---
title: "Laser Cannon"
id: 57242
type: "computer_media"
slug: "laser-cannon"
url: "http://localhost/computer_media/laser-cannon/"
markdown_url: "http://localhost/computer_media/laser-cannon.md"
published_at: "2024-10-04T07:10:57+00:00"
modified_at: "2026-03-30T21:19:05+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/132_LsrC.png"
excerpt: "Guide your cannon with three keys and blast 21 incoming targets before five slip past — but watch your fuel supply carefully."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
genre:
  - name: "Arcade"
    slug: "arcade"
    taxonomy: "genre"
    url: "http://localhost/type/arcade/"
  - name: "Game"
    slug: "game"
    taxonomy: "genre"
    url: "http://localhost/type/game/"
media_contents:
  - id: 56734
    title: "Timex Sinclair Public Domain Library Tape 1003"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1003/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/132_LsrC.png"
media_type_tags: "Arcade, Game"
---

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.

***

## 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.

## Source Code

```
   5 REM LASER CANNON
   6 REM 5/7KEYS MOVE YOU AND 8 FIRES THE LASER
  10 LET A=0
  20 LET J=100
  30 LET K=10
  40 LET G=0
  50 LET X=INT (RND*18)+2
  60 LET A=A+1
  70 IF A=21 THEN GOTO 260
  80 LET Y=30
  90 PRINT AT K,0;CHR$ 130;CHR$ 128;AT X,Y;"X"
 100 IF J<0 THEN GOTO 150
 110 IF INKEY$="7" THEN LET K=K-1
 120 IF INKEY$="6" THEN LET K=K+1
 130 IF INKEY$="8" THEN PRINT AT K,2;"********************"
 140 IF INKEY$="5" THEN LET J=J-1
 150 LET Y=Y-1.5
 160 IF Y=3 THEN LET G=G+1
 170 IF G=5 THEN GOTO 240
 180 IF Y=3 THEN GOTO 50
 190 IF INKEY$="8" AND K=X AND Y<21 THEN GOTO 220
 200 CLS 
 210 GOTO 90
 220 PRINT AT X,Y+1;CHR$ 189
 230 GOTO 50
 240 PRINT "DESTROYED"
 250 STOP 
 260 PRINT "YOU WIN"
 270 PRINT "FUEL LEFT= ";J
 300 SAVE "1013%2"
 310 RUN
```