Date   : Mon, 17 Oct 1983 22:07:16 EDT
From   : David Towson (CSD) <towson@amsaa>
Subject: Re: DSDD can it read SSSD disks

Greg - Yes, a double-side double-density DISK DRIVE can read and write
single-side single-density disks.  HOWEVER, there is more to this than
the disk drive alone.  The controller and software must also work with
the single-side single-density format.  As far as the drive itself is
concerned, single-side single-density just uses one head and a different
encoding scheme.  The bit-rate is the same for both single and double
density, but the bits are used "more economically" in double density,
giving an increase in storage capacity by a factor of about 1.8(not
actually 2).  It is the controller that encodes and decodes the data, and
it is also the controller that determines which head is used at any
given time.  Some double-side systems treat each side as a separate disk
having its own separate directory.  Other double-side systems treat both
sides as a single volume having a single directory.  The difference is
determined by the software that does disk I/O.  In summary then,the drive
is able to handle either format, and the single-chip disk controllers
that I know of for double-density can also handle single density(however,
I don't claim to know about all available single-chip controllers), but the
software in a particular system may or may not be able to switch the disk
controller to either format.

                                                    Dave