---
title: "Prime Numbers"
id: 76573
type: "computer_media"
slug: "prime-numbers"
url: "http://localhost/computer_media/prime-numbers/"
markdown_url: "http://localhost/computer_media/prime-numbers.md"
published_at: "2026-11-11T08:02:24+00:00"
modified_at: "2026-11-11T08:02:24+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2026/11/prime-numbers-1.png"
excerpt: "List every prime number below 1000 on a ZX80, using trial division by the odd numbers up to 31."
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  - name: "Sinclair ZX80"
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indiv:
  - name: "Phyllis Richmond"
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    slug: "phyllis-richmond"
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    url: "http://localhost/indiv/phyllis-richmond/"
download_url: "https://archive.org/download/timex-sinclair-software-archive/Prime%20Numbers%20%281981%29%28Richmond%2C%20Phyllis%29%28ZX80%29%28US%29%28Program%29.zip"
mediadate: "1981"
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  - url: "http://localhost/wp-content/uploads/2026/11/prime-numbers-1.png"
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  - id: 19663
    title: "Prime Numbers"
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    url: "http://localhost/article/prime-numbers/"
media_type_tags: "Mathematics"
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---

# Prime Numbers

Prints the prime numbers from 1 to 999. For each number the program tries the divisors from 3 to 31 (skipping the even ones), and prints the number when the quotient falls to 1 without an even division.

The ZX80 blanks its screen while it computes, so the list appears only when the program finishes; press BREAK to see the primes found so far.

Published in Syntax September 1981 ([Prime Numbers](https://timexsinclair.com/article/prime-numbers/)).

## Source Code

```
  10 PRINT "PRIME NUMBERS"
  20 FOR N=1 TO 999
  30 FOR K=3 TO 31
  40 LET Q=N/K
  50 LET R=N-Q*K
  60 IF Q=1 THEN GO TO 200
  70 IF R=0 THEN GO TO 250
  80 IF N=1 AND K=31 THEN GO TO 400
  90 LET K=K+1
  95 NEXT K
 200 PRINT " ";N;
 250 LET N=N+1
 300 NEXT N
 350 STOP
 400 PRINT "1 2";
 410 LET N=N+1
 420 NEXT N
```