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From: ted@loft.tnolan.com (Ted Nolan <tednolan>)
Newsgroups: comp.os.linux.misc,alt.folklore.computers
Subject: Re: Hmmm ... Downloaded Xenix - But It's *41* Floppies Worth
Date: 26 Aug 2025 21:12:05 GMT
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In article <108l56v$77t$2@gal.iecc.com>, John Levine  <johnl@taugh.com> wrote:
>According to Ted Nolan <tednolan> <tednolan>:
>>>That also depends on one's definition of "proper MMU".  The 286 had a 
>>>segmented MMU, but lacked a paged MMU.  Paging was not added until the 
>>>386.  And there are some that define "proper MMU" as "paged MMU".
>>
>>I don't know the MMU details for the 286, but my undestanding (formed
>>at the time) is that it was "proper" in that it could actually protect
>>running programs from each other.
>
>It could, but if your programs used more than one segment for code
>or data, the switching was extremely slow and painful.  Since the
>segments were of variable size, that meant operating systems had
>to do free space compaction that paging systems don't need.
>
>>  PC-IX and I presume Xenix worked
>>on the 8088/8086 by having the C compiler emit code which stayed
>>in a segment -- so programs wouldn't interfere with each other
>
>There was 286 Xenix that used multiple segments in protected mode.
>I never used it.
>
>>What the 286 couldn't do was virtual memory, which the 386 could.
>
>Sure it could.  The system could mark segments as nonresident and
>take a fault and swap them in as needed.  I wouldn't call that
>very good virtual memory, but it's definitely virtual memory.
>

Thanks!

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