Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!petrus!hammond
From: hammond@petrus.UUCP
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: 24 bit vs. 32 bit address space
Message-ID: <336@petrus.UUCP>
Date: Thu, 25-Apr-85 08:43:15 EST
Article-I.D.: petrus.336
Posted: Thu Apr 25 08:43:15 1985
Date-Received: Fri, 26-Apr-85 07:11:48 EST
References: <983@watdcsu.UUCP> <2385@nsc.UUCP> <730@amdcad.UUCP> <523@inset.UUCP> <5351@utzoo.UUCP> <5522@utzoo.UUCP> <854@peora.UUCP>
Organization: Bell Communications Research, Inc
Lines: 37
Xref: watmath net.arch:1117 net.micro.16k:325 net.micro.68k:732

> 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.

J. Eric Roskos comments:
> 
> Why?  If you make pipes do this, then what you have is equivalent to a
> shared memory segment. ...

No, pipes have synchronization, which you don't get with shared memory alone.
Pipes also have a "stream" semantics which shared memory doesn't have without
extra user level code.
> 
> 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; ...

Wait, memory-mapped I/O is not the same as viewing a device as a segment.
Mapping a disk into an address space is OK, but I am planning on getting
3 Gigabyte optical disks this year, that's 1.5 x 2**31.  If I want to copy
from one to another by mapping them both into memory I need 33 bits
of address space!!!!  Does that mean we go to 64 bit address spaces?
And what does that do the average program which doesn't need more than
about 16 bit pointers?  

I submit that the stream view of the world has the useful feature that it
maps onto any size of real objects, while even 32 bit addresses will soon
have objects that can't be mapped entirely into the address space and thus
will require 2 (TWO) views of the world, depending on the size of the real
object.

Rich Hammond	{ucbvax, decvax, ihnp4, allegra} !bellcore!hammond