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From: nparker@cie-2.uoregon.edu (Neil Parker)
Newsgroups: comp.sys.apple2.programmer
Subject: Re: comp.sys.apple2
Date: 8 Nov 1996 08:45:32 GMT
Organization: University of Oregon Campus Information Exchange
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Message-ID: <55urvc$pbt@pith.uoregon.edu>
References: <55shd4$23l@elna.ethz.ch>
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In article <55shd4$23l@elna.ethz.ch> GUDATH@EZINFO.VMSMAIL.ETHZ.CH (Henrik
'Ratte' Gudat) writes:
>The 3.5" drive has two heads/sides. There're functions to select the upper or
>lower head. That's fine. However, I'm not sure about which side is which. Sides
>are usually referred to as "side 1" (=0) and "side 2" (=1). Is side 1 read by
>the upper head or the lower head?
>
>I thought side 1 was the upper head and side 2 the lower one. Then I was
>looking at Neil Parker's great documentation of the IWM and it looks like I was
>mistaken.
>
>Any suggestions?

Most of the information in that documentation was gathered from _Inside_
_Macintosh_ and the _Macintosh_Family_Hardware_Reference_.  If these sources
can be believed (which isn't guaranteed--I've found some major errors in
_Inside_Macintosh_), then side 1 is the lower side, and side 2 is the upper
side.

This is consistant with what I've read about the workings of other disk
drives.  From what I've seen, it appears to be traditional for single-sided
drives to use the underside of the disk.  Thus it makes sense that when
somebody first realized that disks have two sides, they put the second head
on the upper side.


If you really want to find out for sure, nothing beats experimentation with
and actual disk in an actual drive.  Using the track-dump code from my 3.5-
inch drive documentation, read a track from a disk, and look at the side
number in the sector address fields.  The side number is the third byte after
the address field marker ("D5 AA 96")--the values 96 and 97 indicate side 1,
and D6 and D7 indicate side 2.

              - Neil Parker
--
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker

     "I was going to be a neo-deconstructivist but Mom wouldn't let me."
                                     -- Calvin and Hobbes, 7/13/1995