Re: C64 MMU POC

On 7/11/19 6:58 PM, Mia Magnusson wrote:
> Den Thu, 11 Jul 2019 17:18:00 +0200 skrev Gerrit Heitsch
> <gerrit_at_laosinh.s.bawue.de>:
>> On 7/11/19 2:37 AM, Jim Brain wrote:
>>> On 7/10/2019 1:32 PM, laughton_at_cyg.net wrote:
>>>>
>>>> Sorry to tease everyone -- it was the '816 I was thinking of.  My
>>>> notion was to run it in parallel with the 6510, not to replace
>>>> it.  In other words, the '816 address bus etc would connect to
>>>> nothing, but during reads (code fetches) its data bus would have
>>>> the 6510 data bus copied to it.  Then the oh-so-convenient VPA/VDA
>>>> signals will reveal when the opcode fetches occur.
>>>>
>>> As SMF notes, illegal but non "JAM" opcodes will cause the CPU to
>>> desync.  Maybe using a NMOS 6502 would work for the approach, but
>>> that requires finding a NMOS '02
>>
>> They should be easy enough to find. For example I have a number of
>> 1541 with dead R/W heads. Since that's fatal for use as a disk drive
>> they are now only good for parts. One of them even has a 6502C as
>> CPU, that one should be good for 4 MHz.
> 
> Side track: Are there generally more working 1541 motherboards than
> working drive mechanisms around?

The Newtronics drive mechanics tend to fail with a dead R/W head quite 
often, so I would say yes. And a broken board can be fixed most of the 
time, especially the older ones where there is only one custom chip (the 
gate array), everything else is standard chips you can easily get. The 
very short board with the hybrid might be a different story. If the 
SMD-chip on the 1541-II board dies (I have one of those), it's almost 
impossible to find a replacement of this R/W amp.


> If so, it seems like a good idea to build an interface that is like the
> reverse of the electronics on a Shugart compatible disk drive.

There are people doing other things... Like hooking up an SD-card to the 
digital part of the 1541 logic board with some microcontroller as 
interface. Once it works, the code running on the 1541 (original or 
speeder) should not be able to notice a difference to a real mechanical 
drive.

Like the Gotek, but specialized for the 1541.

  Gerrit