---
title: "Block Art"
id: 56853
type: "computer_media"
slug: "broken-program-1"
url: "http://localhost/computer_media/broken-program-1/"
markdown_url: "http://localhost/computer_media/broken-program-1.md"
published_at: "2024-09-29T07:44:51+00:00"
modified_at: "2026-03-30T21:20:44+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/09/321_Unk.png"
excerpt: "A self-contained rectangle generator that picks random dimensions, fills the interior with a character derived from its own height, and redraws on every keypress."
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: 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/2024/09/321_Unk.png"
media_type_tags: "Demo"
---

This program draws a randomly sized rectangle on screen using inverse-video block characters for the border and a fill character derived from the rectangle’s own height value. The dimensions are generated with RND: width T is scaled to 54 columns and height D to 8 rows, with a bias toward smaller heights (line 6 re-rolls D if it exceeds 4). The rectangle is rendered in three passes — a top border row, D middle rows each with side borders and an interior filled with CHR$ D, and a bottom border row — all positioned with AT and TAB expressions based on T and D. After drawing, the program waits for a keypress via CODE INKEY$ before looping back to draw a new rectangle.

***

## Program Analysis

### Program Structure

The program is a tight loop spanning lines 4–110. Lines 4–6 generate random dimensions; lines 7–100 render the rectangle; line 105 provides a keypress gate; and line 110 restarts the loop. Lines 120–140 are bootstrap/save code that runs once on initial load.

1. **Line 4–6:** Randomise width `T` (0–54) and height `D` (0–8), re-rolling `D` if it exceeds 4 to bias toward shorter rectangles.
2. **Lines 7–35:** Position the cursor with `AT` and print the top border row of `T` inverse-space characters.
3. **Lines 50–70:** Print `D` middle rows, each starting and ending with an inverse-space border character, with `T-2` interior cells filled by `CHR$ D`.
4. **Lines 75–100:** Print the bottom border row, mirroring the top.
5. **Line 105:** Waits until no key is pressed (`NOT CODE INKEY$` is true when INKEY$ is empty), then returns — though there is no corresponding `GOSUB`, so `RETURN` here would cause an error; in practice the condition is reversed and the loop simply falls through to `GOTO 4`.

### Dimension Generation

`RND**54` uses the exponentiation operator applied twice (i.e. `RND * *54` is parsed as `RND ^ 54` — raising a 0–1 random number to the 54th power), which strongly biases `T` toward very small values near zero. Similarly `RND**8` biases `D` toward small heights. The re-roll on line 6 (`IF D>4 THEN LET D=RND**8`) further skews the height distribution downward.

### Rendering Technique

The border character is the inverse-video space (`%` in the source escaping), producing a solid block. The interior fill character is `CHR$ D`, meaning the actual character displayed depends on the rectangle’s own height — for small `D` values this will be a control or graphics character, giving the interior a somewhat unpredictable appearance.

Positioning uses the expression `(1-T/2)` and `(1-D/2)` inside `AT` and `TAB`. Since these are likely to produce values between 0 and negative numbers (given `T` can reach 54 and the screen is 32 columns wide on ZX81/TS1000), the coordinate arithmetic is approximate centering that may wrap or clip on smaller screens.

### Key BASIC Idioms

- `NOT CODE INKEY$` — tests whether `INKEY$` is the empty string (CODE of “” = 0, NOT 0 = 1), used as a “no key pressed” sentinel.
- `RND**N` — raising RND to a power to shape the probability distribution toward smaller values.
- Repeated `TAB (1-T/2)` calls before each row to re-establish the horizontal position after each `PRINT` statement.

### Bugs and Anomalies

| Line | Issue |
| --- | --- |
| `105` | `RETURN` is used without a preceding `GOSUB`. When `NOT CODE INKEY$` is true the program will attempt to return from a non-existent subroutine call, causing a RETURN without GOSUB error. The intent was likely `IF CODE INKEY$ THEN GOTO 4` or similar. |
| `4` | `RND**54` is parsed as `RND ^ 54`, not `RND * 54`. If the intent was a width scaled to 54, the correct expression would be `RND*54`. The power form produces values extremely close to zero nearly all the time. |
| `7` | The `AT` row/column expressions `(1-D/2)` and `(1-T/2)` will frequently yield zero or negative values, which may be clamped or cause errors depending on the interpreter’s AT bounds checking. |

## Source Code

```
   4 LET T=RND**54
   5 LET D=RND*8
   6 IF D>4 THEN LET D=RND**8
   7 PRINT AT (1-D/2),(1-T/2);
  10 FOR X=1 TO T
  20 PRINT "% ";
  30 NEXT X
  35 PRINT TAB (1-T/2);
  50 FOR X=1 TO D
  55 PRINT TAB (1-T/2);
  56 PRINT "% ";
  57 FOR U=1 TO T-2
  60 PRINT CHR$ D;
  65 NEXT U
  66 PRINT "% ";
  70 NEXT X
  75 PRINT TAB (1-T/2);
  80 FOR X=1 TO T
  90 PRINT "% ";
 100 NEXT X
 105 IF NOT CODE INKEY$ THEN RETURN 
 110 GOTO 4
 120 CLEAR 
 130 SAVE "1032%1"
 140 RUN
```