IBM Type 1

IBM Type 1 is a type of 8" floppy disk used by IBM for the 33FD drive used in the 1970s and 1980s on some mainframes and minicomputers such as the 370 and System/34. It uses FM encoding at a track density of 48 tracks per inch and a bit density of 3,268 bits per inch, on single-sided single-density media. There are three subtypes, with different sector layouts and slightly different total storage capacity.

242 KB
This format has 73 tracks (cylinders) with 26 sectors storing 128 bytes each, for a total of 1,898 sectors holding 242,944 bytes.

284 KB
This format has an index cylinder of 26 128 byte sectors, and 74 regular data cylinders of 15 sectors of 256 bytes each, totalling 1,110 data sectors with a capacity of 284,160 bytes.

303 KB
This format has an index cylinder of 26 128 byte sectors, and 74 regular data cylinders of 8 sectors of 512 bytes each, totalling 592 data sectors with a capacity of 303,104 bytes.

Links

 * Computer History Museum: 33FD drive
 * IBM System/34 5340 System Unit Maintenance Manual
 * Some references to the drive in a book on IBM 360 and 370 systems