---
title: "Experiment with OVER"
id: 55162
type: "computer_media"
slug: "experiment-with-over"
url: "http://localhost/computer_media/experiment-with-over/"
markdown_url: "http://localhost/computer_media/experiment-with-over.md"
published_at: "2024-06-17T01:00:13+00:00"
modified_at: "2026-03-30T21:41:50+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/06/SCR-20240616-sfhu.png"
excerpt: "Type any two characters and watch their pixel patterns merge on screen — a hands-on demonstration of how the OVER attribute combines character bitmaps."
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/"
genre:
  - name: "Demo"
    slug: "demo"
    taxonomy: "genre"
    url: "http://localhost/type/demo/"
  - name: "Graphics"
    slug: "graphics"
    taxonomy: "genre"
    url: "http://localhost/type/graphics/"
media_contents:
  - id: 54965
    title: "ISTUG Public Domain Library 5"
    type: "computer_media"
    url: "http://localhost/computer_media/istug-public-domain-library-5/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/06/SCR-20240616-sfhu.png"
media_type_tags: "Demo, Graphics"
---

# Experiment with OVER

This program demonstrates the OVER 1 print attribute by overlaying two user-supplied characters on screen. The user enters any two characters, and the program prints the first character, steps back one position using CHR$ 8 (the cursor-left control code), then prints the second character with OVER 1 active, producing a combined glyph that is the bitwise OR of both character bitmaps. Input validation checks for exactly two characters and uses a space as the exit sentinel. The SAVE line stores the program with an auto-run directive targeting line 0.

***

## Program Analysis

### Program Structure

The program is a short interactive loop with straightforward flow control:

1. Line `10`: Print a title banner.
2. Line `20`: Prompt the user for a two-character string.
3. Line `30`: Exit on a space character (sentinel value).
4. Line `40`: Validate input length; re-prompt if not exactly two characters.
5. Line `50`: Display the overlay effect and a textual description.
6. Line `60`: Loop back to prompt for more input.

### The OVER 1 Technique

The core of the demonstration is in line `50`. The sequence `q$(1);CHR$ 8; OVER 1;q$(2)` works as follows:

- `q$(1)` is printed at the current cursor position.
- `CHR$ 8` moves the print cursor one position to the left, back over the character just printed.
- `OVER 1` switches on the OVER attribute, which causes subsequent pixels to be XORed (effectively ORed onto a blank cell) with whatever is already on screen.
- `q$(2)` is then printed on top, combining both characters’ bitmaps into a single cell.

The remainder of line `50` prints a plain-text description of the operation (`" = ";q$(1);" OVER ";q$(2)`), which also implicitly resets the OVER attribute back to 0 for normal printing, since each new `PRINT` item using a literal string does not carry the attribute forward across the semicolons without an explicit re-statement.

### Input Validation

Two guard conditions protect the main display line. Line `30` checks whether `q$` equals `CHR$ 32` (space) and halts the program, providing a clean exit without requiring a keyword typed at the prompt. Line `40` uses `LEN q$<>2` to reject any input that is not precisely two characters long, looping back to line `20` if the condition is true.

### Notable Techniques

- **CHR$ 8 cursor control:** Using `CHR$ 8` within a `PRINT` statement to reposition the cursor is a well-established idiom for in-place character manipulation without relying on `AT` coordinates.
- **Space as sentinel:** Using a single space to signal termination is a simple and intuitive exit mechanism that avoids special keyword input.
- **Inline attribute reset:** The OVER attribute set mid-statement is scoped to that print item, so subsequent items in the same `PRINT` statement revert to normal rendering, making explicit reset unnecessary.
- **Title string quoting:** Line `10` uses doubled quotes (`""`) inside the string to display literal quotation marks around the word `OVER`, and the trailing `'''` block graphic characters form a simple visual underline/border decoration.

### Potential Anomalies

If the user enters exactly one space character, line `30` triggers a `STOP` as intended. However, if the user simply presses Enter with no input, `q$` will be an empty string (`LEN q$ = 0`), which is not equal to `CHR$ 32`, so the check on line `40` correctly catches it and re-prompts — no bug here. There is no check to ensure that both characters are printable, so control characters entered as part of a two-character string could produce unexpected screen effects, which is arguably part of the experimental spirit of the program.

### Variable Summary

| Variable | Type | Purpose |
| --- | --- | --- |
| `q$` | String | Holds the two-character input from the user |

## 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
```