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SubjectRe: 48-bit Drives Incorrectly reporting 255 Heads?
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On Wed, 20 Aug 2003 22:38:24 PDT, jw schultz <jw@pegasys.ws>  said:

> A 240 head drive would have to have multiple heads per
> surface or the stack of disks on the spindle would be about
> 5 feet tall.

That's an old Jedi mind trick, dating back at *least* as far as the IBM 3350
disk drive from 1976 (the 23xx drives from the S/360 and the 3330/3340 series
had single heads per surface) . 8 14" platters, 317.5M. 16 physical surfaces,
but it reported 30 heads, because each arm had an "inside" and "outside" head :

====\=====\ (arm)
=================| (platter)
====/=====/ (arm)

The outside heads were tracks 0-15 and servo-1, the inside heads were 16-29
and servo-2. It was also available with a second fixed-access arm that had enough
heads to read the first 5 cylinders without seeking at all - so that's another 150
heads. So that gets us to 180... not *quite* 240, but....

And it wasn't 5 foot tall, by a long shot: http://www.columbia.edu/acis/history/mss.html
The last picture has guys standing around a string of 3350 disk drives (the
actual drive was contained in the white head, the blue bottom part was power
supplies and control logic and the like).

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