Date   : Wed, 24 Jun 2009 17:45:30 +0100
From   : dl.harper@... (David Harper)
Subject: Co-Processor RAM Available to Languages

Ian Wolstenholme wrote:

> Is there a way of making RAM in an internal co-processor available
> to the current language whilst the co-processor is disabled?
>
> A bit like the way CPFS uses the Master 512's RAM as a RAM disc and
> printer buffer, perhaps?
>
> It would be nice to have 16MB of RAM available to the likes of Inter-Word
> and Edit whilst the co-processor is present but NOTUBEd.  These won't
> work with the co-processor active as they aren't ARM code.

This is what I would like to have done when I first wrote CPFS, but I
couldn't think of a sensible way of slotting the extra RAM into the Beeb's
memory map.

It is not obvious how to use even the Master's standard SW-RAM with the
current language, unless the language is specifically written to use it. BBC
Basic and View, for instance, will not use SW-RAM for programs/documents
unless you have somehow tweaked them to do so. Making co-processor memory
available is going to be at least as awkward as that.

I eventually decided that the best way of making the co-processor memory
generally usable was to allow it to be accessed as a RAM disc. (The printer
buffer was a later addition, suggested by Robin Burton once the RAM disc was
working.)

No doubt a similar technique could be used to access the memory of any
co-processor. I had originally had it in mind to create a system that would
work whatever co-processor was attached, but working with the M512 there
were a lot of fiddly things to get round, and CPFS finished up very specific
to the 80186.

David Harper