The Complete Spectrum ROM Disassembly
Sinclair's 16 KB, printed and annotated line by line
Ian Logan and Frank O'Hara's 1983 Melbourne House book listed every byte of the ZX Spectrum ROM with commentary — turning the machine's most important 16 KB from a black box into a documented library. It was the fourth book in a run that started on the ZX80, and it is still the reference forty years on.
The Complete Spectrum ROM Disassembly, by Ian Logan and Frank O’Hara, was published by Melbourne House in 1983 at £9.95. It does exactly what the title says: it prints the whole of the ZX Spectrum’s 16 KB ROM as annotated Z80 assembly, routine by routine, with commentary explaining what each one is for and how to call it.
That is a strange thing to publish, and it is worth pausing on how strange. The ROM was Sinclair’s code. Nobody outside Sinclair had a source listing. Logan and O’Hara reconstructed one from the bytes, gave every routine a name, worked out the calling conventions and the register usage, and sold the result in bookshops to teenagers.
The effect was to convert the Spectrum from a machine you programmed around into a machine you programmed with. Everything the ROM already knew how to do — printing a character, scrolling the display, the whole floating-point calculator — became a subroutine you could call from your own code, at a documented address, with documented arguments. On a 48K machine, not rewriting what was already in ROM was not a nicety. It was the difference between a game fitting and not.
Fast facts
- Authors: Ian Logan and Frank O’Hara
- Publisher: Melbourne House — Richmond, Surrey and South Melbourne, Victoria
- Published: 1983
- Price: £9.95
- Covers: the entire 16 KB ROM at
$0000-$3FFF
The three books before it
The Spectrum volume is usually described as though it appeared from nowhere. It was the fourth in a sequence, and the sequence is the interesting part:
| Book | Author | Price | Coverage |
|---|---|---|---|
| Understanding Your ZX81 ROM (1981) | Logan | £8.95 | The ZX81 monitor, plus calling ROM routines from BASIC |
| ZX81 ROM Disassembly Part A | Logan | £7.00 | 0000H-0F54H — everything but the floating-point calculator |
| ZX81 ROM Disassembly Part B | Logan and O’Hara | £8.00 | 0F55H-1DFFH, including the floating-point calculator |
| The Complete Spectrum ROM Disassembly (1983) | Logan and O’Hara | £9.95 | The whole 16 KB |
Two things follow. First, the Logan-O’Hara partnership formed on the ZX81, specifically on Part B — the floating-point calculator was the hard part, and it was the part that needed a second mathematician. Second, the format itself — a commercial, line-by-line, annotated disassembly of an entire ROM, sold at book-shop prices to hobbyists — was worked out across two smaller machines before anyone tried it on the Spectrum. By September 1982 Melbourne House was already advertising the forthcoming Spectrum volume as “sure to become the definitive book on the Spectrum”.
How it was received, and for how long
The book stayed the reference for the rest of the machine’s commercial life and well past it. CRASH was still routing readers to it in July 1988 — five years after publication — as “a classic when it came out in 1983”, and did so in the ordinary course of answering a technical question, which is the better compliment.
Its reach was not only British. MC-microcomputer reviewed it in Italy in December 1983. In 1988 a contributor to the Brazilian magazine Micro Sistemas, publishing tape-loader routines, pointed readers at the book and credited it with everything he knew about the Spectrum’s ROM. A book about one British machine’s 16 KB was, five years and one hemisphere later, still the thing you were told to read.
Why it matters here
Three reasons, in ascending order of importance.
It is a primary source for Spectrum work, and one that can be checked. Where this project cites a ROM entry point, the disassembly is where that address is accountable to.
It is the reason the Spectrum is teachable as a system rather than as a set of tricks. A machine whose ROM is documented can be explained; a machine whose ROM is folklore can only be imitated.
And it is the clearest demonstration in the British 8-bit era that documentation is a thing someone has to choose to do. Sinclair did not publish this. Two people outside the company decided the machine deserved it, spent years on it, and a publisher decided it would sell. None of that was inevitable, and platforms where it did not happen look different in retrospect.
See also
- ZX Spectrum ROM disassembly — the routines that matter most for games.
- Ian Logan — who also ended up writing part of Sinclair’s shadow ROM.
- Melbourne House — the publisher, and the four-book run.
- Machine code for beginners — the tradition this sits at the far end of.
- Sinclair ZX Spectrum