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File Format
Name IBM 23FD
Released 1971

IBM 23FD was a read-only 8" floppy disk drive used with early-1970s IBM mainframes. Since it was read-only (obviously, some sort of device existed at IBM to write to those disks so that they had something to read in the first place, but the drives available to end-users only could read) it was used for loading system software (in place of punched cards) rather than storing user data as other disk drives could do. The 33FD (IBM Type 1), 43FD (IBM Type 2), and 53FD (IBM Type 2D) drives came later, and could both read and write.

The disks were single-sided and single-density, with a 32 track per inch density and 1,594 bits per inch, stored with FM encoding. There were 32 tracks (cyllnders), with 8 sectors per track and 319 bytes per sector (an unusual number; most disk sectors hold multiples of 128 bytes). There were a total of 256 sectors for a capacity of 81,664 bytes.


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