Software Vault: The Gold Collection

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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).