8-Inch Floppy Disk Formats

I have been using 8-inch floppy disk drives since 1978. My initial drives were SSDD Siemens FDD 120-8. Later I also acquired some DSDD FDD 200-8 drives. A pair of these drives are now installed in a Compupro Disk Cabinet I bought many years ago. I also have, but have never used, Shugart SSDD SA-801 drives.

I have ordered some 50-pin IDC centronics connectors. I plan to use these to standardize the connections between my 8-inch drive cabinets and the computers that use them. I have received these connectors. They require a slightly larger hole than a DD-50 connector so I will have to enlarge the holes on the drive and both machine cabinets.

I have not found the skew factor for all these formats. I have figured the skew factors for the Compupro from its BIOS and for my denser disk formats from my format program. Unfortunately, the IBM Document GA21-9182 does not address sector skew. Systems implementors are left to adjust the sector skew to provide the best disk speed with their system.

I am currently trying to figure out how the various systems handle the DS disks. I wrapped around the inside of the disk with tracks above 77 being on side 1 (over and under). I believe Compupro and IBM do cylinders where they handle the sectors on head 0 then the sectors on head 1 before they step the cylinder number. This how IBM handles disks on MSDOS and has become the defacto standard.

The standard formats have track 0 head 0 always formatted SD with 26 128 byte sectors. If the disk is DSSD, then track 0 head 1 is formatted SD with 26 128 byte sectors, otherwise track 0 head 1 is formatted DD with 26 256 byte sectors. This information came from IBM Document GA21-9182, "Diskette General Information Manual", July 1980.

Here are the IBM 8-inch FDD standard formats:

Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
Disk 1 0 SSSD 0 128 26 1 6 26 NA 128 B SD Format
Disk 1 1-76 SSSD 0 128 26 1 6 1950 249600 128 B SD Format
Disk 1 1-76 SSSD 1 256 15 1 ??? 1125 288000 256 B SD Format
Disk 1 1-76 SSSD 2 512 8 1 ??? 600 307200 512 B SD Format
Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
Disk 2 0 DSSD 0 128 26 2 6 52 NA 128 B DSSD Format
Disk 2 1-76 DSSD 1 128 52 2 6 3848 492544 128 B DSSD Format
Disk 2 1-76 DSSD 1 256 30 2 ??? 2220 568320 256 B DSSD Format
Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
Disk 2D 0 DSSD 0 128 26 H0 6 26 NA 128 B SSSD Format
Disk 2D 0 DSDD 1 256 52 H1 ??? 26 NA 256 B SSDD Format
Disk 2D 1-76 DSDD 1 256 104 2 ??? 3848 985088 256 B DSDD Format
Disk 2D 1-76 DSDD 2 512 60 2 6 2220 1136640 512 B DSDD Format
Disk 2D 1-76 DSDD 3 1024 32 1 ??? 1184 1212416 1024 B DSDD Format

Compupro uses only the standard 3740 SD format and the three standard IBM System 34 DD formats. It supports both Single- and Double-sided disks in each format.

Compupro reads the Two Sided Flag from the result of a read to determine the number of sided on the floppy. It then figures out FM or MFM by changing read command bit MFM until it reads without error. It then reads the CHRS from track 2 to determine N. This give a disk type of 0-7 which matches the format command.

This was common practice to allow the inclusion of an "Identity Sector " which contained information necessary for the OS to identify the format of the floppy disk. Some people, like me, used T0H0S1 as an "Identity Sector" to identify the disk type in the BIOS LOGON routine. On my systems I actually record the DPB for the CPM disk on the Identity Sector and copy it into the DPB. Microsoft DOS diskettes all contain a format type byte for the format of the disk. This includes 8-inch disks!

All the 8-inch formats actually reserve 2 tracks for the system, leaving 75 tracks for data. With the allocation of several blocks to the directory, the actual data storage area is less. I normalize the disks below to exclude the reserved tracks/cylinders.

The Compupro Disk 1/1A uses the 8272/765 FDC. To format a track the format track command is issued and the 4 bytes CHRN are sent for each sector in the order you want them written (skew) when the FDC requests data. All the real track formatting is handled by the FDC.

The Jade DD Disk Controller uses the WD 179X FDC. To format a track with this device the format track command is used and each byte must be sent until the track is complete. The 179X does accept special bytes to generate Index and Address Marks, CRCs, etc.

Here are the Compupro 8-inch FDD standard formats:

Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
All 0 SSSD 0 128 26 1 6 26 NA 128 B SD Format
0 1-76 SSSD 0 128 26 1 6 1950 249600 128 B SD Format
1 1-76 SSDD 1 256 26 1 9 1950 499200 256 B DD Format
2 1-76 SSDD 2 512 15 1 4 1125 576000 512 B DD Format
3 1-76 SSDD 3 1024 8 1 3 600 614400 1024 B DD Format
4 1-76 DSSD 0 128 52 2 6 3900 499200 NA
5 1-76 DSDD 1 256 104 2 9 3900 998400 NA
6 1-76 DSDD 2 512 60 2 4 2250 1152000 NA
7 1-76 DSDD 4 1024 32 2 3 1200 1228800 NA

I have played with violating the IBM System 34 DD format on my systems to add more sectors by reducing the gap lengths specified by the standard. This is a violation of the standard and may NOT work with other floppy disk controllers. This does work with the WD 179X and 8272 FDCs. These are also normalized to eliminate the reserved tracks/cylinders.

Here are my non-standard formats:

Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
All 0 SSSD 0 128 26 1 6 26 NA 128 B SD Format
2 SS 1-76 SSDD 1 256 30 1 10 2250 576000 NA
3 SS 1-76 SSDD 2 512 17 1 6 1275 652800 NA
4 SS 1-76 SSDD 3 1024 9 1 3 675 691200 NA
2 DS 1-76 DSDD 2 256 120 2 10 4500 1152000 NA
3 DS 1-76 DSDD 2 512 68 2 6 2550 1305600 NA
4 DS 1-76 DSDD 3 1024 36 2 3 1350 1382400 NA

5.25-Inch Floppy Disk Formats

I have been using the 5.25-inch drives since they became available. This includes SSDD, DSDD, DSQD, and DSHD drives. I have 3 3"x 6"x 9" Bud Boxes each containing a matched pair of 5.25-inch drives.

I have standardized on a DC-37 connector to connect these drives with old and new systems. This is discussed here.

Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
160K 0-39 SSDD 2 512 8 1 ??? 320 163840 512 8 SSDD Format
180K 0-39 SSDD 2 512 9 1 ??? 360 184320 512 9 SSDD Format
320K 0-39 DSDD 2 512 16 2 ??? 640 327680 512 8 DSDD Format
360K 0-39 DSDD 2 512 18 2 ??? 720 368640 512 9 DSDD Format
1.2M 0-79 DSHD 2 512 30 2 ??? 2400 1228800 512 15 DSHD Format

3.5-Inch Floppy Disk Formats

I have been using the 3.5-inch drives since they became available. This includes DSDD and DSHD drives. I have a 3"x 6"x 9" Bud Box containing a matched pair of 3.5-inch drives.

I have standardized on a DC-37 connector to connect these drives with old and new systems. This is discussed here.

Format Track Mode N Sec Size Sec/Cyl Heads Skew Total Sec Size (Bytes) Track Format
720K 0-79 DSDD 2 512 18 2 ??? 1440 737280 512 9 DSDD Format
1.44M 0-79 DSHD 2 512 36 2 ??? 2880 1474560 512 18 DSHD Format

Last Modified: 28 Dec 2019