Download BBC Microcomputer Advanced User Guide

15 Paged ROMs
Paged ROMs are ROMs that fit in the four rightmost ROM
sockets on the BBC microcomputer circuit board. They all have
the following features in common:
They exist in the address space &8000 to &BFFF; thus only one
can be active at a time, the rest being ‘paged’ out.
They are scanned by the operating system under certain
circumstances, usually associated with an occurrence which is
not understood by the operating system, eg. inexplicable
interrupts, unrecognised operating system commands.
The operating system has the software to handle up to 16 paged
ROMs, although the hardware can only handle four.
A paged ROM is used in the following applications:
Filing systems, such as disc operating systems, or the Econet
system.
Languages, such as BASIC, FORTH, BCPL, etc.
Utilities, such as word processors, debugging aids, file utilities
etc.
Hardware drivers, for personalised hardware on the 1MHz bus,
or the lightpen.
A paged ROM must be recognised by the operating system. To
be recognised the first few bytes must conform to:
00-02
03-05
06
07
08
09..
vv..
nn-nn+3
JMP language entry
JMP service entry
ROM type
Copyright offset pointer (=nn)
Binary version number
Title string, printed on selection as a
language.
Optional version string, preceded by &00.
&00, &28 ‘(‘, &43 ‘C’, &29 ‘)’
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