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From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: 24 bit vs. 32 bit address space
Message-ID: <854@peora.UUCP>
Date: Wed, 24-Apr-85 08:17:47 EST
Article-I.D.: peora.854
Posted: Wed Apr 24 08:17:47 1985
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henry@utzoo.UUCP (Henry Spencer at U of Toronto Zoology), writing about
mapping files into the address space, says:

> The point is, you can map files into a process's address space on
> a sufficiently-cooperative machine (I doubt that the 80*86
> qualifies), but it is very hard to make pipes and devices act
> like files if you do this.

Why?  If you make pipes do this, then what you have is equivalent to a
shared memory segment.  The major distinction between pipes and shared
memory in this regard is that pipes are unidirectional and force sequential
access to the data (this is, in fact, a problem with a lot of the current
interprocess communication schemes).  If you map the pipe into the address
space on the output side of one process, and into the address space on the
input side of another process, then you have a shared memory segment
(except that you might make it write-only on the output side and read-only
on the input side in order to make it more pipe-like, to restrict the flow
of data to one direction).

Making devices be mapped into a process's address space actually would solve
many problems.  It is already done for some sequential devices, such as
serial ports, on "memory-mapped I/O" machines.  Making a disk be addressable
as part of the address space would simplify access to it; and that is
essentially how the secondary storage in demand-paging type systems works
(which is probably where the idea came from originally, as a generalization
of that approach).  And of course, primary memory is a random access device
that is almost always mapped into the address space (except on micromachines
such as PE's 3200 series, where you address it in much the way you address
a disk, by writing the address you want to access, then waiting for the
data to become available).
-- 
Full-Name:  J. Eric Roskos
UUCP:       ..!{decvax,ucbvax,ihnp4}!vax135!petsd!peora!jer
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