External ROM (dev. #A1)

External ROM (dev. #A1)

External diagnostic ROM for the ZX Spectrum computer, with the EPROM 27C512 chip with a capacity of 64 kB (4 x 16 kB).

Allows to boot four different computer ROMs instead of the originally built-in one. Designed for service diagnostics of ZX Spectrum computers: memory, ULA, keyboard, EAR/MIC, image and sound. It also allows to run ZX BASIC alternative ROMs, improved from the original 1982 version.

ZX Spectrum ROM programs

The ZX Spectrum computer comes with a firmware program installed in ROM, which is only 16 kB in size. In such a small volume, the manufacturer managed to put a complete BASIC interpreter, a typeface, some useful library functions, and left some free space.

The original 1982 ROM installed in every ZX Spectrum computer, producer has not been intentionally modified or patched. Even despite the fact that the manufacturer knew some important errors – he never decided to correct them.

Today, several corrected and supplemented versions of the original software have been created, as well as several diagnostic programs can be placed in the ROM memory, and run immediately after the system starts.

At this address you can view extensive library of the original and modified versions of the ZX Spectrum ROM programs.

Below I have selected few, most popular ROM programs published on the Internet. Let’s look at them…

1). ZX Spectrum Diagnostics (2018)
Propably most popular diagnostic program writen by Brendan Alford and Dylan Smith. Published on GitHub. There are: user manual, source codes, and compiled machine code.

2). Retroleum DiagROM (2022)
Diagnostic program, writen by Phil Ruston (retroleum.co.uk). This is a ROM  image, Phil originally made to help him diagnose faults I had in a Spectrum which had failed massively (dead ROM, bad memory, stuck keys etc). It is supplied with his own SMART Card, but can be written to any suitable ROM replacement system. Program has been published at this website. There are online manual, and binaries.

3). Dr Ian Logan Test ROM (1983)
Probably the oldest diagnostic program for the ZX Spectrum computer. Primary available for official Sinclair Repair Engineers. Interestingly, its machine code of this program is only 8192 bytes long!

4). Gosh Wonderful ROM (2004-2017)
Probably the most popular modification of the original ZX Spectrum ROM. Writen by Geoff Wearmouth. A widely liked replacement for the original firmware.

The most noticeable change is the so-called „tokenizer”. This is the ability to type BASIC code character-by-character. There are also BASIC extensions like STREAMS, lines DELETE, lines RENUMBER, and improved some original ROM bugs, such as NMI.

Primary published on Geoff’s website. Currently it can be found e.g. here.