---
title: "Bouncing Text"
id: 54970
type: "computer_media"
slug: "input"
url: "http://localhost/computer_media/input/"
markdown_url: "http://localhost/computer_media/input.md"
published_at: "2024-06-10T01:44:45+00:00"
modified_at: "2026-03-30T21:29:16+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2019/02/20230809-041332.jpg"
excerpt: "Type any short phrase and watch it bounce across the screen paired with its inverse-video twin in an endless, scrolling light show."
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/"
media_contents:
  - id: 54964
    title: "ISTUG Public Domain Library 6"
    type: "computer_media"
    url: "http://localhost/computer_media/istug-public-domain-library-6/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/06/SCR-20260330-myhz.png"
media_type_tags: "Demo"
---

# Bouncing Text

This program creates a bouncing-text effect by repeatedly printing a user-supplied string (up to 20 characters) paired with its inverse-video mirror across the screen. The POKE at address 23692 disables the “scroll?” prompt, preventing the display from pausing when it reaches the bottom. Row position `e` bounces between 0 and 20, and column position `d` bounces between 0 and 30, each reversing sign when they exceed their boundary. The combined normal and INVERSE 1 rendering doubles the visual width of the text on each print.

***

## Program Analysis

### Program Structure

The program is organized into three logical phases: initialization (lines 10–40), display (line 50), and position update with wraparound (lines 60–100). The main loop runs from line 50 back to line 50 via `GO TO 50` at line 100, producing a continuous animation with no exit condition.

1. **Lines 10–20:** Set display attributes (black ink, white paper, border 7), clear screen, and collect the user’s string into `a$`.
2. **Line 30:**`POKE 23692,255` sets the scroll counter to its maximum value, suppressing the “scroll?” prompt.
3. **Lines 40–50:** Initialize column (`d`) and row (`e`) to zero, then print the string twice — once normally, once in `INVERSE 1` — at the current screen position.
4. **Lines 60–100:** Increment row and column counters, reverse their sign when they exceed the screen boundaries, and loop back.

### Bouncing Algorithm

The bounce logic uses a sign-flip technique rather than a direction flag. When `e` exceeds 20 it is replaced with `-e`, and when `d` exceeds 30 it becomes `-d`. While this does reverse direction, the approach has a notable anomaly: the counters can take on negative values, which are valid `AT` row/column arguments only so long as they resolve to legal screen coordinates. Negative `AT` coordinates will cause an error (“B Integer out of range”) once the magnitude drifts outside the 0–20 / 0–31 screen bounds, so in practice the bounce only works cleanly during the initial positive sweep.

### Key BASIC Idioms

- `POKE 23692,255` — the standard system-variable poke to defeat the scroll prompt; the value decrements with each line scrolled, so 255 gives the maximum headroom before it must be re-poked.
- `INVERSE 1` inline within a `PRINT` statement — toggles inverse video mid-statement without needing a separate `PRINT` line, producing the two-tone text pair.
- The `INPUT` statement uses the `'` separator to issue a newline before the input cursor, keeping the prompt and entry on separate lines.

### Notable Techniques

Printing at a fixed `AT e,d` position on every iteration means the program does not clear the screen between frames; old text remains visible, giving a trail effect rather than a clean animation. This is intentional for the “light show” aesthetic but means the display fills quickly.

### Bugs and Anomalies

| Line | Issue | Effect |
| --- | --- | --- |
| `70` | `LET e=-e` when `e>20` can produce a negative row such as `-21` | Subsequent `PRINT AT e,d` will raise error “B Integer out of range” once the negated value is used |
| `90` | Same sign-flip issue for column `d` when `d>30` | Same potential out-of-range error on the column axis |
| `30` | Scroll counter poked only once, not inside the loop | After 255 scroll events the “scroll?” prompt will reappear |

## Source Code

```
   10 OVER 0: INK 0: PAPER 7: BORDER 7: CLS 
   20 INPUT "Enter 1 - 20 characters"'a$
   30 POKE 23692,255
   40 LET d=0: LET e=0
   50 PRINT AT e,d;a$; INVERSE 1;a$
   60 LET e=e+1
   70 IF e>20 THEN LET e=-e
   80 LET d=d+1
   90 IF d>30 THEN LET d=-d
  100 GO TO 50
```