---
title: "Corrected Spectrum ROM"
id: 50504
type: "computer_media"
slug: "2068-spectrum"
url: "http://localhost/computer_media/2068-spectrum/"
markdown_url: "http://localhost/computer_media/2068-spectrum.md"
published_at: "2023-06-18T15:23:03+00:00"
modified_at: "2026-03-31T14:59:46+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2019/02/20230809-041332.jpg"
excerpt: "Fixes a documented bug in the original Spectrum ROM, making the NMI reset mechanism actually work."
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: "Emulation"
    slug: "emulation"
    taxonomy: "genre"
    url: "http://localhost/type/emulation/"
  - name: "ROM"
    slug: "rom"
    taxonomy: "genre"
    url: "http://localhost/type/rom/"
media_type: "ROM"
download_url: "https://archive.org/download/timex-sinclair-software-archive/2068%20Spectrum.zip"
related_products:
  - id: 50656
    title: "2068 Spectrum ROM"
    type: "product"
    url: "http://localhost/product/2068-spectrum/"
media_type_tags: "Emulation, ROM"
---

This ROM corrects a well-known bug in the Spectrum’s NMI handler — one so notorious that the ROM disassembly flags it with an inline comment: “Note: This should have been ‘JR Z’!”

The NMI routine (at `0066h`) is designed to allow a controlled system reset: if the system variable `NMIADD` holds a non-zero value when the NMI line fires, the CPU is supposed to jump to that address and reset. But in the original Spectrum ROM, a single wrong opcode — `JR NZ` (`20h`) instead of `JR Z` (`28h`) at `006Dh` — inverts the logic entirely, making the reset trigger when it shouldn’t and suppressing it when it should fire. The mechanism is effectively dead in every unmodified Spectrum.

This ROM replaces `20h` with `28h`, applying the canonical one-byte correction. With `JR Z` in place, the routine behaves as intended: `NMIADD = 0000h` safely bypasses the reset, while any non-zero value causes the jump — giving NMI-capable hardware a working reset hook for the first time.