---
title: "Sine Wave"
id: 57275
type: "computer_media"
slug: "sine-wave"
url: "http://localhost/computer_media/sine-wave/"
markdown_url: "http://localhost/computer_media/sine-wave.md"
published_at: "2024-10-04T07:58:26+00:00"
modified_at: "2026-04-03T07:58:38+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/163_Sine.png"
excerpt: "A compact sine curve plotter that precomputes 64 sample points and pauses for a keypress — revealing a neat busy-wait input technique worth studying."
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/2024/10/163_Sine.png"
media_type_tags: "Demo"
---

This program plots a sine curve on the screen using the low-resolution PLOT command. It precomputes 64 sample points of a full sine wave cycle into array C, scaling the values to span a vertical range centred around y=22 with an amplitude of 20 pixels. After plotting, it calls a subroutine at line 1000 that busy-loops until a key is pressed before returning, effectively pausing execution twice. Six blank PRINT statements then scroll the display downward.

***

## Program Analysis

### Program Structure

The program is divided into three logical phases:

1. **Initialisation (lines 10–40):** Allocates a 64-element array `C` and fills it with precomputed sine values.
2. **Plotting (lines 100–145):** Iterates over the array, PLOTs each point, waits for two keypresses via subroutine, then scrolls the display with blank PRINTs.
3. **Keypress wait subroutine (lines 1000–1010):** Busy-loops on `INKEY$` until any key is held down, then RETURNs.

### Sine Wave Calculation

Line 30 computes each sample as:

`C(V) = 22 + 20 * SIN((V-1)/32 * PI)`

With `V` running from 1 to 64, the argument `(V-1)/32 * PI` sweeps from 0 to just under 2π, producing one complete cycle. The vertical centre is at pixel row 22 and the amplitude is ±20 pixels, giving a total excursion of 40 pixels. Precomputing into array `C` avoids recalculating the sine for each plot, which is a sensible optimisation given the expense of trigonometric evaluation.

### Plotting Loop

Lines 100–115 step through all 64 array entries and call `PLOT G-1, C(G)`, offsetting `G` by 1 so x coordinates run 0–63. The curve therefore occupies the left quarter of the 256-pixel-wide display. No `DRAW` is used between points, so the curve is rendered as discrete dots rather than a connected line; at 64 samples this is generally dense enough to appear continuous.

### Keypress Wait Subroutine

The subroutine at lines 1000–1010 implements a busy-wait loop:

- Line 1000 checks `INKEY$`; if it is not empty (a key is pressed), it RETURNs immediately.
- Line 1010 loops back to 1000, spinning until a keypress is detected.

This subroutine is called twice in succession (lines 117 and 125), requiring two separate keypresses before execution continues. This is a common idiom for requiring a deliberate key-press-and-release cycle, since a single call might return on the same keypress that triggered any previous action.

### Notable Techniques

- Precomputing into a `DIM` array separates data preparation from rendering, a good structural practice even in short BASIC programs.
- The double `GOSUB 1000` ensures the user must press a key once to acknowledge and once to proceed, guarding against accidental continuation.
- The six blank `PRINT` statements (lines 135–145) scroll any text area output without clearing the graphics area, providing visual separation.

### Anomalies and Dead Code

| Line | Issue |
| --- | --- |
| `2000` | `STOP` is never reached; execution halts within the keypress loop or falls through the blank PRINTs into an implicit end. |
| `2020` | `RUN` after `SAVE` would restart the program, but this line is only reached as part of the save sequence and is not part of the normal runtime flow. |

## Source Code

```
   1 REM "SIN CURV"
  10 DIM C(64)
  20 FOR V=1 TO 64
  30 LET C(V)=22+20*(SIN ((V-1)/32*PI))
  40 NEXT V
 100 FOR G=1 TO 64
 110 PLOT G-1,C(G)
 115 NEXT G
 117 GOSUB 1000
 125 GOSUB 1000
 135 FOR H=1 TO 6
 140 PRINT 
 145 NEXT H
 1000 IF INKEY$<>"" THEN RETURN 
 1010 GOTO 1000
 2000 STOP 
 2010 SAVE "1016%3"
 2020 RUN
```