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Path: utzoo!linus!decvax!tektronix!reed!bart
From: bart@reed.UUCP (Bart Massey)
Newsgroups: net.micro.mac
Subject: Re: 800k drives faster?
Message-ID: <2355@reed.UUCP>
Date: Fri, 24-Jan-86 02:54:47 EST
Article-I.D.: reed.2355
Posted: Fri Jan 24 02:54:47 1986
Date-Received: Thu, 6-Feb-86 04:51:46 EST
References: <216@a.sei.cmu.edu>
Organization: Reed College, Portland, Oregon
Lines: 31

> 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. 
> ...
> 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)

     I can't speak for the Mac directly, but on every other machine
I've ever seen with dual heads, the sectors are on alternate sides, so
that 0 and 1 are on opposite sides of the disk, then 2 and 3, then 4
and 5, etc.  On the Mac floppy, I believe a logical block (the
smallest unit readable by user-level software) is two sectors, so the
speedup would occur for all disk access patterns, and buffering would
be straightforward.  If this is the source of the speedup, there would
be none for old disks, but presumably one would only care about
single-sided disks for compatibility reasons, not for day-to-day
operation.  Finally, no write operations should fail regardless of
buffering, as the determination of the "sidedness" of the disk should
occur at mount time... 

					Bart Massey