Geometries & Capacities of Digital[-ish] Floppy Diskette Formats

Table of Diskette Formats

Geometry and capacities of Digital (and Digital-inspired) diskette formats
	Name	Format	Cylinders	Heads	Sectors	Sector Size	Spindle Speed (rpm)	Transfer Rate (kb/s)^A	Encoding	Blocks^B	KB^C	Notes
	Diskette Format		Geometry							Unformatted Capacity		Notes
8″	RX01	SSSD	77¹	1	26	128	360	250	FM	494	250.25	DEC does not use track zero in their 8″ diskette formats (to retain compatibility with IBM's formats). Equivalent to the IBM 33FD.
	RX01-2^D	DSSD	77¹	2	26	128	360	250	FM	988	500.50	DEC does not use track zero in their 8″ diskette formats (to retain compatibility with IBM's formats). Non-standard format; theoretical double-sided RX01. Equivalent to the IBM 43FD.
	RX02	SSDD	77¹	1	26	256	360	500	MFM²	988	500.50	DEC does not use track zero in their 8″ diskette formats (to retain compatibility with IBM's formats).
	RX03^D	DSDD	77¹	2	26	256	360	500	MFM²	1976	1001	DEC does not use track zero in their 8″ diskette formats (to retain compatibility with IBM's formats). Non-standard format; theoretical double-sided RX02, only implemented by third-party device manufacturers — has driver support in RT-11 v4. Equivalent to the IBM 53FD.
5¼″	RX18	SSDD	40	1	9	512	300	250	MFM	360	180	Diskette format/geometry used by the VT-180 “Robin” CP/M machine.
	RX31	DSDD	40	2	9	512	300	250	MFM	720	360
	RX33	DSHD	80	2	15	512	360	500	MFM	2400	1200
	RX36^D	DSED	80	2	30	512	180³	500³	MFM	4800	1200	Special IBM format used for the microcode diskettes of the 3174. Does not correspond to any of the common PC-/MS-DOS diskette formats.
	RX50	SSQD	80	1	10	512	300	250	MFM	800	400	Does not correspond to any of the common PC-/MS-DOS diskette formats.
	RX55^D	DSQD	80	1	10	512	300	250	MFM	1600	800	Non-standard format; theoretical double-sided RX50. Does not correspond to any of the common PC-/MS-DOS diskette formats.
3½″	RX23J^D	DSDD	80	2	8	512	300	250	MFM	1280	640	Non-standard format; this diskette geometry was used in the NEC PC-98, and does not correspond to any of the common PC-/MS-DOS diskette formats.
	RX23	DSDD	80	2	9	512	300	250	MFM	1440	720
	RX24J^D	DSTD	77⁴	2	8	1024	360	500	MFM	2464	1232	Non-standard format; this diskette geometry was used in the NEC PC-98, and does not correspond to any of the common PC-/MS-DOS diskette formats.
	RX24	DSHD	80	2	18	512	300	500	MFM	2880	1440
	RX25^D	DMF	80	2	21	512	300	500	MFM	3360	1680	Non-standard format; format is equivalent to the Microsoft “Distribution Media Format” diskettes.
	RX26	DSED	80	2	36	512	300	1000	MFM	5760	2880

The transfer rate is measured in decimal kilobits per second. That is: one thousnad bits per second.
“Blocks” are five hundred and twelve bytes. The size which is used by several Digital operating systems.
Binary kilobytes, i.e. one thousand and twenty-four bytes. (Which, according to the SI, is properly called a “kibibyte”; however the author personally finds that name to sound stupid.)
This name was invented by the author, as the listed format is not one used by DEC.

In IBM's 3740 Data Entry System and the related 3540 Diskette Input/Output Unit only tracks zero through seventy-three (that is, the first seventy-four tracks) were used, with the remainder being ignored. Whereas DEC makes uses of seventy-six tracks (skipping track zero), and MS-/PC-DOS makes use of all seventy-seven tracks.
To be correct, the RX02 (and semi-standard RX03) formats are not in MFM encoding, but in “Modified MFM” (MMFM). DEC's MMFM encoding is such that the sector headers are recorded in single density FM, while the sector contents are in a tweaked MFM encoding (with the tweak being a change to the encoding to prevent the accidental creation of a sector header in the data area).
IBM achieved “extended density” on the 3174 Establishment Controller microcode diskette by running running the spindle motor of the Hitachi diskette drive (a model HFD532EIU apparently) at half speed (180 rpm), which allows for the diskette to be read reliably using a 500 kb/s transfer rate (the maximum rate supported by Western Digital WD1770, or NEC µPD675A — when abused with a doubled clock speed, as on the IBM PC/AT). Writing would not be reliable at that combination of spindle speed and transfer rate. However, a diskette controller that supports the 1 Mb/s data rate of the 3½″ extended density floppy (such as a Western Digital WD37C65C or Intel 82077) could reliably write a 3174 microcode diskette — by running the spindle motor at full speed.
The NEC PC-98 “1.2M” format uses the same standard 3½″ drives as other PC formats, as such, the PC-98 has access to eighty tracks, but the innermost three tracks are left unused.

Geometries & Capacities of Digital[-ish] Floppy Diskette Formats

Table of Diskette Formats

The ImageDisk File Format

The Teledisk File Format