🚀 go-pugleaf

[ Why do Macs have variable-speed drives? ]

The basic reason is because using a variable-speed drive was the most
reliable way of packing a lot of information onto a disk when the
drive was chosen.

With a fixed-speed drive, there are three approaches to putting lots
of data on a disk (well, besides single-sided vs. double-sided):

  (1) Using a more compact encoding scheme.  This is how Apple ][s
      went from 13-sector to 16-sector drives.  The hardware for this
      can get expensive, though, and current systems come pretty close
      to "perfect" as far as this goes (at least, without $$$ decoding
      hardware).

  (2) Putting more sectors on a track--driving the read/write head
      faster.  This physically packs data closer together, at the
      cost of needing a faster R/W head setup.  Also, some magnetic
      media which can support a high density is needed.

  (3) Putting more tracks on the disk--putting them closer together.
      This requires more accuracy in positioning the head, a "smaller"
      head, and high-density media.

Options (2) and (3) require high-density media with very high quality-
control standards, because the bits are actually closer together.

Using a variable-speed drive mechanism means that you can put more
sectors on outer tracks, without packing data closer together on the
inner tracks.  In general, as long as the speed control works, this
will be more reliable than (2) or (3) because a larger area of the
disk is magnetized for each disk.

  (Does anyone have wear figures for 800K disks vs. HD disks?  I
looked through some back ASLE issues but couldn't find any...)
   
+----------------------------------+------------------+
| Anton Rang (grad student)        | rang@cs.wisc.edu |
| University of Wisconsin--Madison |                  |
+----------------------------------+------------------+