Average Users Monthly Vol 1 Issue 1 - FILE 5
A brief glossary of commonly used computer terms.
Compiled by: Todd Clayton
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).
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.
ANSI
American National Standards Institute
ASCII
American Standard for Coded Information Interchange.
AUTOMATIC BACK UP OF FILES
This gives a user the security to make changes to a file without worrying about accidentally 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.
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 disc 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.
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.
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 receiver.
BUS
A length of parallel conductors that forms a major interconnection route between the computer system CPU and its peripheral subsystems. Depending on its design, a bus may carry data to and from peripheral's addresses, power, and other related signals.
BYTE
A sequence of adjacent BINARY digits or BITS considered as a unit, 8 bits in length. One byte is sufficient to define all the alphanumeric characters. There are 8 BITS in 1 BYTE. The storage capacity of a disc drive is commonly measured in MEGABYTES, which is the total number of bits storable, divided by eight million.
CACHE MEMORY
Cache Memory allows the system to load bytes of data from the hard disc to memory. The system may then refer to memory for information instead of going back to the hard disc, thereby increasing the processing speed.
CENTRAL PROCESSOR UNIT
(CPU). The heart of the computer system that executes programmed instructions. It includes the arithmetic logic unit (ALU) for performing all math and logic operations, a control section for interpreting and executing instructions, fast main memory for temporary (VOLATILE) storage of an application program and its data.
CONTROLLER
A controller is a printed circuit board required to interpret data access commands from host computer (via a BUS), and send track seeking, read/write, and other control signals to a disc drive. The computer is free to perform other tasks until the controller signals DATA READY for transfer via the CPU BUS.
CYCLIC-REDUNDANCY-CHECK
(CRC). Used to verify data block integrity. In a typical scheme, 2 CRC bytes are added to each user data block. The 2 bytes are computed from the user data, by digital logical chips. The mathematical model is polynomials with binary coefficients. When reading back data, the CRC bytes are read and compared to new CRC bytes computed from the read back block to detect a read error. The read back error check process is mathematically equivalent to dividing the read block, including its CRC, by a binomial polynomial. If the division remainder is zero, the data is error free.
DAISY CHAIN
A way of connecting multiple drives to one controller. The controller drive select signal is routed serially through the drives, and is intercepted by the drive whose number matches. The disc drives have switches or jumpers on them which allow the user to select the drive number desired.
DISC OPERATING SYSTEM
(DOS). A computer program which continuously runs and mediates between the computer user and the APPLICATION PROGRAM, and allows access to disc data by DISC FILE names.
ESDI
ENHANCED SMALL DEVICE INTERFACE. A set of specifications for the drives. See also SCSI.
FILE ALLOCATION TABLE
FAT: What the operating systems uses to keep track of which clusters are allocated to which files and which are available for use. FAT is usually stored on Track-0.
FIRMWARE
A computer program written into a storage medium which cannot be accidentally erased, e.g., ROM. It can also refer to devices containing such programs.
FORMAT
The purpose of a format is to record "header" data that organize the tracks into sequential sectors on the disc surfaces. This information is never altered during normal read/write operations. Header information identifies the sector number and also contains the head and cylinder ADDRESS in order to detect an ADDRESS ACCESS error.
G
A G is a unit of force applied to a body at rest equal to the force exerted on it by gravity. Hard disc drive shock specifications are usually called out in Gs. A shock specification of 40 Gs non-operating means that a drive will not suffer any permanent damage if subjected to a 40 G shock. This is roughly equivalent to a drop of the drive to a hard surface from a distance of 1 inch.
HEAD LANDING AND TAKEOFF
In Winchester drives, the head is in contact with the platter when the drive is not powered. During the power up cycle, the disc begins rotation and an "air bearing" is established as the disc spins up to full RPM (rotations per minute). This air bearing prevents any mechanical contact between head and disc.
HEAD LANDING ZONE
An area of the disc set aside for takeoff and landing of the Winchester heads when the drive is turned on and off.
INTERLEAVE FACTOR
The ratio of physical disc sectors skipped for every sector actually written.
INTERLEAVING
The interleave value tells the controller where the next logical sector is located in relation to the current sector. For example, an interleave value of one (1) specifies that the next logical sector is physically the next sector on the track. Interleave of two (2) specifies every other physical sector, three (3) every third sector and so on. Interleaving is used to improve the system throughout based on overhead time of the host software, the disc drive and the controller; e.g., if an APPLICATION PROGRAM is processing sequential logical records of a DISC FILE in a CPU time of more than one second but less than two, then an interleave factor of 3 will prevent wasting an entire disc revolution between ACCESSES.
INTERRUPT
A signal, usually from a peripheral device to a CPU, to signify that a commanded operation has been completed or cannot be completed.
KILOBYTE
(KBYTE). 1) 1024 bytes (two to the tenth power); 2) 1000 bytes; 1024 bytes is the normal definition.
LAN
Local Area Network
LANDING ZONE
The landing zone is where the read/write head sits when it is not active. If the system features a dedicated landing zone, the head will rest on the same track each time.
LOW LEVEL FORMAT
The first step in preparing a drive to store information after physical installation is complete. The process sets up the "handshake" between the drive and the controller. In an XT system, the low level format is usually done using DOS's debug utility. In an AT system, AT advanced diagnostics is typically used. Other third party software may also be used to do low level format on both XTs and ATs.
MEGABYTE
One million bytes (usually exactly 1,024,000 bytes). Abbreviation: MB or Mbyte.
MODIFIED FREQUENCY MODULATION
(MFM). A method of recording digital data, using a particular CODE to get the flux reversal times from the data pattern. MFM recording is self-clocking because the CODE guarantees timing information for the playback process. The controller is thus able to synchronize directly from the data. This method has a maximum of one bit of data with each flux reversal. (See NRZ, RLL).
MULTITASKING
The ability of a computer system to execute more than one program or program task at a time.
NRZ
NON-RETURN TO ZERO 1) User digital data bits; 2) A method of magnetic recording of digital data in which a flux reversal denotes a one bit, and no flux reversal a zero bit, NRZ recording requires an accompanying synchronization clock to define each cell time unlike MFM or RLL recording). No Seagate drives use NRZ recording methods.
PARITY
A computer data checking method using an extra bit in which the total number of binary 1's (or 0's) in a byte is always odd or always even; thus, in an odd parity scheme, every byte has eight bits of data and one parity bit. If using odd parity and the number of 1 bits comprising the byte of data is not odd, the 9th or parity bit is set to 1 to create the odd parity. In this way, a byte of data can be checked for accurate transmission by simply counting the bits for an odd parity indication. If the count is ever even, an error is indicated.
PARTITIONING
Method for dividing an area on disc drive for use by more than one disc operating system or for dividing large disc drives into areas which the File Allocation Table (FAT) can deal with when in use. The current IBM DOS maximum partition size is 32 MB for the XT and AT. This limit can be overridden using partitioning software written expressly for this purpose.
PATH
The DOS term "path" has three definitions and each definition involves directories. A PATH may be defined as: 1) the names of the chain of directories leading to a file; 2) the complete file or directory name; 3) a DOS command.
PLATTER
The round magnetic disc surfaces used for read/write operations in a hard disc system.
PREVENTIVE MAINTENANCE
A method of doing a scheduled routine observation or exchanging a part, prior to a breakdown of a piece of equipment.
RANDOM ACCESS MEMORY
(RAM) Memory where any location can be read from or written to in a random order. Random access memory usually refers to volatile memory where the contents are lost when power is removed. The user addressable memory of a computer is random access memory.
REDUCED WRITE CURRENT
A signal input (to some older drives) which decreases the amplitude of the write current at the actual drive head. Normally this signal is specified to be used during inner track write operations to lessen the effect of adjacent bit "crowding." Most drives today provide this internally and do not require controller intervention.
RLL
(RUN LENGTH LIMITED CODE). 1) A method of recording digital data, whereby the combinations of flux reversals are coded/decoded to allow greater than one (1) bit of information per flux reversal. This compaction of information increases data capacity by approximately 50 percent; 2) a scheme of encoding designed to operate with the ST412 interface at a dial transfer rate of 7.5 megabit/sec. The technical name of the specific RLL CODE used is "two, seven".
ROM
(READ ONLY MEMORY) A chip that can be programmed once with bits of information. This chip retains this information even if the power is turned off. When this information is programmed into the ROM, it is called burning the ROM.
SCSI
Small Computer Systems Interface. The current "high end" CPU-to-drive interface. Pronounced "Scuzzy."
SECTOR
A sector is a section of a track whose size is determined by formatting. When used as an address component, sector and location refer to the sequence number of the sector around the track. Typically, one sector stores one user record of data. Drives typically are formatted from 17 to 26 sectors per track. Determining how many sectors per track to use depends on the system type, the controller capabilities and the drive encoding method and interface.
SOFTWARE
APPLICATION PROGRAMS, disc operating systems and other programs (as opposed to HARDWARE). The instructions or programs, usually stored on floppy or hard discs, which are used to direct the operations of a computer, or other hardware.
SOFTWARE PATCH
Software modification which allows or adds functions not otherwise available using the standard software program.
STEPPER MOTOR
The stepper motor is the electro-mechanical part of the disc drive that positions the heads by step pulse on the tracks of the disc to read and write data.
TPI
Tracks per inch.
TRACKS PER INCH
Track density, number of tracks per inch.
VOLATILE
Memory that will be erased if power is lost. Typically, MAIN MEMORY is volatile, and AUXILIARY MEMORY is non-volatile and can be used for permanent (but changeable at will) storage of programs and data.
WINCHESTER DRIVE
A disc drive with a Winchester head and non-removable (fixed) discs sealed in a contaminant-free housing.
