Path: utzoo!attcan!uunet!mcvax!unido!tub!tubopal!alderaan
From: alderaan@tubopal.UUCP (Thomas Cervera)
Newsgroups: comp.sys.atari.st
Subject: Re: Multitasking on the ST
Message-ID: <679@opal.tubopal.UUCP>
Date: 13 Aug 89 11:43:01 GMT
References: <8908021826.AA05333@ucbvax.Berkeley.EDU> <15627@watdragon.waterloo.edu> <652@opal.tubopal.UUCP> <471@tw-rnd.SanDiego.NCR.COM> <1066@philmds.UUCP> <675@opal.tubopal.UUCP> <1070@philmds.UUCP>
Reply-To: alderaan@tubopal.UUCP (Thomas Cervera)
Organization: Technical University of Berlin, Germany
Lines: 32

In article <1070@philmds.UUCP> leo@philmds.UUCP (Leo de Wit) writes:
=In article <675@opal.tubopal.UUCP> alderaan@tubopal.UUCP (Thomas Cervera) writes:
=|  But isn't there any software solution to that ? (I'm looking on primitive
=|MMU versions on PDPs where the operating system does a part of context-
=|saving work on failed EMT or TRAP recovery). The LSI11 processors are very
=|much like the M68k family. Or is this really impossible on 680x0 ? (Remember,
=|I'm only a dumb physicist :-) )
=
=The problem is that for instance a page fault can emerge whilst in the
=middle of an instruction. Whereas the 68010, 68020 etc. store much
=information on the stack at a BUSERR (29 words if I'm correct), the
=68000 only stores 7 words, which is not sufficient to resume each type
=of instruction.  For instance:
=
=    movem.l   (a3),a2-a5
=
=Since this instruction modifies the contents of a3, it cannot be resumed
=when a bus error occurs after a3 has been modified (and the instruction
=has not yet completed).

  I'm not that familiar with the 68000's hardware and pin configuration.
But isn't there a pin telling the non-existent MMU 'shut up, I'm working on
an instruction right now !' ?
  Sure, the MMU must be able to hold it's BUSERR signal back until the CPU
drops this line and tries to fetch the next instruction.
  If this is possible, error recovery on BUSERR should not be a problem.

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