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│ Hard Disk Tech Specs(tm) v3.0 │
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Hard Drive Solutions Family(tm)
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.
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.
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)
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.
Gigabyte: 1,073,741,824 bytes.
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.
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.
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.
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 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.
Slave: * see "Master."
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."
SMD: Storage Module Drive. An interface used primarily on
high-capacity drives.
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.
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.
