KarbosGuide.com. Module 4a.
Drives are storage media
A drive is the 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 medium is:
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, it will not be accessible to the operating system. However, some external drives contain special soft-ware, allowing them to be connected during operation.
Some examples of drives
|Storage media||Drive letter|
|Floppy disks||A: B:|
|Hard disk||C: D: E:|
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: 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|
|Magneto optic||High end drives|
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:
|IDE and EIDE||Hard disks (currently up to 40 GB)
|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:
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.
The floppy controller
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.
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.
Read Module 6a about file systems.
Copyright (c) 1996-2005 by Michael B. Karbo. www.karbosguide.com.