---
title: "Bat-and-Ball"
id: 56855
type: "computer_media"
slug: "broken-program-2"
url: "http://localhost/computer_media/broken-program-2/"
markdown_url: "http://localhost/computer_media/broken-program-2.md"
published_at: "2024-09-29T07:53:09+00:00"
modified_at: "2026-03-30T21:20:43+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/09/323_Unk.png"
excerpt: "A minimalist bat-and-ball game where a bouncing block and a player-controlled paddle face off using nothing but BASIC and inverse-video characters."
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: "Game"
    slug: "game"
    taxonomy: "genre"
    url: "http://localhost/type/game/"
media_contents:
  - id: 56738
    title: "Timex Sinclair Public Domain Library Tape 1007"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1007/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/09/323_Unk.png"
media_type_tags: "Game"
---

This program implements a simple bat-and-ball game on the ZX81/TS1000, where a ball bounces horizontally across the screen and the player controls a bat using keys. The ball’s position is tracked by column variable D, which reverses direction when it hits the screen edges (columns 7 and 17), while the bat is drawn at row A using inverse-video space characters for solid blocks. The SCROLL command at line 80 is used to erase the previous ball position before redrawing, a common ZX81 screen-update trick. Key codes 50 and 63 correspond to the ‘2’ and ‘/’ keys respectively, used for left/right bat movement. Line 140 uses RETURN to exit the game loop when no key is pressed, serving as a termination condition.

***

## Program Analysis

### Program Structure

The program is a short bat-and-ball game contained in a single loop from lines 60 to 150. Initialization occupies lines 10–50, and lines 160–180 handle save/restart. The main game loop redraws the ball, reads the keyboard, moves the bat, updates the ball’s horizontal position, and repeats.

### Variable Roles

| Variable | Initial Value | Role |
| --- | --- | --- |
| `K` | 2 | Ball direction multiplier (+2 or −2) |
| `A` | 10 | Bat row (fixed vertical position) |
| `B` | 10 | Bat column (player-controlled) |
| `C` | 20 (A+A) | Ball row (fixed at row 20) |
| `D` | 6 | Ball column (moves each frame) |

### Game Loop Mechanics

Each iteration of the loop (lines 60–150) performs the following steps in order:

1. Draw the ball at row `C`, column `D` (line 60).
2. Erase the bat’s previous position by printing a space at row `A`, column `B` (line 70).
3. `SCROLL` the screen up one line (line 80), which also erases the top ball row indirectly.
4. Read the keyboard: key code 50 (‘2’) moves the bat right; key code 63 (‘/’) moves it left (lines 90–100).
5. Redraw the bat at its new position (line 110).
6. Bounce the ball if it reaches column boundaries (lines 120–130).
7. Exit via `RETURN` if no key is held (line 140), otherwise loop back (line 150).

### Key BASIC Idioms and Techniques

- **Inverse-video blocks:** The `%` sequences in PRINT statements render as solid inverse-video space characters, creating visible bat and ball sprites without UDGs or machine code.
- **Direction flip:**`LET K=-K` at line 120 neatly reverses the ball’s horizontal direction by negating the step value between +2 and −2.
- **RND* *K:** Line 130 uses `RND**K` — `RND*` on the ZX81 gives a random integer, and multiplying by `K` (±2) adds a variable-speed horizontal step each frame.
- **RETURN as exit:** Line 140 uses `RETURN` when no key is pressed. Since the main loop is not called via `GOSUB`, this will cause an error or jump to a calling context, effectively acting as a game-over or halt mechanism.
- **SCROLL for animation:** The `SCROLL` at line 80 shifts the entire display up, functioning as a crude erase for the ball row rather than blanking individual positions.

### Bugs and Anomalies

- The bat movement keys are asymmetric: code 50 (‘2’) increases `B` (moves right), while code 63 (‘/’) decreases it (moves left). On a standard keyboard layout this is non-intuitive; ‘5’ and ‘8’ would be more natural for left/right.
- There is no collision detection between the bat (row 10) and the ball (row 20); the game has no win/lose condition based on interception.
- The bat column `B` is never clamped, so it can move off-screen without restriction.
- `SCROLL` unconditionally shifts the whole display each frame, causing the entire screen to scroll upward continuously, which distorts any persistent display elements.
- Lines 160–180 (`CLEAR`, `SAVE`, `RUN`) are unreachable from the main loop and appear to be leftover development/save boilerplate.

## Source Code

```
  10 LET K=2
  20 LET A=10
  30 LET B=A
  40 LET C=A+A
  50 LET D=6
  60 PRINT AT C,D;"% % % % % % "
  70 PRINT AT A,B;"% "
  80 SCROLL 
  90 IF CODE INKEY$=50 THEN LET B=B+1
 100 IF CODE INKEY$=63 THEN LET B=B-1
 110 PRINT AT A,B;"% "
 120 IF D>17 OR D<7 THEN LET K=-K
 130 LET D=D+RND**K
 140 IF NOT CODE INKEY$ THEN RETURN 
 150 GOTO 60
 160 CLEAR 
 170 SAVE "1032%3"
 180 RUN
```