home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Collection of Tools & Utilities
/
Collection of Tools and Utilities.iso
/
dskut
/
seg_drvs.zip
/
GLOSSARY.TXT
< prev
next >
Wrap
Text File
|
1988-11-28
|
56KB
|
2,315 lines
ACCESS
Refers to the process of obtaining data from, or placing data into a
disc storage device, register, or RAM. (i.e. accessing a
memory location).
ACCESS TIME
Time required to perform an ACCESS. Usages, e.g.: 1) seek to
location on a disc, 2) amount of time to read or write to a memory
location, 3) the time to position to the correct location in a disc
drive and carry out a read or write operation. ACCESS TIME is often
defined as the time from the leading edge of the first step pulse
received to SEEK COMPLETE (including settling).
ACTUATOR
See HEAD POSITIONER. The two basic types of actuators are
steppers and voice coils. Open-loop steppers generally cannot
achieve tracks per inch (TPI) as high as the closed-loop system
because of the lack of feedback on track positioning accuracy.
In open-loop stepper drives mechanical tolerances are one of the
most significant factors in limiting TPI enhancement.
ADDRESS
(physical) A specific location in memory where a unit record, or
sector, of data is stored. To return to the same area on the disc,
each area is given a unique address consisting of three components:
cylinder, sector, and head. CYLINDER ADDRESSING is accomplished by
assigning numbers to the disc's surface concentric circles (cylinders).
The cylinder number specifies the radial address component of the data
area. SECTOR ADDRESSING is accomplished by numbering the data records
(sectors) from an index that defines the reference angular position of
the discs. Index records are then counted by reading their ADDRESS
MARKS. Finally, HEAD ADDRESSING is accomplished by vertically numbering
the disc surfaces, usually starting with the bottom-most disc data
surface. For example, the controller might send the binary equivalent
of the decimal number 610150 to instruct the drive to access data at
cylinder 610, sector 15, and head 0.
ADDRESS MARK
Two byte address at the beginning of both the ID field and the data
field of the track format. The first byte is the "A1" data pattern,
the second byte is used to specify either an ID field or a data
field.
ADJUSTABLE INTERLEAVE
Interleaving permits access to more than one memory module, e.g., if
one memory module contains odd-numbered address and another
even-numbered address, they can both be accessed simultaneously for
storage. If the interleave is adjustable, the user may select which
ranges or areas are to be accessed each time.
ANSI
American National Standards Institute
APPLICATION PROGRAM
A sequence of programmed instructions that tell the computer how to
perform an end use task (i.e. accounting, word processing or other
work for the computer system user). To use a program, it must first
be loaded into MAIN MEMORY from some AUXILIARY MEMORY such as a floppy
diskette or hard disk.
AREAL DENSITY
Bit density (bits per inch, or BPI) multiplied by track density
(tracks per inch, or TPI), or bits per square inch of the disc
surface. Bit density is measured around a track (circumferential on
the disc), and track density is radially measured.
ASCII
American Standard for Coded Information Interchange.
ASME
American Society of Mechanical Engineers
ASYNCHRONOUS DATA
Data sent usually in parallel mode without a clock pulse. Time
intervals between transmitted bits may be of unequal lengths.
AUTOMATIC BACK UP OF FILES
This gives a user the security to make changes to a file without
worrying about accidently destroying it; there is always another copy.
One weakness of this method is that files take up twice the room on a
disc.
AUXILIARY MEMORY
Memory other than main memory; generally a mass storage subsystem, it
can include disc drives, backup tape drives, controllers and buffer
memory. Typically, AUXILIARY MEMORY is non-volatile.
AUXILIARY STORAGE DEVICE
Devices, generally magnetic tape and magnetic disk, on which data can
be stored for use by computer programs. Also known as secondary
storage.
AVERAGE ACCESS TIME
The average track access time, calculated from the end of the
CONTROLLER commands to access a drive, to drive "seek complete" time
averaged over all possible track locations at the start of ACCESS,
and over all possible data track ADDRESSES. Typically, the minimum
average access time including carriage settling for open loop
actuators is less than 85 ms and for voice coil disc drives is less
than 40 ms. As technology improves these times will continue to
decrease.
AZIMUTH
The angular distance in the horizontal plane, usually measured as an
angle from true track location.
BACKUP DEVICE
Disc or tape drive used with a fixed Winchester disc drive to make
copies of files or other data for off line storage, distribution or
protection against accidental data deletion from the Winchester
drive, or against drive failure.
BACKUP FILE
File copies made on another removable media device (disc, tape or
sometimes a remote hard dsic system) and kept to ensure recovery of
data lost due to equipment failure, human errors, updates, disasters
and the like.
BAUD RATE
A variable unit of data transmission speed equal to one bit per
second.
BDOS
The Basic Disk Operating System (BDOS) controls the organization of
data on a disk. BDOS is usually pronounced "B-DOS".
BIDIRECTIONAL BUS
A buss that may carry information in either direction but not in both
simultaneously.
BINARY
A number system like the decimal numbers, but using 2 as its base and
having only the two digits 0 (zero) and 1 (one). It is used in
computers because digital logic can only determine one of two states
- "OFF" and "ON." Digital data is equivalent to a binary number.
BIOS
(BASIC INPUT OUTPUT SYSTEM) A collection of information (firmware)
that controls communication between the Central Processor and its
peripherals.
BIT
The smallest unit of data. Consists of a single binary digit that can
take the value of 0 or 1.
BIT CELL LENGTH
Physical dimension of the bit cell in direction of recording along
the disc circumference of a track.
BIT CELL TIME
The time required to pass one bit of information between the
controller and the drive. Cell time is the inverse of the drive's
data rate; nominally 200 nsec for 5 Mhz drives.
BIT DENSITY
Expressed as "BPI" (for bits per inch), bit density defines how many
bits can be written onto one inch of a track on a disc surface. It is
usually specified for "worst case", which is the inner track. Data is
the densest in the inner tracks where track circumferences are the
smallest.
BIT JITTER
The time difference between the leading edge of read and the center of
the data window.
BIT SHIFT
A data recording effect, which results when adjacent 1's written on
magnetic discs repel each other. The "worst case" is at the inner
cylinder where bits are closest together. BIT SHIFT is also called
pulse crowding.
BLOCK
A group of BYTES handled, stored and accessed as a logical data unit,
such as an individual file record. Typically, one block of data is
stored as one physical sector of data on a disc drive.
BOOT
(Short for bootstrap). Transfer of a disc operating system program
from storage on diskette or hard disc drive to computer's working
memory.
BUFFER
A temporary data storage area that compensates for a difference in
data transfer rates and/or data processing rates between sender and