Date   : Fri, 18 Mar 2011 12:23:03 +0000
From   : jgh@... (J.G.Harston)
Subject: How much sideways ROM is possible ?

David Hunt wrote:
> A bit of a long shot, but can the OS use all 8 bits of the value of ROMSEL
> or would the OS need patching, that would give you 256 ROMs (4M bytes!) :)

The OS would need hugely patching. You'd need to extend the ROM info
table at &2A1-&2B0, and as everything assumes it starts at &2A1 you'd
have to shift everything at &2B1-&3A0 out of the way, including the
VDU workspace at &300-&37F which everything assumes is as &300-&37F;
and you'd have to extend the ROM workspace table at &DF0-&DFF which
is specified as starting at &DF0, so you'd have to patch absolutely
everything that uses sideways ROM workspace at &E00 to move everything
out of the way.

Also, parts of the OS use a ROM number with bit 7 set to indicate
'absent', and parts of the OS use bit 6 to indicate 'no relocation',
eg see the BASIC ROM number at http://mdfs.net/Docs/Comp/BBC/BASIC/Osbyte187

Assuming you could shift huge amounts of workspace out of the way to extend
the ROM info and ROM workspace tables, it would only be realistically
possible to use ROM numbers 0-63.

-- 
J.G.Harston - jgh@...