Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!ut-sally!im4u!oakhill!davet From: davet@oakhill.UUCP (Dave Trissel) Newsgroups: comp.sys.m68k Subject: Re: Virtual Machine (was: Apollo (was: MINIX port to A1000)) Message-ID: <1049@oakhill.UUCP> Date: Thu, 26-Nov-87 09:32:27 EST Article-I.D.: oakhill.1049 Posted: Thu Nov 26 09:32:27 1987 Date-Received: Sun, 29-Nov-87 16:21:53 EST References: <8711050534.AA25885@jade.berkeley.edu> <7862@ism780c.UUCP> <8965@utzoo.UUCP> <1856@bloom-beacon.MIT.EDU> <8046@ism780c.UUCP> Reply-To: davet@oakhill.UUCP (Dave Trissel) Organization: Motorola Inc. Austin, Tx Lines: 36 Keywords: virtual machine, emulation, 68010 In article <8046@ism780c.UUCP> tim@ism780c.UUCP (Tim Smith) writes: >< >The generic solution to this is not to let the user's operating system >< >meddle with the frames. Keep the *real* frame somewhere private. Give >< >him a fake one ... >< There is still a real problem here. If the kernel keeps a copy of the >< real frames, it will have to keep them *forever* ... I think there is an easier way out. Think about the reasons for a bus error frame being created in the first place: 1) As an access to currently paged out memory 2) As an illegal access to unassigned memory For case 1 the frame will shortly be referenced again by a RTE as soon as memory is paged in. Thus a virtual machine operating system storing outstanding frame images in a table would find such entries quickly removed. Case 3, on the other hand, would never receive a de-referencing RTE. But how many case 3's are you going to get? They only occur when a process fatally aborts. So the trick is to have a table large enough to sufficiently handle all frames generated during the worst latency of a page-fault recovery. And the illegal access frames slowly fade away into that great bit-bucket in the sky. A virtual O/S capable of running other operating systems can extend this concept by keeping a unique table for each subordinate O/S. The table would simply be saved and restored as part of the subordinate's state if rebooting of the master O/S was required. -- Dave Trissel - Motorola Semiconductor - Austin, Texas ihnp4!ut-sally!im4u!oakhill!davet