Hardware Reference
In-Depth Information
Note
The original Shugart SA400 floppy interface made for 5 1/4-inch floppy drives supported up
tofourdrivesonasinglecable.However,IBMmodifiedthecontrollerpinouttosupportonly
two drives and eliminate the need to change DS jumpers on the drive.
How the OS Uses a Floppy Disk
To the OS, data on your PC disks is organized in tracks and sectors, just as on a hard disk
drive.
Tracks
are narrow, concentric circles on a disk;
sectors
are pie-shaped slices of the
individualtracks.A31/2-inch1.44MBfloppydiskdrivehasthefollowingspecifications:
• Bytes per sector: 512
• Sectors per track: 18
• Tracks per side: 80
• Track width (mm): .115
• Sides: 2
• Capacity (KiB): 1,440
• Capacity (MiB): 1.406
• Capacity (MB): 1.475
The floppy disk's capacity can actually be expressed in various ways. For example, what
we call a 1.44MB disk really stores 1.475MB if you go by the correct decimal prefix
definition for a megabyte. The discrepancy comes from the fact that in the past floppies
were designated by their kilobinary (1,024-byte) capacities, which were originally (and
improperly) abbreviated as KB. To prevent ambiguities in binary versus decimal number
interpretations, the International Electrotechnical Commission (IEC) has designated KiB
as the correct abbreviation for kilobinary.
Despite the IEC standards, the traditional method when discussing floppy drives or disks
is such that a floppy disk with an actual capacity of 1,440KiB is instead denoted as a
1.44MB disk, even though it would really be 1.406MiB (megabinary bytes) or 1.475MB
(million bytes) if we went by the correct definitions for MiB (mebibyte) and MB (mega-
byte).
Note
As with hard disk drives, using the same prefixes for both decimal and binary multiples has
resulted in a great deal of confusion. The IEC prefixes for binary multiples were designed
to eliminate this confusion. For more information on prefixes for binary multiples, see
ht-
