31 Aug
2019
31 Aug
'19
3:13 a.m.
On Fri, 30 Aug 2019 20:58:13 -0400 Clem Cole <clemc(a)ccc.com> wrote:
Actually not in lock step. They were independent. One was called the
executor and the other the fixer. When a fault was detected the executor
was sent wait stated while the fixer handled the fault and refilled the
TLB. Once the TLB was set to instruction was allowed to complete. Btw
when the 68010 was released the pals on the board were changed to allow the
executor to actually take the fault and do something else while the fixer
replaced the TLB entry
As I remember, the issue with 68000 was that instructions were
not restartable so in case of accessing memory that didn't
exist, you couldn't take a segfault and do anything useful.
This is why you needed a second processor to deal with an
external MMU. There would have been no TLB unless you actually
added an external TLB -- but an external CAM would've been
very expensive. May be a direct map?
What we did at Fortune was to utilize a 4 entry external map:
text, data, extra and stack. When a new function was invoked
it would do a 'probe'. If the probe caused a segfault, stack
was extended in the handler. The probe didn't have to be
restartable. So we didn't need a second 68k. This logic may
have been in the V7 port we started from.