[rescue] [OT] S: IBM /370 or /390 card

[Joshua Boyd]

On 9/12/23 16:37, Dave McGuire via rescue wrote:
> On 9/12/23 16:25, Mouse via rescue wrote:
>>>> Wasn't the XT/370 primarily made with 68000s with custom microcode?
>>>> [...] I'd love to see someone work out how to redo that on a newly
>>>> made board.
>>> Since the 68K's control store is in mask ROM on the chip, I think
>>> that would be a challenge.
>>
>> I can't imagine that there would be any difficulty beyond possible lack
>> of documentation in reimplementing a 68k with writable microcode in a
>> modern FPGA.
>
>   How I interpreted Josh's question was the modification of a 68K's 
> microcode.


I was originally thinking modify the microcode in the chip, but making a 
cycle accurate microcode compatible 68K in an FPGA to run altered 
microcode would also be cool.  Extra cool if the FPGA's image was 
derived from a scan of the 68k silicon.  Square the cool if that was 
then run through on one of Google's Open MPW shuttles.

I haven't dug into it too deeply, but I think that when people talk 
about emulating 68000s as part of, say, the MiSTer project, they are 
using a fairly high level emulation of 68000s, not something that is 
executing original microcode. I guess it would be interesting to know 
what is really happening there.  Also how far down the accuracy hole the 
MAME 68k core goes.