Xref: utzoo comp.sys.mac:37981 comp.sys.mac.programmer:8942
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!mtxinu!taniwha!paul
From: paul@taniwha.UUCP (Paul Campbell)
Newsgroups: comp.sys.mac,comp.sys.mac.programmer
Subject: Re: upgrading MacII to a 68030 machine
Message-ID: <412@taniwha.UUCP>
Date: 11 Sep 89 04:40:16 GMT
References: <763@swbatl.UUCP> <950@cbnewsk.ATT.COM> <653@mit-amt.MEDIA.MIT.EDU> <410@taniwha.UUCP> <22032@cup.portal.com>
Reply-To: paul@taniwha.UUCP (Paul Campbell)
Organization: Taniwha Systems Design, Oakland
Lines: 33

In article <22032@cup.portal.com> ts@cup.portal.com (Tim W Smith) writes:
>< Yes it does have VM, but like most other Unixes, even if it didn't have
>< VM (ie it swapped), it would still an MMU to do process address space
>< switching and to protect processes and the kernel from each other.
>
>Since when do processes need protection from the kernel?  Is A/UX that
>unstable or something?  And how does the MMU accomplish this, since
>the kernel can reprogram the MMU?  A kernel that is out to get a process
>ain't gonna be stopped by an MMU!

The kernel can use user mode access to a process (for example when a
user does a read system call) to transfer data. If a user asks for a
write to a read-only part of their address space (code for example)
the system can do one of 2 things - either check every access page by
page - or use the MMU to take a fault (from the kernel) to report the
problem. Of cource this is not really protecting a process from the
kernel, really from itself - I should have put up a smiley with my
original comment, I assumed people would see the humor in it :-)

A/UX is not unstable - this system (taniwha) has run for the past
year almost continuously without any problems, (except when I am doing
driver development :-). 

	Paul




-- 
Paul Campbell    UUCP: ..!mtxinu!taniwha!paul     AppleLink: D3213
"Free Market": n. (colloq.) a primitive fertility goddess worshipped by an
obscure cult in the late 20th C. It's chief priest 'Dow Jones' was eventually
lynched by an enraged populace during an economic downturn (early 21st C).