Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!ucsd!ucsdhub!hp-sdd!hplabs!hp-sde!hpcea!hpausla!cjh
From: cjh@hpausla.HP.COM (Clifford Heath)
Newsgroups: comp.arch
Subject: Re: memory speed & futurology
Message-ID: <2220001@hpausla.HP.COM>
Date: 26 Sep 88 06:57:28 GMT
References: <2179@ditmela.oz>
Organization: HP Australian Software Operation
Lines: 29

>	It would seem that the access time on hard or optical disks is
>	limited by rotational speed in the long run. You can add sectors
>	and tracks to reduce the track to track seeks, but there is that
>	limiting factor. Depending who you believe, it is obvious that
>	the WORM has a higher limit than the magnetic disk, or exactly
>	the opposite.

(Drifting slightly...)

Given the (perhaps almost) unlimited density of optical disks, the
limiting factor on rotational speed is the speed of the encoder/decoder.
If you want to spin a disk with 1 million bits/track at 1000
revs/second, you've got to detect that information at 1Gbit/second.
That's about two orders of magnitude more than is required by current
magnetic technology.  The example given is probably within reach, but it
still gives a rotational latency of .5 ms, which although faster than
current devices it's still not that fast.  To go beyond that you
eventually reach a limit at which electronics isn't fast enough to
detect and serialize the information any more.  Note that the optical
read head is capable of these speeds, although the write head may find
it harder (because of the use of heating effects).  Optics in general
are capable of speeds far in excess of what electronics can handle,
n'est ce pas?
(Blue sky on... I'm looking forward to optical computers, built as
optical chips mounted directly on the fixed head of a super-fast
optical disk.  Pocket super-computers, here we come!)

Clifford Heath, Hewlett Packard Australian Software Operation.
(UUCP: hplabs!hpfcla!hpausla!cjh, ACSnet: cjh@hpausla.oz)