out-of-mainstream minis

Johnny Billquist bqt at update.uu.se
Sat Jul 4 11:07:10 CDT 2015

On 2015-07-04 17:08, Jules Richardson wrote:
> On 07/03/2015 06:59 PM, Johnny Billquist wrote:
>> I find it hard to believe it was a plain 68K in there. That CPU have some
>> serious issues that makes it close to impossible to implement virtual
>> memory or proper usermode protection.
>> (Yes, it can be done, but the amount of hardware required means most did
>> not. I think SUN did it with their own MMU, and an extra CPU in there.
>> Trying to remember who else - I seem to remember one other company who
>> actually used a plain 68K, and it was not Unisys.)
>> Essentially, the 68010 is pretty much the minimum of processor needed to
>> make a sane system to run Unix on.
> I've got a few Torch boards which use a plain ol' m68k (either 8 or
> 10MHz depending on the variant) and they were capable of running Uniplus+.
> There's very little other logic on the boards, although they do have a
> local Z80 (they were designed to run Torch's CP/M variant too), so I
> suppose it's possible that could be employed somehow to provide the
> necessary support for the m68k.

How hard can it be to understand/believe that the 68000 cannot be used 
without some serious hardware hacking? Not something most companies did.
The problems revolve around the fact that instructions cannot be 
properly restarted on the 68000. Not enough context is saved. This in 
turn means you cannot do demand paging, as that will cause a memory 
exception trap, from which you cannot recover.
(The tricks done by those who did fix this consists of having a second 
processor which gets interrupted when you get a page fault, and the 
second processor do all the work related to the page fault, while the 
primary processor just stalls until the memory is available, at which 
point it can continue. There is no limits to how long the CPU can wait 
for memory to return data on a read.)

Also, there is no MMU from Motorola for the 68000, so you would have to 
design your own.

In addition, there is also a potential privilege escalation problem with 
the 68K if I remember right. You always have full access to the whole 
processor status word in that CPU. I can't remember what the scope of 
that issue is. It might only be information leak, or it might be that 
you can elevate yourself as well.

These problems were all addresses in the 68010.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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