6800 memory addressing

Chris Elmquist chrise at pobox.com
Sun Aug 7 07:55:35 CDT 2016

sorry for top post-- coming from Android thing.

you definitely do not want RAM at $8000 because it will collide with the I/O block that is decoded there.  If you have mp-b back plane as you stated then i/o occupies $8000 to $9fff and so RAM needs to start at $a000.

The modified mp-m is to make it decode at $a000 or above because the stock circuit decoded on 4k boundaries from $0000 to $7000 only.


On August 7, 2016 1:56:14 AM CDT, Brad H <vintagecomputer at bettercomputing.net> wrote:
>So I have the 6800 almost fully working, however I've hit one snag.  
>I loaded some of the memory diags as a first test, from txt files
>posted online.  The first couple worked ok, but the third kept hitting
>an error immediately on start.  
>I thought little of it and was eager to try a bigger program.  I tried
>Blackjack.  However I noticed when it was loading.. instead of just the
>cursor remaining beside my L until loaded, it began dumping random
>stuff out.
>So that's when I went back to that RAM test.  I noticed it has having
>the same issue with a certain range.  I realized then that my single
>RAM board was only covering from (I think) A000 up to but not including
>So I used the jumpers on my DRC 16k boards to fill in the blanks, so to
>speak.  But I'm stuck on the last one.  The memory check now crashes at
>address 8000.  Because my last MPM board was modified, I'm not sure how
>to get that particular range.  I'm also wondering why I only get from
>A000 to C000 on my other MP-M board.  The 16k boards can be set for
>wide ranges.. say 8000 through I think FFFF (can't look at the docs as
>SWTPC.COM went down tonight).  I'm assuming the smaller RAM boards
>cover less?  With three of the four installed, I can load a program via
>txt file without error but cannot run it.. if I hit G it just freezes
>there.  So I'm not sure if I need to configure a ram board for $8000 up
>to $A000 or if I potentially have another problem.  And because the
>MP-M boards are both modified.. I don't know what they are/were set up
>to do.
>Sent from my Samsung device

Chris Elmquist

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