SWTPC 6800 weirdness

Brad H vintagecomputer at bettercomputing.net
Wed Sep 7 10:04:08 CDT 2016

-----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.


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
>> >
>> >
>> >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
>> >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:
>> 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
>> 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
>> 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
>>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?
>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..

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.

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