home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Software Vault: The Gold Collection
/
Software Vault - The Gold Collection (American Databankers) (1993).ISO
/
cdr23
/
hdts_2.zip
/
TERMS.LST
< prev
next >
Wrap
File List
|
1993-06-08
|
22KB
|
410 lines
Hard Disk Tech Specs (tm) v2.0
Copyright (c) 1991,92 All rights reserved.
Octagon Micro Devices - Clearwater, FL. USA
Address Mark: The first two bytes in ID fields and data fields.
The first byte is always 10100001 (binary), while
the second byte identifies whether the field is
of the type ID or data.
Areal Density: BPI (bits per inch) multiplied by TPI (tracks per
inch), resulting in bits per square inch. Track
density is measured radially, while bit density
is measured circumferentially (around the track).
Average Latency: The average time that the drive must wait before
the requested information passes under the read/
write heads. Because most drives rotate at 3600
RPM, the average latency for most drives is
8.4 ms (milliseconds).
Azimuth: The angular distance in the plane of the disk surface
expressed as an angle from the correct track location.
BFI: Bytes From Index. Often used when mapping defects
(bad sectors). The index referred to is the timing
index.
BPI: Bits Per Inch, or bit density. This refers to how
many bits can be placed sided-by-side within one
inch of track space, while maintaining their unique
states (0 or 1). Since data is densest on the
innermost track, the BPI measurement refers to this
track. The BPI measurement is actually greater on
the outermost tracks, although controllers almost
never attempt to pack bits more densely onto outer
tracks.
Capacity: The total storage space expressed in megabytes.
There are two distinct types: formatted, and
unformatted capacity. The formatted capacity is
the space usable after the disk has been low-level
formatted. The unformatted capacity is higher,
due to the fact that sector-boundary information
occupies space when the drive is formatted.
Cluster: The number of sectors that an operating system
accesses at one time. A 32 Mb partition may have
a cluster size of 4K, while a 512 Mb drive may
have clusters 32K in size. This simplifies and
speeds up I/O for your operating system, though
large cluster sizes often waste space when small
amounts of data are written.
Controller: The electronics that allow the system BIOS to issue
standard commands to access the hard drive's data.
On drives such as IDE (Integrated Drive Electronics),
the controller is actually located on the drive itself,
with only a gateway interface needed to connect to
the system's I/O bus.
CRC: Cyclic Redundancy Check. A method commonly used to
provide a checksum for data. As data is written
to the disk, its calculated CRC is also written.
When data is read from the disk, the CRC is also
read, and then compared against a calculated CRC.
Cylinder: A reference to all tracks at a given radius. If a
drive has three platters, a given cylinder will
consist of six complete tracks, as long as each
side of every platter has its own read/write head.
DTR: Data Transfer Rate. This measurement includes
the time needed to transfer data from the CPU, to
the controller, to the actual drive. Often
expressed in BPS (bits per second), an ST-506/412
interface typically achieves transfer rates of 5
Mbits/sec, while most ESDI interfaces can reach
speeds of 24 Mbits/sec.
Dedicated Servo System: Disk systems employing a dedicated servo
system reserve an entire disk surface for
reliably and accurately tracking data with
the read/write heads.
Drive Type: On AT class machines, the system BIOS has a built-in
drive table indexed by a drive type. For example,
a common Phoenix BIOS used in many 286-class machines
recognizes that drive type 44 has 820 cylinders, 6
heads, and 17 sectors per track.
ECC: Error Correction Code. Many controllers have built-
in ECC hardware that is used to correct a single burst
error of up to 11 bits.
Embedded Servo System: Drives employing an embedded servo system
use servo (or tracking) information that
is embedded or coexists with data. This
can be contrasted against a dedicated
servo system, which reserves an entire
disk surface for servo information.
ESDI: Enhanced Small Device Interface. This interface is an
enhanced version of the ST-506 interface, capable of
transfer rates as high as 24 Mbits/sec. Error checking
and correction is significantly improved over the
ST-506 interface. In addition, ESDI controllers are
capable of handling floppy drives, tape backup drives,
and direct file transfers for any of these storage
devices.
FAT: File Allocation Table. This table is set up by an
operating system for tracking the allocation of
clusters to files. Most operating systems place the
FAT on the first track of a given partition. DOS
maintains two identical FATs for each partition.
Errors or discrepencies in the FAT(s) can result in
cross-linked and truncated files.
Format: A low-level format places cylinder and sector in-
formation onto the disk surface. This information
is not altered by a read or write operation to the
disk. A high-level format is performed by an
operating system. This process can place a boot
program, FATs, and system files onto special areas
of the disk, for use only by the operating system.
Form Factor: The physical width dimension of the hard drive.
Typically, 5.25, 3.50, or 8.00 inch.
Head: The element that actually reads or writes data to the
disk surface. Hard drives usually have two or more
heads that are stepped from track to track in unison
by a head actuator (stepper motor, voice coil, or
electronic). Currently, there are three types of
heads:
- Monolithic ( 8,000 BPI)
- Composite (12,000 BPI)
- Thin Film (25,000 BPI)
Head Crash: During normal operation, the read/write heads ride on
an extremely thin cushion of air, making no contact
with the actual disk surface. If the drive is bumped,
or if a foreign particle gets lodged between the
heads and the disk surface, the heads can crash onto
the disk surface, resulting in physical damage.
Usually, data is lost in the process.
Head Parking: Since heads normally ride on a cushion of air created
by the spinning of the disk, the heads make contact
with the disk surface only when the drive is powered
down. To prevent the heads from contacting an area
of the disk that contains data, the heads can be
parked, or placed over a track that does not contain
data (Landing Zone).
IDE: Integrated Drive Electronics. IDE drives have a built-
in controller, and only need a gateway interface to
the I/O bus on the main system board. Therefore, all
command interpretation is done by the electronics on
the drive.
ID Field: The area of every sector that contains the address
information (Head, Cylinder, and Sector). The ID
Field is created when the hard disk is low-level
formatted.
Gigabyte: 1,073,741,824 bytes.
Index Pulse: Created during a low-level format. Used by the
controller to sense the first sector of each track.
Index Time: The time necessary for the hard disk to complete one
revolution. The controller measures the index time
based upon the Index Pulse.
Interleave: The scheme used to number sectors in a track. With
slower controllers, the first sector is read and
transferred to the computer while the second sector
continues to rotate past the read/write head. Then,
the controller has to wait one full revolution until
the second sector is again under the read/write head.
This is very inefficient, so the sectors, during a
low-level format, can be numbered so that the first
sector is sector 1, the third sector is sector 2,
the fifth is sector 3, etc. (2 to 1 interleave).
This interleave factor can be adjusted so that the
next desired sector is always approaching the read/
write head when the controller is prepared to read
the sector. Most newer controllers, especially those
with a built-in track buffer, are capable of reading
an entire track in one revolution if the sectors are
numbered consecutively, which is a 1 to 1 interleave.
Landing Zone: A cylinder, usually one greater than the last usable
cylinder, to which the read/write heads are moved
when the drive is powered down or parked.
Low-Level Format: * see "Format."
Master: The primary drive. "Slave" is used to refer to
the secondary drive. Typically used to describe
the relationship between two IDE hard drives.
MCA: MicroChannel Architecture. Used in the IBM PS/2
series, the MCA allows only two drives to be
attached to the bus, even when two controllers are
installed that each are capable of handling two
drives. It is interesting to note that the system
BIOS will choose which drive will be C: based upon
the interface used. For example, when an ESDI and
an ST-506/412 controller are present, the system
will assign C: to the ESDI drive.
Megabyte: 1,048,576 bytes. The standard method for expressing
the capacity of a hard drive.
MFM: Modified Frequency Modulation. A fixed-length
encoding method used by many hard drives. All
bits are evenly spaced, with error-correction
information stored with the data.
MTBF: Mean Time Before Failure. This is the average amount
of time, usually expressed in thousands of hours, that
the drive will operate before requiring repair or
replacement.
Media: The substance used to record information. On most hard
drives, the media is magnetic, while newer drives rely
on an optical media.
Media Defect: Physical damage or flawed magnetic areas of a disk's
surface.
MZR: Multiple-Zone Recording. This is a method of media
optimization where zones of neighboring tracks are
given progressively more sectors per track as the
zones move toward the outside of the platter. For
example, a drive that has 612 cylinders and employs
an MZR 3-zone scheme might group 304 tracks in each
zone, with the innermost zone having less sectors
per track than the middle zone, and with the outer-
most zone having more sectors per track than the
middle zone.
NRZ: Non-Return to Zero. An encoding method that requires
a synchronizing clock to establish the position of
bits within a sector.
Optimization: Relating to a drive's interleave, this is the process
of adjusting the interleave factor to achieve the
greatest transfer rate. Defragmentation is a form of
optimization where files are re-written to disk in
a sequential fashion that reduces the number of seeks
needed to read a particular file, thusly "optimizing"
the drive's performance.
Parity: An error-detection method that uses an extra bit for
each byte, indicating whether the total number of 1's
or 0's is odd or even.
Partition: A range of cylinders that is designated to perform
its own record keeping chores, thusly it acts as a
separate drive. Each partition has its own FAT.
Platter: A circular piece of aluminum, usually coated or plated
with a magnetic oxide material. Platters are normally
stacked atop one another to increase storage capacity.
Recalibration: The act of returning a drive's read/write heads to
track 0.
RWC: Reduced Write Current. Expressed as the cylinder
from which point the current used to write to the
disk surface is reduced in order to maintain the
separate identity of bits in a sector. This is
sometimes necessary to compensate for the smaller
sectors found on the inner tracks.
RLL: Run Length Limited. An encoding method used to
limit the length of data written in order to
increase the capacity of the drive. In RLL2,7,
the run length of 0's may be no longer than seven.
Most drives designed for use with MFM controllers
can be used with RLL controllers, resulting in a
significant gain in storage capacity (this is
against the manufacturer's suggestion).
SASI: Shugart Associates Systems Interface. Pioneered
by Al Shugart, this interface is the predecessor
to the SCSI interface.
SCSI: Small Computer System Interface. An interface
that allows up to seven intelligent devices to be
attached to the system's bus. Currently, hard
drives, tape backup units, and printers are the
predominant SCSI devices available.
SCSI-Mac: Small Computer System Interface - Macintosh
Sector: A logical section of a track. Most drives pack 512
bytes into a sector.
Seek: The action of moving the read/write heads to a
specified cylinder.
Seek Time: The average seek time, expressed in milliseconds,
indicates the average time necessary for the read/
write heads to find or seek to a specified cylinder.
Another rating is track-to-track seek time. This
is the time necessary to seek from one track to an
adjacent track, also expressed in milliseconds.
Servo Track: A track that is recorded at the factory, and is used
to control the precise movement of the read/write
heads.
Skew: Sectors sometimes are written during a low-level
format in a way that the first few bits of a
sector are closer to an adjacent outer track,
while the remainder of the bits move gradually
toward an adjacent inner track in order to
compensate for track-to-track seek time, avoiding
an extra disk revolution while the read/write
heads are swithing from track to track.
Slave: * see "Master."
Spindle: The rotating shaft to which the platters are secured.
Spindle motor: The motor that rotates the spindle, and thusly the
platters. Most often, the drive rotates at 3600 RPM.
ST-506/412: An interface developed by Shugart in 1980 for use with
their ST-506 drive (5 Mb). In 1981, a buffered seek
was added to handle their new ST-412 drive (10 Mb).
A typical ST-506/412 interface is capable of transfer
rates of 5 Mbits/sec.
Stepper Motor: The motor that steps the read/write heads from track
to track. The shaft of the stepper motor rotates in
fixed degrees, allowing accurate head positioning.
* see also "Voice-Coil Actuator."
Step Pulse: A pulse sent to the stepper motor, resulting in the
heads being stepped in or out one track.
SMD: Storage Module Drive. An interface used primarily on
high-capacity drives.
Thin Film Head: A read/write head that is composed of a tiny integrated
circuit (IC) as opposed to an actual coil of wire.
Terminating Resistor: A small package of resistors that is used to
terminate the signals on the drive cable,
which prevents the signals from echoing. In
addition, controllers expect a certain amount
of electrical load, whether or not a drive is
actually attached. Without a terminating
resistor , the controller could suffer
electrical damage. Normally, the drive at
the end of the drive cable should have its
terminating resistor in place. If a second
drive is present in the middle of the drive
cable, its terminating resistor is normally
removed.
TPI: Tracks Per Inch. This number indicates the quality of
the recording media by indicating how many tracks can
reside within one radial inch of the disk surface.
Track: A circle existing at a given radius on one side of a
platter. The collection of tracks existing at a
given radius for all platters is referred to as a
cylinder. Tracks are divided into sub-units called
sectors.
Voice-Coil Actuator: A solenoid used to control the positioning of
the read-write heads. Instead of stepping to
the appropriate track, as is the case with a
stepper motor, the voice-coil actuator can
seek directly to the desired track using servo
information. * see also "Stepper Motor."
Winchester Drive: IBM originally made a drive for mainframes that
had 30 MB of fixed media, and 30 MB of removable
media. The Winchester gun factory makes a rifle
with a 30-30 calibre, thus the name.
Write Precomp: Write Precompensation. The gradual reduction in write
current from the outermost to innermost tracks. This
occurs over the entire range of cylinders, as opposed
to RWC which starts at a pre-defined cylinder.
XSMD: Extended Storage Module Drive. An enhanced version of
the SMD interface.
ZBR: Zone-Bit Recording. Developed by Seagate, this is a
type of media optimization that varies the number of
sectors per track. The outer tracks have more sectors
than the inner tracks.
ZCAV: Zoned Constant Angular Velocity. The "Constant Angular
Velocity" portion of the term refers to the disk
spinning at a steady rate. The "Zoned" part refers to
an increase in the number of sectors per track as the
tracks get longer (the outer tracks have a larger
circumference than the inner tracks, and can thusly can
hold more sectors).