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From: tgl@a.sei.cmu.edu (Tom Lane)
Newsgroups: net.micro.mac
Subject: 800k drives faster?
Message-ID: <216@a.sei.cmu.edu>
Date: Tue, 21-Jan-86 12:31:45 EST
Article-I.D.: a.216
Posted: Tue Jan 21 12:31:45 1986
Date-Received: Fri, 24-Jan-86 09:36:08 EST
Organization: Carnegie-Mellon University, CS/RI
Lines: 20

Apparently the new 800k drives are supposed to be twice as fast
as the old drives.  Does anybody know how Apple did this?

I can think of two possibilities: either the disc actually spins
twice as fast, or they have somehow doubled the throughput by
simultaneously reading/writing on both heads.  Neither of these
thoughts is appealing.  The first alternative is likely to break
some copy-protection schemes (anything that's timing-sensitive
could have trouble); the implication is that some copy-protected
programs could not be loaded from an 800k drive.  (Not to mention
purely hardware considerations such as reduced error margins...)
The second alternative would imply some rather tricky buffering
schemes; the speedup would only exist for a few standard disc
access patterns, and there would be NO speedup on old
(single-sided) discs.  And if they buffer writes, there is a
risk of failing to complete the write operation...

Can anyone on the net shed more light on this question?

		tom lane    (ARPA: lane@a.cs.cmu.edu)