NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/sys/3b1/3842

Text File | 1992-11-17 | 1.9 KB | 37 lines

Newsgroups: comp.sys.3b1
Path: sparky!uunet!ukma!darwin.sura.net!zaphod.mps.ohio-state.edu!n8emr!colnet!res
From: [e-mail] (Rob Stampfli)
Subject: Re: Unix system V question
Message-ID: <[e-mail]>
Organization: Little to None
References: <[e-mail]> <[e-mail]>
Date: Tue, 17 Nov 1992 03:03:21 GMT
Lines: 26

>There are 1024 4K pages, thus giving the 4MB virtual memory limit.
>Along with the 12 address bits for each page, there are 4 other protection
>bits.  I believe these are read, write, and execute permission, plus a
>page modified bit.

The MMU is configured into 1024 16 bit words, which map the 1024 pages of
virtual memory to their physical counterparts.  Ten of these 16 bits contain
MA21-MA12, the high order bits of the physical address the MMU translates a
virtual memory reference to.  Another 2 bits (PS0 and PS1) contain state
information about the status of the virtual memory:  whether the page is
(0) unallocated, (1) allocated (valid) but not yet accessed, (2) *has*
been accessed (which should be used to determine which pages are part of
the processes' working set, but which in reality is ignored by the kernel),
or (3) *has* been written-to (dirtied).  In addition, there is a bit which
indicates whether the memory is writeable.  Three bits are unused and
available for future expansion.  They are nominally zero, and I don't know
what the effect of setting them is.  The MMU is located at physcial address
x400000 - x4007FF and consists of high speed static RAM.

Note that there is no read or execute bits.  The kernel can read or execute
from any allocated page.  A user process can do the same except that there
is a hardware prohibition against a user process reading from the first 1/2
Meg of virtual memory.
--
Rob Stampfli  [e-mail]      The neat thing about standards:
614-864-9377  HAM RADIO: [e-mail]  There are so many to choose from.