Path: utzoo!attcan!uunet!lll-winken!ames!mailrus!csd4.milw.wisc.edu!nic.MR.NET!umn-cs!berlin!grg
From: grg@berlin.acss.umn.edu (George Gonzalez)
Newsgroups: comp.sys.mac
Subject: Re: Virtual memory init
Summary: Isn't it just paging?
Message-ID: <288@berlin.acss.umn.edu>
Date: 25 Jan 89 13:51:29 GMT
References: <1542@csuna.UUCP> <76000334@p.cs.uiuc.edu> <13871@cup.portal.com>
Organization: U of M MicroGroup, Minneapolis
Lines: 17


I don't understand all the amazement about this virtual memory INIT.
It would be amazing if it did full-blown virtual memory, with variable
sized segments, rings of protection, gates, access levels, et. al.

But if it is to be transparent to the current Mac OS and applications, it
can't be that fancy.  It sounds like it's just doing *paging* of fixed
sized blocks between RAM and the disk.  This is less than trivial, but 
perhaps only 20% of the complexity of full virtual memory.  Not all that
difficult to do.

It also does not sound that computer-sciency.  It probably involved more
trial-and-error experimenting than pure computer science.  I'm sure there
was a lot of cut-and-try, as the Mac OS's internals are not too well
documented.  As others have mentioned, there are some tricky areas if
a page fault occurs during a disk operation!  They probably attacked the
problem by patching the Mac I/O system, not by referring to Knuth.