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Retrocomputing

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    I'm sure there are annotated disassemblies of the Spectrum ROM, as there are such for the ZX81. What did your research and investigation reveal? Commented Jan 2, 2024 at 8:40
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    You wrote "it seems unlikely that the entire extra 8K, fully half the Spectrum ROM, is dedicated entirely to color graphics" then state that you know 1 KB of it's unused. So there's only 7 KB. Meanwhile, have you compared the list of Spectrum keywords to the ZX81's list? Commented Jan 2, 2024 at 9:08
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    There used to be an annotated disassembly published as a book, if I remember correctly from Melbourne House? The book exists, I used to own it :) Commented Jan 2, 2024 at 13:29
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    You say the ZX81 uses the floating-point code path in all cases, but from Reusing code, reverse engineering and collaboration, it states that, building on the ZX80 BASIC, the ZX81 introduced For the floating point routines, Vickers used a format almost identical to the 40-bit version of MBF. But he added the ability to store integer numbers in the range -65535 to 65535. The user doesn’t have to specify variable types. And it’s faster than BASICs that only store numbers as floating-point, such as Commodore’s. Commented Jan 4, 2024 at 1:21
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    @ABMK - The Complete Spectrum ROM Disassembly, by Logan, Ian; O'Hara, Frank. Published by Melbourne House, 1983. ISBN 10: 0861611160 / ISBN 13: 9780861611164 Commented Jan 4, 2024 at 3:30