---
title: "Microworld"
id: 57333
type: "computer_media"
slug: "microworld"
url: "http://localhost/computer_media/microworld/"
markdown_url: "http://localhost/computer_media/microworld.md"
published_at: "2024-10-04T08:22:10+00:00"
modified_at: "2026-03-30T21:18:32+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/09/181_MW.png"
excerpt: "A procedurally generated pixel landscape with a scrolling animated header and a customisable inverse-video title — all in a compact BASIC program."
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: "Demo"
    slug: "demo"
    taxonomy: "genre"
    url: "http://localhost/type/demo/"
media_contents:
  - id: 56735
    title: "Timex Sinclair Public Domain Library Tape 1004"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1004/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/09/181_MW.png"
media_type_tags: "Demo"
---

# Microworld

This program generates a randomly seeded landscape of vertical bar-chart columns across the screen, then overlays a scrolling animated border and a title in inverse video. The landscape is drawn in FAST mode using PLOT to render columns of pixels, with each column reaching a random height between 1 and 41 pixels. The title string T$ (defaulting to “MICROWORLD”) is printed in inverse video by adding 128 to each character code, centred on line 21. The scrolling effect is created by repeatedly printing a four-character string S$ (built from CHR$ 0, 9, 8, 9) across row 0 in an infinite loop, producing a moving pattern of ZX81 block-graphic characters.

***

## Program Analysis

### Program Structure

The program divides into four logical phases:

1. **Initialisation (lines 10–50):** Dimensions a three-character string `E$`, builds the scrolling pattern string `S$`, and sets the title `T$`.
2. **Landscape generation (lines 60–130):** Runs in `FAST` mode, iterating over 61 columns (M = 0 to 60) and plotting a random-height column of pixels in each.
3. **Title and border setup (lines 140–210):** Prints the title in inverse video on row 21, then enters the scroll loop for row 0.
4. **Scrolling loop (lines 180–220):** An infinite `GOTO` loop that sweeps `S$` across positions 0–28 of row 0, creating a marquee-like animation.

### Landscape Generation

The outer loop (`M = 0 TO 60`) steps through horizontal pixel columns. For each column, the inner loop plots from pixel row 0 up to `INT(RND*40)+1`, giving a random height of 1–41 pixels. Using `FAST` mode suppresses the display driver during drawing, dramatically speeding up the `PLOT` calls. The `RAND` at line 70 (no argument) seeds the random number generator from the system clock, so each run produces a different landscape.

### Inverse-Video Title

The ZX81 encodes inverse characters as the normal character code plus 128. Lines 150–170 loop over each character of `T$`, printing `CHR$(CODE T$(N)+128)` to produce each letter in inverse video. The `AT 21, 15-(LEN T$/2)` at line 140 attempts to horizontally centre the title; because `LEN T$/2` uses integer division the centering is approximate, and for a 10-character title like “MICROWORLD” it evaluates to column 10 — one cell left of true centre on a 32-column display.

### Scrolling Header

The string `S$` is assembled as `CHR$ 0 + CHR$ 9 + CHR$ 8 + CHR$ 9`. On the ZX81, character codes 0–10 are space and graphic block characters; this four-character sequence forms a small animated pattern. Printing it at successive columns 0–28 of row 0 in a tight `FOR/NEXT`/`GOTO` loop creates a leftward-sweeping marquee. `E$` (three spaces, from `DIM E$(3)`) is printed at column 29 to clear the tail end of row 0 each cycle.

### Key Variables

| Variable | Purpose |
| --- | --- |
| `T$` | Customisable title string, printed in inverse video |
| `S$` | Four-char graphic pattern used for the scrolling header |
| `E$` | Three-space string used to blank the trailing characters of row 0 |
| `M` | Horizontal pixel column counter (0–60) for landscape drawing |
| `N` | Reused: pixel row counter during plotting; character index during title print; scroll position during animation |

### Notable Techniques and Observations

- Reusing loop variable `N` for three distinct purposes (plotting height, title character index, scroll position) is a common ZX81 memory-saving idiom.
- The landscape is drawn with `PLOT` rather than character-cell graphics, giving pixel-level resolution on the 64×44 pixel grid.
- The title is user-customisable via the `REM`-documented `T$` assignment at line 50, a common convention for configurable BASIC programs of this era.
- The scroll loop has no exit condition; the program runs indefinitely until interrupted by the user.
- The centering formula `15-(LEN T$/2)` performs integer division, so for odd-length titles the result is biased one column to the right.

## Source Code

```
  10 DIM E$(3)
  20 LET S$=CHR$ 0+CHR$ 9+CHR$ 8+CHR$ 9
  30 REM T$ IS THE TITLE.
  40 REM IT CAN BE CHANGED.
  50 LET T$="MICROWORLD"
  60 FAST 
  70 RAND 
  80 FOR M=0 TO 60
  90 FOR N=0 TO INT (RND*40)+1
 100 PLOT M,N
 110 NEXT N
 120 NEXT M
 130 SLOW 
 140 PRINT AT 21,15-(LEN T$/2);
 150 FOR N=1 TO LEN T$
 160 PRINT CHR$ (CODE T$(N)+128);
 170 NEXT N
 180 FOR N=1 TO 28
 190 PRINT AT 0,N;S$
 200 NEXT N
 210 PRINT AT 0,29;E$
 220 GOTO 180
 230 SAVE "1018%1"
 240 RUN
```