Computer Architecture : Auxiliary Memory

Auxiliary memory also known as secondary memory or external memory, is a non-volatile memory and used to data storage. The auxiliary memory is not directly accessible by the CPU, because it is not accessed via the input/output channels (it is an external device). An auxiliary memory or secondary storage is one which does not lose stored data when the device is powered off unlike RAM which is a primary memory.

Auxiliary memory is much larger in size than main memory but is slower. It normally stores system programs, instruction and data files. It is also known as secondary memory. It can also be used as an overflow/virtual memory in case the main memory capacity has been exceeded. Secondary memories cannot be accessed directly by a processor. First the data/information of auxiliary memory is transferred to the main memory and then that information can be accessed by the CPU. Auxiliary memory usually consists of magnetic disk and tape, which is much less expensive than RAM, but also much slower. Disks are used to store programs and data that are not currently in use. When a program is executed, it is loaded from disk into main memory. The program may then load data from disk into memory for processing. Main memory is arguably the most used memory. When discussing computer algorithms such as quick sort, balanced binary sorted trees, or fast Fourier transform, one typically assumes that the algorithm operates on data stored in the main memory. The main memory is reasonably fast, with access speed around 100 nanoseconds. It also offers larger capacity at a lower cost.

Some of the examples of Auxiliary Memory is :

Optical Disc : A storage medium from which data is read and written by lasers. Optical disks can store much more data up to 8 gigabytes more than most portable magnetic media, such as floppies. There are three basic types of optical disks: CD/DVD/BD-ROM (read-only), WORM (write-once read-many) & EO (erasable optical disks).

Magnetic Disk : A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material. Both sides of the disk are used and several disks may be stacked on one spindle with read/write heads available on each surface. Bits are stored on the magnetized surface in spots along concentric circles called tracks. Tracks are commonly divided into sections called sectors. Disks that are permanently attached and cannot be removed by the occasional user are called hard disks. A disk drive with removable disks is called a floppy disk drive.

Magnetic Tapes : A magnetic tape transport consists of electric, mechanical and electronic components to provide the parts and control mechanism for a magnetic tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on tape along several tracks. Seven or nine bits are recorded to form a character together with a parity bit. R/W heads are mounted in each track so that data can be recorded and read as a sequence of characters.

Flash Memory : An electronic non-volatile computer storage device that can be electrically erased and reprogrammed, and works without any moving parts. Examples of this are flash drives, memory cards and solid state drives. A version of this is implemented in many notebook and some desktop computers.

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