Disk Doctoring

by P.D. Coker

Over the past few years, I have accumulated a number of computers either as bits and boards or as more or less working entities. One of my more recent acquisitions was a strange beast called a Samurai (produced by the Kokusai Electric Co. of Japan). It was sold by Matmos as a 16-bit small business computer, has 8″ drives and runs both MS-DOS and CP/M-86. Matmos supplied some software and a few disks and, by one of those strange quirks of fate (like getting a reply from certain 80-BUS manufacturers), I was offered from another source, a couple of hundred 8″ disks, used once, one careful owner, etc. Without hesitation, I bought them, or rather did a deal…

So, what’s all this got to do with 80-BUS machines ? Read on..

The usual type of 8″ drive operates in single-sided single-density (SSSD) mode and has a capacity of about 243 kbytes. More modern versions sometimes occur in equipment or on the second hand market which are capable of operating in either SSSD or double-sided: double-density (DSDD) mode. In the latter case, this gives a vastly increased capacity (round about 1.14 MBytes). Several 80-BUS FDC cards are available which cater for these, and Bioss can be modified to take advantage of the 8″ drives, so if you can get hold of cheap 8″ disks, why not have a go !

A few years ago Dave Hunt wrote a very interesting article on disks in 80-BUS News in which he pointed out that the disk base material was coated with magnetic material on both sides and that selection for single- or double-sided disks was made afterwards. This means that it is always worth trying to format both sides of a single-sided floppy disk – in my experience well over 90% will function quite happily as double-sided.

Standard 5.25″ disks have the index hole in the same place, regardless of whether they are single- or double-sided but 8″ disks don’t. If an 8″ disk is designed to be used in SSSD mode, the index hole is almost vertically above the read/​write slot (in fact, a few millimetres to the right). If it is a double-sided, double-density type, the index hole is offset to the right by 20 mm or so. Another peculiarity of 8″ disks is that the write-protect notch is on the bottom edge and has to be covered if you wish to write to the disk – devotees of this format say that the 5.25″ system with its write-protect notch on the side is peculiar – but who’s worried.