---
title: "OVERS 1"
id: 55860
type: "computer_media"
slug: "overs-1"
url: "http://localhost/computer_media/overs-1/"
markdown_url: "http://localhost/computer_media/overs-1.md"
published_at: "2024-07-06T17:27:01+00:00"
modified_at: "2026-03-30T21:44:23+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2019/02/20230809-041332.jpg"
excerpt: "See what happens when two characters share the same screen cell — this interactive tool visually demonstrates XOR overlay printing with instant labeled results."
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 2068"
    slug: "ts2068"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts2068/"
model:
  - name: "Timex/Sinclair 2068"
    slug: "ts-2068"
    taxonomy: "model"
    url: "http://localhost/model/ts-2068/"
indiv:
  - name: "Roger Valentine"
    slug: "roger-valentine"
    taxonomy: "indiv"
    url: "http://localhost/indiv/roger-valentine/"
genre:
  - name: "Demo"
    slug: "demo"
    taxonomy: "genre"
    url: "http://localhost/type/demo/"
media_contents:
  - id: 55879
    title: "ISTUG Public Domain Library 7"
    type: "computer_media"
    url: "http://localhost/computer_media/istug-public-domain-library-7/"
media_type: "Program"
programmers:
  - name: "Roger Valentine"
    slug: "roger-valentine"
    taxonomy: "indiv"
    url: "http://localhost/indiv/roger-valentine/"
download_url: "https://archive.org/download/timex-sinclair-software-archive/OVERS%201%20%28198x%29%28TS2068%29%28US%29%28Program%29.zip"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/07/SCR-20260329-odzj.png"
related_content:
  - id: 20956
    title: "The Timex Sinclair 2068: what can you do with it?"
    type: "book"
    url: "http://localhost/book/the-timex-sinclair-2068-what-can-you-do-with-it/"
media_type_tags: "Demo"
---

# OVERS 1

This program demonstrates the OVER 1 print attribute by overlaying two user-supplied characters on the same screen position using a CHR$ 8 (backspace/cursor-left) control code to reposition the cursor. The user enters any two characters; the first is printed, the cursor backs up one space, OVER 1 is engaged, and the second character is XOR-composited on top of the first. The result and the individual characters are then displayed as a labeled equation. Input validation checks both for a space (stop condition) and for exactly two characters before proceeding.

From page 70 of [Roger Valentine’s The Timex Sinclair 2068](http://localhost/book/the-timex-sinclair-2068-what-can-you-do-with-it/).

***

## Program Analysis

### Program Structure

The program is a short interactive loop with a title display, input, validation, and a single output line before cycling back. There is no initialization beyond the title print at line `10`.

| Line | Purpose |
| --- | --- |
| `10` | Print title banner (uses literal embedded quotes) |
| `20` | Prompt user for a two-character string |
| `30` | Stop if input is a space (CHR$ 32) |
| `40` | Validate input length; re-prompt if not exactly 2 |
| `50` | Demonstrate OVER 1 overlay and print labeled result |
| `60` | Loop back to prompt |

### The OVER 1 Demonstration Technique

Line `50` is the heart of the program. It performs the following sequence in a single `PRINT` statement:

1. Print `q$(1)` — the first character, advancing the cursor one cell right.
2. Print `CHR$ 8` — move the cursor one cell left, back over the printed character.
3. Activate `OVER 1` — switch to XOR pixel-compositing mode.
4. Print `q$(2)` — the second character is XOR-overlaid onto the first in the same cell.
5. Print the remainder of the line as a plain-text label showing the two source characters.

`CHR$ 8` is used instead of `TAB` or `AT` so the overlay demo fits naturally within a streaming `PRINT` statement without needing to know the current cursor column. `OVER 1` remains in effect only for the immediately following item (`q$(2)`); subsequent items in the same statement revert to normal because no explicit `OVER 0` is needed — the attribute is item-scoped within a `PRINT` statement.

### Input Validation

Two guard conditions protect the main display line. Line `30` treats a lone space as a sentinel exit value via `CHR$ 32`, which avoids the user needing to type a command to quit. Line `40` rejects any input whose length differs from 2, looping back to the prompt with `GO TO 20`. Together, these ensure `q$(1)` and `q$(2)` are always safe to subscript at line `50`.

### Notable BASIC Idioms

- Embedded double-quotes in the title string at line `10` are represented by doubled `""` pairs inside the string literal, which is standard Sinclair BASIC string quoting.
- The `INPUT` prompt at line `20` uses the inline string form, combining the prompt and the variable assignment in one statement.
- The program relies on the fact that `OVER` (and other print attributes like `INK`, `PAPER`) placed inline in a `PRINT` statement act as temporary modifiers scoped to the next printable item only.

### Bugs and Anomalies

There are no functional bugs. One minor behavioral note: if the user enters exactly one character followed by pressing Enter, the `LEN` check at line `40` correctly rejects it. However, if `INPUT` is given an empty string (just Enter), `q$` will be empty, the `CHR$ 32` check will fail, and the `LEN` check will also catch it, so the loop still handles this gracefully.

## Source Code

```
   10 PRINT "EXPERIMENTS WITH "" OVER """''
   20 INPUT "Enter any 2 characters (or space to stop) ";q$
   30 IF q$=CHR$ 32 THEN STOP 
   40 IF LEN q$<>2 THEN GO TO 20
   50 PRINT q$(1);CHR$ 8; OVER 1;q$(2);" = ";q$(1);" OVER ";q$(2)
   60 GO TO 20
 9998 SAVE "OVERS 1" LINE 0
```