Quick survey on equipment

dwight elvey dkelvey at hotmail.com
Mon Apr 23 10:15:46 CDT 2007

>From: Dave McGuire <mcguire at neurotica.com>
>On Apr 22, 2007, at 11:52 AM, dwight elvey wrote:
>>>>BTW Can TRS-80 Model 1's or Memotech's run CP/M (I had an "SDX"   disc 
>>>>pack for my Memotech at one time, but that didn't run CP/M;   rather it 
>>>>had a basic "disc filing system" in ROM)?
>>>   I'm pretty sure the TRS-80 Model I is incapable of running   
>>>unmodified CP/M because it has ROM in low memory that cannot be   
>>>remapped...is this correct?
>>Most any machine can be setup to shadow the ROM. On a different  machine,
>>I wrote a few lines of code that would simply read and then write
>>ROM data. I set it up so that writes would go to RAM that on boot were
>>shadowed behind the ROM.
>>After the ROM had been copied, the code was still running from ROM.
>>I had a 7474 that would switch the enables on the ROM and RAM
>>when it saw the first I/O operation.
>>In your case, you'll either need to write separate I/O code for CP/M
>>or rige some way such that on each I/O action of the CP/M, you
>>switched the ROM back into memory. This should work so long as you  don't
>>expect to buffer the I/O data where to ROM was. Both are relatively
>>simple to do.
>   Uhhh...but what if the address lines aren't handled such that the  lower 
>address space isn't remappable?  It's my impression that this  sort of 
>scheme isn't really all that common.  I mean, sure it's easy  to do (I did 
>it on one of my SBCs) but I don't think many  commercially-produced 
>machines were done that way.
>              -Dave

If you mean that the machine wasn't designed that way and it can't
be done without modifications, then you may be right. If you are willing to 
a few cuts and jumpers, then most any machine that I've ever seen can be
The modifiaction that I described required a few cuts, jumper and
a daughter board.
Even without this, most CPU's are socketed. It is not that hard to trap
out specific addresses and change what is sent to the bus. This can be
doen with no modifications to the system. You seem to be lacking imagination
on what can and can't be done.
I think you need to give the problem a little more thought.

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