External diagnostic ROM for the ZX Spectrum computer, equipped with an EPROM 27C512 chip with a capacity of 64 kB (4 × 16 kB).
It allows running four different ROM versions on the computer instead of the originally built-in one. It is designed for service diagnostics of ZX Spectrum computers: memory, ULA chip, keyboard, EAR/MIC, video, and sound. It also enables running alternative versions of ZX BASIC ROM, improved compared to the original 1982 version.
ROM Programs for ZX Spectrum
The ZX Spectrum computer is equipped with firmware stored in a ROM of only 16 kB. In such a small space, the manufacturer managed to fit a complete BASIC interpreter, a font, several useful library functions, and still leave some free space.
The original 1982 ROM, installed in every ZX Spectrum computer, was not intentionally modified or improved by the manufacturer. Even though the manufacturer was aware of some significant bugs, they never decided to fix them.
Currently, several improved and extended versions of the original software have been created, along with several diagnostic programs that can be placed in ROM and run immediately after system startup.
An extensive library of original and modified versions of ROM programs for ZX Spectrum can be found at this address.
Below, I have selected some of the most popular ROM programs available online. Let’s take a look at them…
1). ZX Spectrum Diagnostics (2018)
Probably the most popular diagnostic program, written by Brendan Alford and Dylan Smith. Published on GitHub. The user manual, source code, and compiled machine code are available.
2). Retroleum DiagROM (2022)
A diagnostic program written by Phil Ruston (retroleum.co.uk). This is a ROM image that the author originally created to help diagnose serious ZX Spectrum faults (damaged ROM, faulty memory, stuck keys, etc.). The program comes with its own SMART card, but it can be written to any compatible ROM replacement system. The program was published on this page. An online manual and binary files are available.
3). Dr Ian Logan Test ROM (1983)
Probably the oldest diagnostic program for the ZX Spectrum computer. Originally available exclusively to official Sinclair service technicians. Interestingly, the machine code for this program occupies only 8192 bytes!
4). Gosh Wonderful ROM (2004–2017)
Probably the most popular modification of the original ZX Spectrum ROM. Written by Geoff Wearmouth. A highly valued replacement for the original firmware.
The most noticeable change is the so-called “tokenizer”, which is the ability to type BASIC code character by character. BASIC extensions were also added, such as STREAMS, DELETE line commands, RENUMBER line commands, and some bugs in the original ROM were fixed, e.g., NMI handling.
Originally published on Geoff’s website. It can currently be found, among other places, here.