Loading Acorn (BBC) sideways RAM.

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Thu Sep 16 20:07:49 2004

>From: ard_at_p850ug1.demon.co.uk
>
>This is a really trivial question, but I wasn't really a BBC micro hacker
>in the old days,
>
>I am planning some upgrdes to a BBC Model B+ (the fact that this
>particular B+ lives in the bottom of an Acorn Cambridge shouldn't
>matter...). One of them is to combine some of the ROMs (the B+ can take
>27256 32K byte EPROMs), thus freeing up some ROM sockets and put a RAM
>chip in one of them. I then have to fiddle the write-enable line [1] (should
>be easy) and I have 32K of sideways RAM.
>
>The hardware doens't worry me (I have schematics...). But how do I load a
>ROM image into the sideways RAM? Is there some loader program I need (and
>where do I get it)? Anything I should know about the image file on disk
>(anything about the start/end/entry addreeses in the direxctory, for
>example)?

Hi Tony
 It depends on the form of the image. There are no standards for these
things. One can create wave files to send audio as though there
were a cassette player. I've been involved in creating a number of
methods to bootstrap old computers. Having a built in cassette load
is something that I've not tried. Most cases, I have access to a
monitor program on the target machine and I'm able to enter a small
bootstrapping program.
 In most cases, information about load/end and start address are
missing from many images. One is suppose to know this to use the image.
Often doing some disassembly of the image can recover information
as to where in memory it should exist. Most programs start at the
beginning but not all.
 What kinds of images are we talking about? Are these machine language,
BASIC text or something?
 Of course, I'm not familiar with the term sideways RAM. Do you mean
shadow RAM or just parallel RAM? Could you explain a little?
 I would guess that you may have to write your own loader. At the
simplest, it is rarely more than a few lines of code.

>
>
>[1] I believe it's a good idea to be able to write-protect the sideways
>RAM, but I don't want to drill holes in my Cambridge. Has anoyne ever
>used the cassette motor line for this? Seems like an obvious thing to use.

 This makes sense. Also look for a unused data port bits on I/O chips.
Most of these smaller machines had unused port bits someplace. These
usually had a default value written to them by the machines code
so that you could use the opposite value as a write enable.
Dwight

>
>-tony
>
>
Received on Thu Sep 16 2004 - 20:07:49 BST