Ian Logan
The doctor who disassembled Sinclair's ROMs, then wrote part of one
Ian Logan was the first person to disassemble the ZX80 monitor, and went on to document the ZX81 and ZX Spectrum ROMs across four Melbourne House books — culminating in The Complete Spectrum ROM Disassembly with Frank O'Hara. He then crossed the line from outside to inside, writing part of the ZX Interface 1 shadow ROM for Sinclair.
Dr Ian Logan did the work that made Sinclair’s machines legible. Between 1981 and 1984 he took three undocumented ROMs apart and published the results, and in the middle of that he was hired to write part of a fourth.
He was, as Melbourne House’s own advertisements put it in 1981, “the first person to disassemble the Sinclair ZX80 Monitor”, and co-author of the ZX80 Companion. The period press credits him as Dr Logan throughout, and treats him as an outside authority rather than an industry figure: a reader who took a machine apart thoroughly enough that professionals started citing him.
What he published
Almost all of it through Melbourne House:
- Understanding Your ZX81 ROM (1981, £8.95) — the ZX81 monitor explained, with a section on calling ROM routines from your own BASIC.
- ZX81 ROM Disassembly Part A (£7.00) —
0000Hto0F54H. - ZX81 ROM Disassembly Part B (£8.00, with Frank O’Hara) —
0F55Hto1DFFH, and the floating-point calculator that made a second author necessary. - The Complete Spectrum ROM Disassembly (1983, £9.95, with O’Hara) — the whole 16 KB, and the book he is remembered for.
- Understanding Your Spectrum — which CRASH rated, alongside the disassembly, as “by far the best I’ve seen” on the subject.
- Spectrum Microdrive Book — the best technical account of the Microdrive and Interface 1 there was, and CRASH’s reviewer said so while complaining about the title.
For the 1981 ZX81 work he was given what the advertisements called the Rosetta Stone Award, described there as going to the best independent product, software package or application for the ZX80 or ZX81. ⚠ The award is attested here only by Melbourne House’s own advertising copy, which is not a neutral source; the work is better evidenced than the prize.
Then Sinclair hired him
This is the turn that makes the career worth an entry rather than a footnote. Logan spent two years reverse-engineering Sinclair’s code from the outside. He then wrote part of the ZX Interface 1 shadow ROM — Sinclair’s own firmware, the 8 KB that appears in the address space when a Microdrive command is executed.
CRASH recorded it in November 1985, reviewing the Spectrum Microdrive Book: Logan “actually wrote a part of the shadow ROM”, which is why the book knows so much about Interface 1 — and, the same paragraph notes, why it does not say more still, because his contract with Sinclair prevented it.
There is a neat irony in that. The man who made his name publishing the contents of Sinclair ROMs ended up under an agreement stopping him publishing the contents of one.
The Jupiter Ace, sideways
A smaller story, from the Jupiter Ace User newsletter of December 1982, and a good illustration of method: Logan obtained a listing of the Jupiter Ace ROM by cross-loading it from the Ace to a Spectrum, the two machines’ cassette standards being identical. Faced with a ROM he could not read, he found a machine that could read it and a wire between them. That is the whole disposition in one anecdote.
Why he matters here
Documentation is not a by-product of a platform. It is something particular people decide to do, at length, usually unpaid at the start, and platforms where nobody does it end up harder to learn on for the next forty years. The Spectrum’s reputation as a machine you can understand, rather than only operate, rests substantially on one reader deciding that 16 KB of somebody else’s machine code was worth several years and four books.