > character to skip to line 800.

Variable Summary

Variable	Purpose
A$	Bomb graphic string
C$	Dual-use: input string and blank eraser (5 spaces)
H	Current row of bomb (outer loop, step 2)
X	Current column of bomb (inner loop)
Y	Loop variable for city drawing and explosion
F	Value fetched from display file for collision check