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From: jnw@mcnc.UUCP (John White)
Newsgroups: net.arch
Subject: Re: Page size and the meaning of life
Message-ID: <950@mcnc.mcnc.UUCP>
Date: Thu, 24-Oct-85 22:38:08 EDT
Article-I.D.: mcnc.950
Posted: Thu Oct 24 22:38:08 1985
Date-Received: Sat, 26-Oct-85 04:29:05 EDT
References: <926@decwrl.UUCP> <931@lll-crg.ARpA> <146@opus.UUCP>
Organization: Microelectronics Center of NC; RTP, NC
Lines: 35

> > Would anyone care to comment on why we need virtual memory at all
> > with a 256 meg real memory being available in the near future?
> 
> First response:  How near?  1 Mbit chips are real but not quite big-time
> commercial stuff yet (that means: not CHEAP yet), but suppose that they
> are.  256 Mb = 256*8 = 2K of these chips, which is a fair space-heater in
> any technology.  In larger machines, maybe yes; we're a few years away in
> small machines.

The 1Mb chips will probably use about as much power as todays 256k chips.
Acording to some specs I have lying around, 256k chips take 70ma grinding
and 4.5ma idle. If you have a 32bit processor, you will have 32 chips
grinding and 2048-32=2016 chips idle. This gives 11.3 amps or 57 watts.
I would hate to have to heat my house with that! :-)
Of course, you will need more power than this because of refresh, parity,
bus drivers, etc. I expect 100 watts would do it, though.
As for "how near?", with 4 jram cards you can put 8Mbytes in a PC.
When the 4Mbit chips come out in a couple of years, 128Mbytes will
fit in a PC. This is enough to avoid demand-paging on a single user system
for most aplications.

> ... VM sets the "hard limit" of
> a process address space independently of the actual physical memory on the
> machine, ...

Main memory of a given size will not cost much more than a paging disk of
the same size in a few years. (At least compared to system cost).
Then, there will be no advantage to having a limit set by a disk rather
than by main memory. Main memory is much faster than disk, and complex
demand paging hardware will not be needed.

Of course, if you replace the paging disk with main memory and main memory
with cash ...

-John N. White {jnw@mcnc, jnw@duke}