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- From: celeste%express@freedom.msfc.nasa.gov (Celeste)
- Subject: Re: 68000 & Lisa VM (was Re: Motorola's 68060 for the Mac)
- Message-ID: <celeste-270193080343@128.158.16.248>
- Followup-To: comp.sys.mac.hardware
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- Organization: AEGIS
- References: <D2150056.oaq0me@erics.infoserv.com> <77386@apple.apple.COM> <1jv3a7INN49t@boston.crhc.uiuc.edu>
- Date: Wed, 27 Jan 1993 14:03:58 GMT
- Lines: 53
-
- In article <1jv3a7INN49t@boston.crhc.uiuc.edu>, jchandy@crhc.uiuc.edu (John
- Chandy) wrote:
- >
- > In article <77386@apple.apple.COM> north@Apple.COM (Don North) writes:
- > >In article <D2150056.oaq0me@erics.infoserv.com> erics@infoserv.com writes:
- > >>I don't know how this was accomplished. When the 68000 first came out
- > >>it was not designed to support virtual memory. At the company I was
- > >>then working for, we did a lot of looking to see how we might get around
- > >>the limitations that prevented implementing VM. Finally we decided that
- > >>there were some instructions (notably the block move instructions I
- > >>think) for which not enough information was saved on page fault to enable
- > >>them to be restarted. Possibly Apple's compiler never generated these
- > >>instructions? On the systems I was working on, we couldn't guarantee
- > >>that users wouldn't program in assembly language and use these instructions.
- >
- > >The Lisa OS and H/W implemented a segmentation-based VM environment. It
- > >was not a demand-paged system as are most today (ie, Mac VM in Apple's case).
- > >References that might cause a fault - by convention ONLY intersegment
- > >procedure calls - had to use a calling sequence which was KNOWN to be
- > >restartable on a vanilla 68000. Demand paging of data references WAS NOT
- > >supported (ie, would result in a bus error trap to the application). All
- > >this, and a full multitasking OS and GUI on a 5 MHz 68000 in 1 Megabyte!
- >
- >
- > Of course, there were UNIX workstations from Apollo that also
- > supported demand paging on a vanilla 68000. I believe Apollo
- > addressed the problem by using 2 68000's. The second one was only
- > used to load the faulting page into memory and then the other
- > processor could continue. I think the main processor was simply put
- > into a wait state until the page was loaded. Thus no faults were
- > incurred on the main processor.
- >
- > I don't know if HP or Sun used 68000's in their first workstations. I
- > would guess not, since I think they both started with the 020.
- >
- > John Chandy
- > j-chandy@uiuc.edu
-
- The SUN 1's were 68010 based! The orginal Lisa was a 68010!
- I remember an advertisment by Apollo for a 68000 based "workstation"
- for about $10,000.
-
- The "segmentation" method of code virtual memory was the same one as
- in early and current Macs. For you history buffs, go look at the
- PRT, Program Reference Table of the Burroughs 5500. PRT fuctionally
- equlivent to A5 addressing. Both procedure calls and global data
- references are made thought it. The B-5500 did in
- hardware what the Mac and Lisa did in software for segmentation).
- The current VM is simply hardware paging underneth that!
- Can you say "Multix" which had both segmenation and paging!!!!
-
- Celeste
- "A coding pad in one hand and a scope probe in the other."
-