Date   : Fri, 18 Mar 2011 23:19:40 +0100
From   : kortink@... (John Kortink)
Subject: How much sideways ROM is possible ?

On Fri, 18 Mar 2011 12:23:03 +0000, "J.G.Harston" <jgh@...>
wrote:

>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.

If the OS is patched, the arrays can easily be moved, or split
into two parts (i.e. leaving the existing 16 entries where they
are, and moving the rest elsewhere), so none of those problems
would occur. All ROMs that use the arrays directly and use just
their own entry can be accomodated in the first 16 slots.

It only gets tricky when you want to keep supporting ROMs which
use the arrays directly, and access other ROM's entries (and
don't self-limit themselves naturally to the 16 usual ROM
numbers). But very few do anyway.


John Kortink

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