DASD

A DASD (Direct Access Storage Device), which nowerdays we call a disk drive,
has undergone an evolution of development almost as mind boggling as that of integrated circuitry.

Before disk storage was magnetic tape storage which consisted of a long (2400ft) strip of 1/2inch wide flexible material
with a coating of a ferro-magnetic material capable of being magnetised so as to store data.

The magnetic tape was a sequential storage media.
To read any data on the tape, it was necessary to read all the data between where the tape was up to and the required data.
This was fine for a payroll run, when the input data was sorted by employee number and
the stored data on the output magnetic tape was also sorted by employee number,
so the output tape was similarly sorted, and will be the input tape for the next week's payroll run.

Disk storage was introduced by IBM in the 1950's.
It was a number of rigid disks, clamped together with spacers,
so that there is space for a read/write head to move over the surfaces.
Where the head stopped data could be written on the circle on the disk which was called a track.
The tracks on all the surfaces where a read/write head assembly stopped is called a cylinder.

The big difference with the magnetic tape was that data could be accessed
directly by moving the head assembly, then electronically selecting a head
and finally the sequential operation of waiting for the data to arrive under the read/write head.

What follows is a display of some of the artifacts of disk storage
in the collection and their significance in the developement of disk storage.
It is assumed the reader has a basic understanding of how disk storage works.
If not I can recommend two sources of information:

• A good overview to disk storage I found in:
  SCOTT MUELLER'S UPGRADING AND REPAIRING PCs.
  It also has a wealth of information about all the parts that comprise a computer.
  Unfortunately it is not on the WEB, you have to buy the book!
  And it is not cheap, mine cost £39.99 for the 1999 Edition.
  But it is worth every penny.
• For information about disk storage before PC's I found the
  IBM Journal of Research and Development, Vol 25, Sept 1981,
  "A Quarter Century of Disk File Innovation" a mine of information, it is at:
  http://www.research.ibm.com/journal/rd/255/ibmrd2505ZC.pdf

The objective of this WEB page is to take you through the early developement of disk storage
via the artifacts we have in the collection.

This is done via the table below which has a thumbnail of an artifact
which when clicked on opens another WEB page.
The columns of information are hopefully self explanatory,
and show the development of disk storage.

In particular notice the Capacity, the weight in gram/MByte, and the Areal Density in
Mbyte/sq inch over the 47 year period.