Floppy drives
We all know diskettes. Small flat disks, irritatingly slow and with too limited storage capacity.
Yet, we cannot live without them. Very few PC's are without a floppy drive.
Diskettes were developed as a low cost alternative to hard disks. In the 60's and 70's, when
hard disk prices were exorbitant, It was unthinkable to use them in anything but mainframe and
mini computers.
The first diskettes were introduced in 1971. They were 8" diameter plastic disks with a magnetic
An illustrated Guide to disk drives - storage medias.
file:///E|/education/Click & Learn/module4a.htm (2 of 4) [4/15/1999 6:22:43 AM]
coating, enclosed in a cardboard case. The had a capacity of one megabyte. The diskettes are
placed in a drive, which has read and write heads. Conversely to hard disks, the heads actually
touch the disk, like in a cassette or video player. This wears the media.
Later, in 1976, 5.25" diskettes were introduced. They had far less capacity (only 160 KB to
begin with). However, they were inexpensive and easy to work with. For many years, they were
the standard in PC's. Like the 8" diskettes, the 5.25" were soft and flexible. Therefore, they
were named floppy disks.
In 1987 IBM's revolutionary PS/2 PC's were introduced and with them the 3½" hard diskettes
we know today. These diskettes have a thinner magnetic coating, allowing more tracks on a
smaller surface. The track density is measured in TPI (tracks per inch). The TPI has been
increased from 48 to 96 and now 135 in the 3.5" diskettes.
Here you see the standard PC diskette configuration
The floppy controller
All diskette drives are governed by a controller. The original PC controller was named NEC
An illustrated Guide to disk drives - storage medias.
file:///E|/education/Click & Learn/module4a.htm (3 of 4) [4/15/1999 6:22:43 AM]
PD765. Today, it is included in the chip set, but functions like a 765. It is a programmable chip.
It can be programmed to handle all the various floppy drive types: 5.25" or 3.5" drives, DD or
HD etc.
The controller has to be programmed at each start up. It must be told which drives to control.
This programming is performed by the start up programs in ROM (read module 2a). So you
don't have to identify available drive types at each start up, these drive parameters are saved in
CMOS RAM.
The floppy controller reads data from the diskette media in serial mode (one bit at a time. like
from hard disks). Data are delivered in parallel mode (16 bits at a time) to RAM via a DMA
channel. Thus, the the drives should be able to operate without CPU supervision. However, in
reality this does not always work. Data transfer from a diskette drive can delay and
We all know diskettes. Small flat disks, irritatingly slow and with too limited storage capacity.
Yet, we cannot live without them. Very few PC's are without a floppy drive.
Diskettes were developed as a low cost alternative to hard disks. In the 60's and 70's, when
hard disk prices were exorbitant, It was unthinkable to use them in anything but mainframe and
mini computers.
The first diskettes were introduced in 1971. They were 8" diameter plastic disks with a magnetic
An illustrated Guide to disk drives - storage medias.
file:///E|/education/Click & Learn/module4a.htm (2 of 4) [4/15/1999 6:22:43 AM]
coating, enclosed in a cardboard case. The had a capacity of one megabyte. The diskettes are
placed in a drive, which has read and write heads. Conversely to hard disks, the heads actually
touch the disk, like in a cassette or video player. This wears the media.
Later, in 1976, 5.25" diskettes were introduced. They had far less capacity (only 160 KB to
begin with). However, they were inexpensive and easy to work with. For many years, they were
the standard in PC's. Like the 8" diskettes, the 5.25" were soft and flexible. Therefore, they
were named floppy disks.
In 1987 IBM's revolutionary PS/2 PC's were introduced and with them the 3½" hard diskettes
we know today. These diskettes have a thinner magnetic coating, allowing more tracks on a
smaller surface. The track density is measured in TPI (tracks per inch). The TPI has been
increased from 48 to 96 and now 135 in the 3.5" diskettes.
Here you see the standard PC diskette configuration
The floppy controller
All diskette drives are governed by a controller. The original PC controller was named NEC
An illustrated Guide to disk drives - storage medias.
file:///E|/education/Click & Learn/module4a.htm (3 of 4) [4/15/1999 6:22:43 AM]
PD765. Today, it is included in the chip set, but functions like a 765. It is a programmable chip.
It can be programmed to handle all the various floppy drive types: 5.25" or 3.5" drives, DD or
HD etc.
The controller has to be programmed at each start up. It must be told which drives to control.
This programming is performed by the start up programs in ROM (read module 2a). So you
don't have to identify available drive types at each start up, these drive parameters are saved in
CMOS RAM.
The floppy controller reads data from the diskette media in serial mode (one bit at a time. like
from hard disks). Data are delivered in parallel mode (16 bits at a time) to RAM via a DMA
channel. Thus, the the drives should be able to operate without CPU supervision. However, in
reality this does not always work. Data transfer from a diskette drive can delay and
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