A disk consists of one or more metal platters that rotate at 5400, 7200, or 10,800 rpm. A mechanical arm pivots over the platters from the corner. Information is written onto the disk in a series of concentric circles. At any given arm position, each of the heads can read an annular region called a track. Together, all the tracks for a given arm position form a cylinder.
Each track is divided into some number of sectors, typically 512 bytes per sector. On, modern disks, the outer cylinders contain more sectors, than the inner ones. Moving it to a random cylinder typically takes 5msec to 10msec, depending on the drive.
Once the arm is on the correct track, the drive must wait for the needed sector to rotate under the head, an additional delay of 5msec to 10msec, depending on the drive`s rpm. Once the sector is under the head, reading or writing occurs at a rate of 5 MB/sec on low-end disks to 160 MB/sec on faster ones.
The final layer in the memory hierarchy is magnetic tape. This medium is often used as a backup for disk storage and for holding very large data sets. To access a tape, it must first be put into a tape reader; either by a person or a robot (automated tape handling is common at installations with huge databases). Then the tape may have to be spooled forwarded to get to the requested block. This whole process could take some few minutes. The big plus of tape is that it is exceedingly cheap per bit and removable, which is important for backup tapes that must be stored off-site in order to survive fires, floods and earthquakes, etc.
The memory hierarchy is typical, but some installations do all of them, as one goes down the hierarchy, the random access time increases dramatically, the capacity increases equally dramatically, and the cost per bit drops enormously. Consequently, it is likely that memory hierarchies will be around for years to come.
Electrically erasable ROM and flash RAM are also nonvolatile, but in contrast to ROM can be erased and rewritten.
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Writing them takes orders of magnitude more time than writing RAM, so they are used in the same way ROM is, only with the additional feature that it is now possible to correct bugs in programs they hold. CMOS is another kind of memory which is volatile.
