ZX Computing

Spectrum Shadow ROM Disassembly
By Melbourne House
Spectrum 48K

Published in ZX Computing #21

Spectrum Shadow ROM Disassembly

Spectrum Shadow ROM Disassembly by Gianluca Carri, is written in two parts. The first shows how to extend Sinclair Basic to include various useful commands, random file handling on the microdrive, extended BEEP and EDIT commands. Full details on the implementation are given, along with very clear explanations.

It is possible to add extra commands to the Spectrum with an Interface 1 unit, containing the Shadow ROM, because when an invalid command is found, a search is made in the existing Spectrum ROM. If this command is not present, then the Shadow ROM is searched. If the command is not found again, then it is at this point that we can jump to our own routine, and carry out the extra commands.

There are two types of Shadow ROMs, and this book accommodates both. There might even be another release of the Shadow ROM software, but the author still explains fully what to do if this is the case.

The second part of the book gives a full disassembly of the Shadow ROM in the Interface 1, and how to link up and use these various routines. This is ideal literature for the Z80 programmer wishing to either add commands to Basic, access the microdrives via machine code or see what's in the Shadow ROM.

David Harwood