Laser Cannon

Laser Cannon is a simple shooting gallery game where the player controls a cannon along the left edge of the screen and attempts to destroy incoming targets moving from right to left. The player uses keys 5/6/7 to move the cannon vertically and key 8 to fire. The cannon character is rendered using CHR$ 130 and CHR$ 128 (Spectrum block graphics), while CHR$ 189 displays a hit explosion character. The game tracks fuel (variable J, decremented by key 5), ends in victory after destroying 21 targets, or in defeat if 5 targets get through. The target X-position is randomised each round using INT(RND*18)+2, and movement is achieved by decrementing Y by 1.5 each loop iteration without CLS between print calls to create a scrolling effect.

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Program Analysis

Program Structure

The program is a single-file game with no subroutines, relying entirely on GOTO for flow control. The main game loop runs from lines 90 to 210, with a new target initialised at line 50 each time one is destroyed or reaches the left side. End conditions branch to either a win screen at line 260 or a loss screen at line 240.

1. Lines 10–40: Initialise variables (score A, fuel J, cannon row K, targets-missed G)
2. Lines 50–80: Set up a new target at a random row X, advance target counter
3. Lines 90–210: Main display/input/movement loop
4. Lines 220–230: Hit detected — display explosion character, loop to next target
5. Lines 240–270: End-game messages (loss and win)
6. Lines 300–310: Save and restart

Key Variables

Variable	Purpose
A	Target counter; game won when A=21
J	Fuel; decremented by key 5, displayed at win screen
K	Cannon row (vertical position)
G	Targets missed; game lost when G=5
X	Random row of the current target
Y	Horizontal position of the target, starts at 30

Display Technique

The cannon is drawn using CHR$ 130 and CHR$ 128, which are Spectrum block graphic characters, giving it a solid blocky appearance at column 0. The target is a plain "X" character placed via AT X,Y. There is no CLS inside the tightest loop — line 200 issues CLS only once per full loop iteration, meaning the screen is redrawn from scratch each cycle rather than using incremental updates. This causes inevitable flicker but keeps the code simple.

A hit triggers display of CHR$ 189 at the target position (line 220), which is the copyright/explosion-style character used as a visual “bang” effect.

Movement and Hit Detection

The target moves left by decrementing Y by 1.5 each loop (line 150). Because PRINT AT truncates to integer column positions, this gives effective movement every other iteration, producing a smoother visual step than decrementing by 1 would. Hit detection at line 190 checks that key 8 is held, the cannon row K equals the target row X, and the target column Y is less than 21 (i.e., within laser range). The laser itself is drawn as a string of asterisks at line 130 purely cosmetically and has no role in the hit logic.

Notable Techniques and Anomalies

• The INKEY$ at lines 110–140 and 190 are all separate reads in the same loop iteration, so simultaneous key effects are not possible and there is slight timing sensitivity.
• Line 100 checks IF J<0 and skips to line 150, bypassing all input processing when fuel is exhausted — effectively immobilising the player but not ending the game, which is an unusual design choice.
• The condition IF Y=3 at line 160 relies on Y landing exactly on 3 when stepping by 1.5 from 30. Starting at 30 and subtracting 1.5 repeatedly: 30, 28.5, 27, 25.5 … 4.5, 3 — this works correctly since 30 and 3 differ by 27, a multiple of 1.5.
• Key 5 is labelled “move” in the REM but actually decrements fuel J, with movement left handled implicitly by the target approaching; the REM comment is slightly misleading.
• Lines 300–310 save and immediately RUN the program, providing an auto-restart after saving.