7 Sep
2016
7 Sep
'16
8:04 a.m.
-----Original Message-----
From: Chris Elmquist [mailto:chrise at pobox.com]
Sent: Wednesday, September 7, 2016 6:13 AM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>;
Brad H <vintagecomputer at bettercomputing.net>; 'General Discussion: On-Topic
and Off-Topic Posts' <cctalk at classiccmp.org>
Subject: RE: SWTPC 6800 weirdness
Both MIKBUG and SWTBUG need RAM at $a000. Originally this was provided by a 6810,
128-byte SRAM on the MP-A CPU board. To run Flex and other stuff that wanted larger stack
and workspace, people modified a 4K MP-M to reside at $a000 (instead of somewhere below
$8000) and then removed the 6810 from the CPU board. This results in 4K of RAM at $a000
instead of 128 bytes but you need one or the other-- but not both and not neither ? else
the monitor won't run and without 4K, Flex won't run either.
Chris
On September 6, 2016 5:01:53 PM CDT, Brad H <vintagecomputer at bettercomputing.net>
wrote:
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
william degnan
Sent: Tuesday, September 6, 2016 2:52 PM
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at classiccmp.org>
Subject: Re: SWTPC 6800 weirdness
quite
card.
like
Thanks Chris. I figured it was something like that with my first MP-M. I am curious
though why simply setting another board to cover 8000-BFFF won't allow the system to
operate in that board's absence. It bothers me that I'm reliant on that heavily
modded unit to be able to operate.
I have another general question for folks out there. I'm trying to understand exactly
how memory addressing works in relation to these boards. For example, if I understand
correctly, the original MP-M had 16 2102 chips for a total of 4K. The memory address
jumper chose which addresses that 4K applied to. So for example, if I set the jumper to
5, the card would occupy $5000 to $5FFF?
My question is, if I am correct that my second MP-M, having 32 RAM chips, has 8K, and
I've set it the jumper to '4', where does the other 4K of RAM go beyond $4FFF?
It doesn't seem to go to $5000. The instructions for upgrading to the MP-MX spec
don't say anything about having to modify the card to span a greater address range, in
fact, the jumpers for board # are identical to the 4K MP-M. So I'm confused..
what's the benefit of going out to 8K if the board can't address more than 4?
I'm sure it's something I'm missing here.
>
>
>Brad,
>You'll need to make electrical measurements, from the system
checkout in
>the manual. You very possibly will have
marginal components that
need to
>be replaced, but it's best to try to
locate which is bad rather that
to
replace at random.
>A000 is not the same place as 0100. In the 64K space, they're distant.
Eliminate all but the one RAM board, setting it to
0000. Test that
thoroughly, then add the next at the next RAM space beyond the
first Continue
until you have enough RAM for a minimal Flex boot. It
should
tell you in the version of Flex you're using how much that is (24K?)
It's hard to do everything at ?>the same
time, break it down into
chunks..
Bill
Thanks Bill. I've tried working with just single RAM boards, but I said, the only one that will work at all is
this modified board. I
have pics of it here:
https://drive.google.com/folderview?id=0B4pq0-BHd2x6WVFiZHdyMHBlNW8&us
p=
sharing
If I could understand better what it is set up to do, what address
spaces its occupying, I might be able to understand why my 16K DRC
boards don't work when I try to put them to $A000. I'd prefer to
work
with one of those boards first since the chips
are socketed, and then
I could test the chips individually and be sure
one whole board is
good.
I note in one of my pics there, the cap on that modified MP-M looks a
little tarnished on the outside...
Getting a memory map of your system is an important step. You need to
know what
memory addresses each board is attempting to use, so that
there is no
overlap. Also remember that the ROM board has RAM
on it too. You
would
not want to map both boards to the same A000
space, but why do you
need this at all? What wants free RAM there?
One important rule is that you don't want to
overlap RAM.
Can you get to a monitor prompt without any RAM
installed other than
that which is in the ROM board?
b
Based on what I've read, you *have* to have A000 if your CPU card has
been modified for Flex 2.0, which I've verified mine has. When mods on
the older MP-A cards are done apparently it disables the onboard RAM,
and that's where A000 would be. I could reverse the mod but I'm not
sure if I want to forgo FLEX use. So yeah, according to SWTPC.com
since that mod was done, I have to have a board at $A000. However,
setting either of my configurable ram boards to that space doesn't
work. The system will only boot with that weird MP-M in. So there's
more to it than that.. probably mods above and beyond.
I suppose it wouldn't be too bad to just reverse the Flex 2.0 mods and
start there. I'm doubtful if I'd ever use it and I could always
reverse again if I do..