An illustrated Guide to disk drives - storage medias.

Click & Learn. Module 4a. WWW.MKDATA.DK

Drives are storage media's

A drive is the name name for several types of storage media. There are also storage media, which are not drives (RAM, Tape Streamers), but on these pages, we will discuss the drives.

Common to drive media is:

●A file system can be assigned to them.

●They are recognized by the operating system and they are assigned a drive letter.

During start up, drives are typically recognized by the PC system software (ROM-BIOS + operating system). Thus, the PC knows which drives are installed. At the end of this configuration, the appropriate drive letter is identified with each drive. If a drive is not "seen" during start up, if will not be accessible to the operating system. However, some external drives contain special soft-ware, allowing them to be connected during operation.

Here some examples of drives:

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.

Storage principles

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

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An illustrated Guide to disk drives - storage medias.

Interface

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

Let us start evaluating the drives from the easy side:

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

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An illustrated Guide to disk drives - storage medias.

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 configurations:

Diskette drives turn at 300 RPM. That results in an average search time (½ revolution) of 100 ms.

The floppy controller

All diskette drives are governed by a controller. The original PC controller was named NEC

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An illustrated Guide to disk drives - storage medias.

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 sometimes freeze the whole PC, so no other operations can be performed simultaneously.

To continue:

Read Module 4b about hard disks.

Read Module 4c about optical media's (CD-ROM and DVD).

Read Module 4d about MO drives.

Read Module 4e about tape streamers (which are not drives).

Read Module 5c about SCSI.

Read Module 6a about file systems.

To overview

Last revised: 19. May. 1998. Copyright (c) 1996, 1997, 1998 by Michael B. Karbo. WWW.MKDATA.DK.

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