---
title: "Text Tiles"
id: 57240
type: "computer_media"
slug: "text-tiles"
url: "http://localhost/computer_media/text-tiles/"
markdown_url: "http://localhost/computer_media/text-tiles.md"
published_at: "2024-10-04T07:07:30+00:00"
modified_at: "2026-03-30T21:19:06+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/130_TxtT.png"
excerpt: "An infinite stream of randomly generated textile-like patterns floods the screen, mixing normal and inverse characters for a mesmerising woven effect."
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: 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/2024/10/130_TxtT.png"
media_type_tags: "Demo"
---

This program generates an endlessly scrolling screen of pseudo-random textile-like patterns using block graphics and alphanumeric characters. It builds a 9-character string where each character is either a printable ASCII character (codes 32–42) or its inverse-video equivalent, chosen by adding 128 when a second random number exceeds 0.5. The full screen is filled by printing the 9-character string 78 times plus the first 2 characters, totalling 704 characters to fill a 32×22 display. The loop restarts from line 20 each iteration, regenerating the pattern string before redrawing.

***

## Program Analysis

### Program Structure

The program is a tight infinite loop across three phases: string generation (lines 10–40), screen printing (lines 50–90), and restart (line 100). There is no user interaction; it runs continuously until interrupted.

1. **Lines 10–40:** Dimension and fill a 9-character string `G$` with randomly selected characters.
2. **Lines 50–90:** Clear the cursor to the top-left and print the pattern to fill the screen.
3. **Line 100:**`GOTO 20` skips the `DIM` at line 10, regenerating the string without re-dimensioning.

### Character Generation (Line 30)

Each character is chosen by:

- `INT(RND*11)` — selects a base character in the range 0–10, which when added to a minimum of 32 gives codes 32–42 (space through `*`).
- `(RND>.5)*128` — with roughly 50% probability, adds 128 to produce the inverse-video version of that character.

The result is a character that is either a low printable ASCII symbol or its inverse counterpart. The combination of normal and inverse characters at the same base codepoints creates the woven textile appearance.

### Screen Filling Strategy

The ZX81 screen is 32 columns × 24 rows = 768 characters. The program prints `G$` (9 characters) 78 times (702 characters) then prints `G$(TO 2)` (2 characters), for a total of 704 characters. Starting at `AT 0,0`, this fills 22 full rows (704 = 22×32), leaving the bottom status rows untouched — a practical choice to avoid interfering with the system area.

| Action | Characters printed |
| --- | --- |
| `G$` × 78 iterations | 702 |
| `G$(TO 2)` | 2 |
| **Total** | **704 (22 rows)** |

### Notable Techniques

- **Slice printing for exact fill:**`G$(TO 2)` uses a string slice to print exactly the remainder needed after the 78 full copies, avoiding screen scroll or overflow.
- **Skipping DIM on re-entry:**`GOTO 20` bypasses line 10, so the array is only dimensioned once at startup. Re-entering at line 20 simply overwrites the existing string contents.
- **Dual-RND inverse selection:** Using a separate `RND>.5` boolean expression (evaluating to 0 or 1) multiplied by 128 is a compact idiom for a 50/50 attribute flip without needing an `IF` statement.
- **`PRINT AT 0,0;` without clearing:** Printing directly over the previous frame without `CLS` avoids a flash between frames, keeping the animation smooth.

### Potential Anomalies

The base range `INT(RND*11)` produces values 0–10. Without an explicit offset these would be control characters (codes 0–10). However, on the ZX81, `CHR$` of codes below 32 still produces displayable graphic characters rather than control codes, so the output remains visible — this is intentional exploitation of the ZX81 character set rather than a bug.

## Source Code

```
   5 REM TEXT(ILES)
  10 DIM G$(9)
  20 FOR I=1 TO 9
  30 LET G$(I)=CHR$ (INT (RND*11)+(RND>.5)*128)
  40 NEXT I
  50 PRINT AT 0,0;
  60 FOR I=1 TO 78
  70 PRINT G$;
  80 NEXT I
  90 PRINT G$( TO 2);
 100 GOTO 20
 110 SAVE "1013%0"
 120 RUN
```