hardware Some examples of drives

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Some examples of drives

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Storage media	Drive letter
Floppy disks	A: B:
Hard disk	C: D: E:
CDROM/DVD	F:
MO drive	G:
Network drive	M:
RAM disk	O:

On this and the following pages, I will describe the various drive types, their history and technology. The last two drive types in the above table will not be covered.

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Storage principles

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Storage: Magnetic or optic. Data on any drive are digitized. That means that they are expressed as myriads of 0s and 1s. However, the storage of these bits is done in any of three principles:

The physical drive principle	Disk types
Magnetic	Floppy disks Hard disk Syquest disks Zip drive LS-120 disks
Optic	CD-ROM DVD
Magneto optic	High end drives

Interface

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Individual drives are connected to other PC components through an interface. The hard disk interface is either IDE or SCSI, which in modern PCs is connected to the PCI bus. Certain drives can also be connected through a parallel port or the floppy controller:

Interface	Drive
IDE and EIDE	Hard disks (currently up to 40 GB) CD-ROM
SCSI	Hard disks (all sizes) and CD-ROM
ISA (internal)	Floppy drives CDROM and super floppies connected through parallel port

Let us start evaluating the drives from the easy side:

The traditional floppy drive

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We all know diskettes. Small flat disks, irritatingly slow and with too limited storage capacity. Yet, we cannot live without them. Very few PCs are without a floppy drive.
Diskettes were developed as a low cost alternative to hard disks. In the 60s and 70s, 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 coating, enclosed in a cardboard case. They 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 PCs. Like the 8" diskettes, the 5.25" were soft and flexible. Therefore, they were named floppy disks.
In 1987 IBM's revolutionary PS/2 PCs 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 configurations:

Diskette size	Name	Tracks per side	Number of sectors per tracks	Capacity
5.25" Single side	SD8	40	8	40 X 8 X 512 bytes = 160 KB
5.25" Double side	DD9	40	9	2 X 40 X 9 X 512 bytes = 360 KB
5.25" Double side High Density	DQ15	80	15	2 X 80 X 15 X 512 bytes = 1.2 MB
3.5" DD	DQ9	80	9	2 X 80 X 9 X 512 bytes = 720 KB
3.5" HD	DQ18	80	18	2 X 80 X 18 X 512 bytes = 1.44 MB
3.5" XD ( IBM only)	DG36	80	36	2 X 80 X 36 X 512 bytes = 2.88 MB

Diskette drives turn at 300 RPM. That results in an average search time (½ revolution) of 100 ms.
The super floppy drives are described in module 4d.

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The floppy controller

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All diskette drives are governed by a controller. The original PC controller was named NEC 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 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 sometimes freeze the whole PC, so no other operations can be performed simultaneously.

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Learn more

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Module 4b about hard disks.
Read Module 4c about optical media (CDROM and DVD).
Read Module 4d about super diskette and MO drives.
Read Module 4e about tape streamers (which are not drives).
Read Module 5c about SCSI.