Date   : Thu, 12 Nov 1992 01:58:30 +1300
From   : Dave Sainty <terros@...>
Subject: My latest....

Phew, I always get a little nervous when I start playing about with the guts of
my
machine, especially when in trial runs it won't even give the startup message!

But I've managed an interesting addition to my model B today. This won't be of
practical use to master users btw...

The Master has an 8k block of memory from &C000 to &DFFF which is paged with
the VDU routines if I'm correct.... and allows roms to claim this memory
leaving main memory free.

I've done the same to my model B.

At least, the hardware is done, I shall have to write some software to
accomodate the new memory, the main aim is to use the master ADFS with my still
non-functional hard drive, thus saving me the bother of finishing my filing
system which was probably more trouble than it was worth.

Because the vdu routines in the model B are a little smaller, I couldn't
include the full 8k, in particular, the interrupt routines are part of that 8k!
I've used an 8k chip though, and in fact have two 4k blocks pageable to teh
same location (&C000). That will not have much practical use, but could be
useful for my own software wanting workspace I guess.... Good for resident
programs that don't collide with other programs!

The whole circuit is only two chips, a 6264 (8k static ram), and a 74F139.
I don't know whether the 139 need be fast (F) but it was only $2 for the chip!
the 6264 I don't know the price, it was part of my original 16k SRAM design,
where I piggybacked two 6264's. I did the same in this project, and it looks
very ugly! :-) The 6264 sits under the OS rom, and the 139 dangles to the side.
It's a bit crammed, so I shall check later for heat problems. But the ram chip
is CMOS, so keeps pretty cool.

The pages are currently switched by the user port, one bit for selecting ram,
one for selecting which bank of ram.

Do people want the full constructional details posted here? Are people
interested in getting roms running master style on their beebs?

It's not very hard to make, it just took me this afternoon and evening, and I
had some problems as I was basically making it on the fly. But you would have
to be reasonably competent with a soldering iron, and not be too nervous about
performing surgery on their machine. And, or course, I take no responsibility
for any damage caused by people making my project.....

Dave.