The primary computer storage medium, which is made of one or more
aluminum or glass platters, coated with a ferromagnetic material.
Most hard disks are fixed disks, which are permanently sealed in
the drive. Removable cartridge disks such as Iomega's Jaz disks
enable the disk to be removed from the computer and used as backup
or transferred to another machine with the same drive.
Most desktop hard disks are either IDE (also known as EIDE or ATAPI)
or SCSI. The advantage of IDE is their lower cost. The advantage
of SCSI is that up to seven or more devices can be attached to the
same controller board. SCSI drives are typically used in high-end
servers, because SCSI is available as a fault tolerant disk subsystem
(RAID systems), while IDE drives are found in most desktop and laptop
machines. Increasingly, IDE drives are available in RAID configurations
Hard disks provide fast retrieval because they rotate constantly
at high speed, from 5,000 to 15,000 rpm. In laptops, they can be
turned off when idle to preserve battery life.
Back in the 1950s, the very first hard disks held just a few hundred
thousand bytes and used platters 12" in diameter. In the 1980s,
the first personal computer hard disks started at 5MB (see ST506).
Today's hard disks start around 20 gigabytes and generally use 3.5"
platters for desktop computers and 2.5" platters for notebooks.
Hard disks are usually low-level formatted from the factory, which
records the original sector identification on them. See floppy disk,
magnetic disk and format program.
The primary computer storage device, which spins, reads and writes
one or more fixed disk platters. In practice, the terms "hard drive"
and "hard disk" are used synonymously. Hard drives are the storage
medium in desktop and laptop computers as well as all servers and
mainframes throughout the world. They are also used in printers
for storing fonts and print jobs as well as MP3 players and a myriad
of other portable and stationary computer-based devices. Although
removable disks encased in cartridges use the same "hard" disk media
and a similar drive technology, they are mostly called "removable
drives" rather than hard drives.
The term "hard" differentiates high-capacity rigid disks made of
aluminum or glass from low-capacity floppy disks made of plastic.
Megabyte - MB:
One million bytes, or more precisely 1,048,576 bytes.
Gigabyte - GB:
One billion bytes.
The connection and interaction between hardware, software and the
Hardware interfaces are the plugs, sockets, wires and the electrical
pulses traveling through them in a particular pattern. Also included
are electrical timing considerations. Examples are RS-232 transmission,
the Ethernet and Token Ring network topologies and the IDE, ESDI,
SCSI, ISA, EISA and Micro Channel interfaces.
Software, or programming, interfaces are the languages, codes and
messages programs use to communicate with each other and to the
hardware. Examples are the applications that run under the Mac,
DOS and Windows operating systems as well as the SMTP e-mail and
LU 6.2 communications protocols.
User interfaces are the keyboards, mice, commands and menus used
for communication between you and the computer. Examples are the
command lines in DOS and UNIX, and the Mac, Windows and Motif graphical
Interfacing is a major part of what engineers, programmers and
consultants do. Users "talk to" the software. The software "talks
to" the hardware and other software. Hardware "talks to" other hardware.
All this is interfacing. It has to be designed, developed, tested
and redesigned; and with each incarnation, a new specification is
born that may become yet one more de facto or regulated standard.
Also called "data rate," it is the transmission speed of a communications
or computer channel. Transfer rates are measured in bits or bytes
Pronounced "cash." A cache is used to speed up data transfer and
may be either temporary or permanent. Memory and disk caches are
in every computer to speed up instruction execution and data retrieval.
These temporary caches serve as staging areas, and their contents
can be changed in seconds or milliseconds.
Browser caches and Internet caches hold popular Web pages long
periods of time and even for the duration, because caching servers
constantly update the page with the latest version from the Internet
(see Web cache and browser cache). In these cases, the cache database
is actually a folder on the disk.
A disk cache is a section of main memory or memory on the disk controller
board that bridges the disk and the CPU. When the disk is read,
a larger block of data is copied into the cache than is immediately
required. If subsequent reads find the data already stored in the
cache, there is no need to retrieve it from the disk, which is slower
If the cache is used for writing, data is queued up at high speed
and then written to disk during idle machine cycles by the caching
program. If the cache is built into the hardware, the disk controller
figures out when to do it.
Disk access time is an average of the time between initiating a
request and obtaining the first data character. It includes the
command processing, the average seek time (moving the read/write
head to the required track) and the average latency (rotation of
disk to the required sector). This specification must be given as
an average, because seek times and latency can vary depending on
the current position of the head and platter.
Fast hard disks have access times of 10 milliseconds (ms) or less.
This is a common speed measurement, but overall disk performance
is significantly influenced by channel speed (transfer rate), interleaving
(Revolutions Per Minute) The measurement of the rotational speed
of a disk drive. Floppy disks rotate at 300 rpm, while hard disks
rotate from 2,400 to 3,600 rpm and more.
(Basic Input Output System) An essential set of routines in a PC,
which is stored on a chip and provides an interface between the
operating system and the hardware. The BIOS supports all peripheral
technologies and internal services such as the realtime clock (time
On startup, the BIOS tests the system and prepares the computer
for operation by querying its own small CMOS memory bank for drive
and other configuration settings. It searches for other BIOS's on
the plug-in boards and sets up pointers (interrupt vectors) in memory
to access those routines. It then loads the operating system and
passes control to it. The BIOS accepts requests from the drivers
as well as the application programs.
BIOSs must periodically be updated to keep pace with new peripheral
technologies. If the BIOS is stored on a ROM chip (ROM BIOS), it
must be replaced. Newer BIOSs are stored on a flash memory chip
that can be upgraded via software.
(2) (Integrated Development Environment) A set of programs run from
a single user interface. For example, programming languages often
include a text editor, compiler and debugger, which are all activated
and function from a common menu.
(1) (Integrated Drive Electronics) A type of hardware interface
widely used to connect hard disks, CD-ROMs and tape drives to a
PC. IDE is very popular because it is an economical way to connect
peripherals. Starting out with 40MB capacities years ago, 20GB IDE
hard disks have become entry level, costing less than half a cent
With IDE, the controller electronics are built into the drive itself,
requiring a simple circuit in the PC for connection. IDE drives
were attached to earlier PCs using an IDE host adapter card. Today,
two Enhanced IDE (EIDE) sockets are built onto the motherboard,
and each socket connects to two devices via a 40-pin ribbon cable.
Starting with ATA-66 drives, the cable uses 80 wires and 39 pins.
It plugs into the same socket with one pin removed.
The IDE interface is officially known as the ATA (AT Attachment)
specification. ATAPI (ATA Packet Interface) defines the IDE standard
for CD-ROMs and tape drives. ATA-2 (Fast ATA) defined the faster
transfer rates used in Enhanced IDE. ATA-3 added interface improvements,
including the ability to report potential problems. Starting with
ATA-4, either the word "Ultra" or the transfer rate was added to
the name in various combinations. For example, at 33 Mbytes/sec,
terms such as Ultra ATA, Ultra DMA, UDMA, ATA-33, DMA-33, Ultra
ATA-33 and Ultra DMA-33 have all been used. Following are the transfer
rates for the various ATA modes. See Cable Select.
|IDE Drive Type
||Transfer Rate MB/Sec
(Small Computer System Interface) Pronounced "scuzzy." SCSI is a
hardware interface that allows for the connection of up to 15 peripheral
devices to a single board called a "SCSI host adapter" that plugs
into the motherboard, typically using a PCI slot. SCSI peripherals
are daisy chained together. They all have a second port used to
connect the next device in line. SCSI host adapters are also available
with two controllers that support up to 30 peripherals.
Introduced in 1986 and originally developed by Shugart Associates
(see SASI), SCSI is widely used from desktop PCs to mainframes,
although most desktop PCs come with IDE drives. The advantage of
SCSI in a desktop PC is that a scanner and several other drives
(CD-Rs, DVD-RAM, Zip drives, etc.) as well as hard drives can be
added to one SCSI cable chain. However, this has become less important
as alternate interfaces such as USB and FireWire have become popular.
Until the late 1990s, SCSI hard disks were the only ones used in
RAID configurations which provide improved performance and/or fault
tolerance. Since the advent of IDE RAID controllers, SCSI and IDE
have become more equalized, although SCSI continues to be the drive
interface of choice in the server market.
Windows 95/98/NT/2000 and the Macintosh provide internal support
for SCSI, but Windows 3.1 and DOS did not. Installing SCSI in a
Win 3.1 or DOS machine required adding the appropriate SCSI driver.
A SCSI Chain:
The advantage of SCSI is that several peripherals can be daisy chained
to one host adapter, using only one slot in the bus.
ASPI and CAM:
Because internal support for SCSI was not provided by DOS and Windows
3.x, there was no benchmark for a standard implementation. As a
result, hooking up two SCSI devices often meant plugging in two
host adapters, negating SCSI's advantage of connecting multiple
ASPI and CAM were created to resolve these differences and provide
common interfaces between the drivers and the host adapters. Almost
all SCSI products are ASPI and CAM compliant. Windows 95 and higher
does support popular SCSI host adapters directly. It also supports
the ASPI and CAM standards so that older applications and drivers
will run even if Windows does not support that peripheral with a
SCSI Is Like a LAN:
SCSI is a bus structure itself and functions like a mini-LAN connecting
eight or 16 devices. The host adapter counts as one device, thus
up to seven or 15 peripherals can be attached depending on the SCSI
type. SCSI allows any two devices to communicate at one time (host
to peripheral, peripheral to peripheral).
||Bus Width (bits)
|Bus Lengths, Meters for Device
|Wide SCSI, aka
Fast Wide SCSI
|Wide Ultra SCSI
|Wide Ultra SCSI
|Wide Ultra SCSI
|Wide Ultra2 SCSI
|Ultra3 SCSI, aka
|Ultra4 SCSI, aka
12* - LVD was not part of these specs; however, if all devices
are LVD, 12 meters applies. If any device is single ended, then
length in SE column applies.
Information for this chart was obtained from the SCSI Trade
Association (STA), San Francisco, CA (www.scsita.org).
The different SCSI types provide backward and forward compatibility.
If a new SCSI host adapter is used with an older SCSI drive, the
drive will run at its maximum speed. If an older SCSI host adapter
is used with a newer drive, the drive will run at the host adapter's
SCSI and IDE Drives:
You can install SCSI hard disk drives in a PC that already contains
one or two IDE disk drives. The IDE drive will still be the boot
drive, and the SCSI drives will provide additional storage. Follow
the instructions in your SCSI host adapter manual carefully to make
the correct settings. Some SCSI host adapters provide floppy disk
control, which can be disabled.
IDs and Termination:
SCSI devices are daisy chained together. External devices have two
ports, one for the incoming cable and another for the outgoing cable
to the next device. An internal device has a single port that attaches
to a ribbon cable with multiple connectors. Each device must be
set to a unique ID number, which is normally done by flipping rotary
switches on external devices or by setting jumpers on internal ones.
The SCSI ID determines the device priority, which starts at 7 and
goes to 0 and then from 15 to 8. The host adapter defaults to the
highest priority, which is 7.
A subset of Plug and Play, called "SCSI Configured Automatically"
(SCAM), allows IDs to be set by software rather than manually. Both
the host adapter and peripheral must support this.
The device at the end of a SCSI chain must be terminated by either
setting a switch or plugging a resistor module into the open port.
Usually, host adapters default to terminated. If devices are connected
both internally and externally, the host adapter termination must
be removed and termination must be applied to the ends of both chains.
Parallel to SCSI:
There are adapters that allow SCSI peripherals to be connected via
the parallel port. Although the parallel port's transfer rate is
considerably less than the SCSI host adapter, it does provide a
means to hook up SCSI devices to laptops without PC Card slots or
desktop machines without available bus slots.
Each SCSI device can be further broken up into eight logical units,
identified by logical unit numbers (LUNs) 0 to 7. Although most
SCSI disks contain only one disk inside and are addressed as LUN
0, CD-ROM and optical disk jukeboxes contain multiple units. Each
disk in these devices can be addressed independently via LUN numbers;
for example, a four-disk jukebox could be assigned LUN 0 to 3.
Single Ended, Differential and Low Voltage Differential:
There are three types of SCSI signaling. Single-ended SCSI allows
devices to be attached to a total cable length of 6 or 3 meters
for Fast and Ultra SCSI. Single-ended SCSI is not defined for Ultra2
SCSI and higher.
Differential SCSI, or High Voltage Differential SCSI (HVD), is
used when devices are spread across a room, because the total cable
length is increased to 25 meters. Differential devices cost more
than single-ended ones.
Ultra2 SCSI introduced Low Voltage Differential signaling (LVD or LVDS)
that supports cable lengths up to 12 meters. Single-ended SCSI uses a
data line and ground. Both HVD SCSI and LVD SCSI use data low and data
high lines to increase transmission distance. However, LVD requires less
power and is less costly, because the transceivers are built into the
Information source: Techweb.com