IBM Type 2D

IBM Type 2D is a type of 8" floppy disk used by IBM for the 53FD drive used in the 1970s and 1980s on some mainframes and minicomputers such as the 370 and System/34. It uses MFM encoding at a track density of 48 tracks per inch and a bit density of 6,816 bits per inch, on double-sided double-density media. There are three subtypes, with different sector layouts and slightly different total storage capacity.

985 KB
This format has an index cylinder of 26 128 byte sectors on side 0 and 26 256 byte sectors on side 1, and 74 regular data cylinders of 26 sectors of 256 bytes each per side, totalling 3,848 data sectors with a capacity of 985,088 bytes.

1,136 KB
This format has an index cylinder of 26 128 byte sectors on side 0 and 26 256 byte sectors on side 1, and 74 regular data cylinders of 15 sectors of 512 bytes each per side, totalling 2,220 data sectors with a capacity of 1,136,640 bytes.

1,212 KB
This format has an index cylinder of 26 128 byte sectors on side 0 and 26 256 byte sectors on side 1, and 74 regular data cylinders of 8 sectors of 1,024 bytes each per side, totalling 1,184 data sectors with a capacity of 1,212,416 bytes.

Links
33FD/53FD Drive and Attachment (reference manual)