Format 3.5 DD vs HD Floppy Disks in MS-DOS 6.22

In MS-DOS 6.22, formatting a 3.5-inch double-density (DD) versus a high-density (HD) floppy disk involves distinct disk geometry configurations, hardware sensor checks, and command-line switches. While a standard 1.44 MB high-density disk is formatted by default in modern drives, double-density 720 KB disks require the operating system to adjust sector allocations, transfer rates, and track parameters via the FORMAT utility and the Floppy Disk Controller (FDC).

Physical Detection and Hardware Differences

The primary differentiator between 3.5-inch DD and HD disks is physical media construction and detection:

  • Sensor Holes: An HD disk features two square holes at the bottom edge: a write-protect notch on the left and a media-sensor notch on the right. A DD disk features only the write-protect notch.
  • Hardware Detection: When a disk is inserted into a standard 1.44 MB drive, a physical microswitch checks for the presence of the media-sensor hole. If no hole is present, the drive signals the Floppy Disk Controller (FDC) that a DD disk is inserted.
  • Magnetic Coercivity: HD disks utilize a high-coercivity magnetic coating (~720 oersteds) compared to DD disks (~600 oersteds). Consequently, the drive’s read/write heads must adjust their write current depending on the detected density to properly magnetize the substrate.

Disk Geometry and File Allocation

MS-DOS 6.22 structures the file system differently depending on whether it is writing a DD or HD format. Both formats use 512 bytes per sector and 80 tracks (cylinders) across 2 read/write heads (sides), but they differ in sector density:

  • Double-Density (720 KB):

    • Sectors Per Track: 9
    • Total Sectors: 1,440
    • Transfer Rate: 250 kbps
    • Media Descriptor Byte: F9
    • Root Directory Capacity: 112 directory entries
    • Cluster Size: 2 sectors (1,024 bytes per cluster)

  • High-Density (1.44 MB):

    • Sectors Per Track: 18
    • Total Sectors: 2,880
    • Transfer Rate: 500 kbps
    • Media Descriptor Byte: F0
    • Root Directory Capacity: 224 directory entries
    • Cluster Size: 1 sector (512 bytes per cluster)

The MS-DOS 6.22 FORMAT Command

The internal mechanics of the FORMAT.COM utility in MS-DOS 6.22 depend on user-supplied parameters and BIOS reporting.

Default High-Density Formatting

When running the default command:

FORMAT A:

MS-DOS queries the drive via BIOS Interrupt 13h. If an HD disk is inserted into an HD drive, the system detects the drive’s maximum capacity and formats 18 sectors per track, establishing a 1.44 MB capacity.

If this command is run on a DD disk, the hardware sensor will typically prevent it from formatting at 1.44 MB, or the format will fail with sector read/write errors because the drive cannot write reliable 500 kbps signals onto low-coercivity media.

Explicit Double-Density Formatting

To format a 720 KB disk in a 1.44 MB drive under MS-DOS 6.22, the user must explicitly instruct the utility to limit the track layout. This is accomplished using the /F (size) switch:

FORMAT A: /F:720

Alternatively, the legacy track and sector switches produce the identical layout:

FORMAT A: /T:80 /N:9

When these switches are supplied, MS-DOS 6.22 programs the FDC to drop its data transfer rate from 500 kbps to 250 kbps, writes only 9 sectors per track, builds a File Allocation Table (FAT12) corresponding to 720 KB, and inserts the F9 media descriptor byte into the boot sector.