A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks, and a track location that cuts across all platters is called a cylinder. For example, a typical 84 megabyte hard disk for a PC might have two platters (four sides) and 1,053 cylinders.
In general, hard disks are less portable than floppies, although it is possible to buy removable hard disks. A machine that reads data from and writes data onto a disk. A disk drive rotates the disk very fast and has one or more heads that read and write data. There are different types of disk drives for different types of disks. For example, a hard disk drive (HDD) reads and writes hard disks, and a floppy drive (FDD) accesses floppy disks. A magnetic disk drive reads magnetic disks, and an optical drive reads optical disks.
Disk drives can be either internal (housed within the computer) or external (housed in a separate box that connects to the computer).
Hard disk drives
The mechanism that reads and writes data on a hard disk. Hard disk drives (HDDs) for PCs generally have seek times of about 12 milliseconds or less. Many disk drives improve their performance through a technique called caching. Pronounced cash, a special high-speed storage mechanism. It can be either a reserved section of main memory or an independent high-speed storage device. Two types of caching are commonly used in personal computers: memory caching and disk caching.
A memory cache, sometimes called a cache store or
RAM cache, is a portion of memory made of high-speed static RAM (SRAM)
instead of the slower and cheaper dynamic RAM (DRAM) used for main memory.
Memory caching is effective because most programs access the same data
or instructions over and over. By keeping as much of this information as
possible in SRAM, the computer avoids accessing the slower DRAM.
Some memory caches are built into the architecture of microprocessors.
Intel 80486 microprocessor, for example, contains an 8K memory cache, and the Pentium has a 16K cache. Such internal caches are often called Level 1(L1) caches. Most modern PCs also come with external cache memory, called Level 2 (L2) caches. These caches sit between the CPU and the DRAM. Like L1 caches, L2 caches are composed of SRAM but they are much larger.
Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.
When data is found in the cache, it is called a cache hit, and the effectiveness of a cache is judged by its hit rate. Many cache systems use a technique known as smart caching, in which the system can recognize certain types of frequently used data. The strategies for determining which information should be kept in the cache constitute some of the more interesting problems in computer science. There are several interface standards for passing data between a hard disk and a computer. The most common are IDE and SCSI.
IDE - Abbreviation of either Intelligent Drive Electronics or Integrated Drive Electronics, depending on who you ask. An IDE interface is an interface for mass storage devices, in which the controller is integrated into the disk or CD-ROM drive.
Although it really refers to a general technology,
most people use the term to refer the ATA specification, which uses this
technology. Hard disk drives are sometimes called Winchester drives, Winchester
being the name of one of the first popular hard disk drive technologies
developed by IBM in 1973. Sample Drives Polaris 2000
External 2GB SCSI-2 Hard Drive
In a market where manufacturers continue to cut corners to stay competitive, Microtech refuses to compromise on the details that add up to quality. This is evident in the Polaris line of affordable hard drives. Polaris offers an effective combination of affordability, in capacities ranging of 2GB and 4GB. Microtech Polaris drives ship with top name mechanisms from our partners including Quantum, Seagate, and IBM. These mechanisms are fully tested to assure optimum performance. Each drive ships with CharisMac's acclaimed Anubis formatting utility, as well as consise, easy to understand documentation.
2GB external SCSI drive with software and accessory cables Affordable, incremental storage capacity for Mac OS systems
4500RPM disk rotation speed o12 ms average seek time o10.4 MB/sec burst transfer rate o5.6 MB/sec sustained transfer rate o128 K onboard cache speeds disk performance
Absolute Plug-n-Play simplicity with quick setup guide Preformatted for Mac OS, ready to use Compatible with Mac OS 8.0, System 7.x, and System 6.x Includes SCSI system cable and termiator for easy setup oIncludes CharisMac Anubis utility software for formatting and partitioning oVertical or horizontal orientation, user selectable o40 watt power supply Electronic registration via 800# installed on drive oLifetime toll free technical support