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Primary and secondary memory



[1] The term 'memory' is usually used to refer to the internal storage 1

locations of a computer. It is also called real storageor primary memory,and is expressed as quantities of K. For example, computers are advertised as having memories of 16K or 152K, depending on their storage capacity. Each K is equal to 1,024 bytes,and each byte is equal 5 to 8 bits.Some modern computers measure their memory in megabytes (Mb) - a megabyte is equal to 1048576 bytes.

[2] Primary memory is closely associated with the CPU because it stores
programs and data temporarily, thus making them immediately
available for processing by the CPU. To facilitate processing, two things 10
are needed: random accessand speed. The former means that any part
of the memory may be read, or accessed,equally quickly. This is made
possible by the system of addressesin primary memory, where the
storage locations are like a series of tiny compartments, each having its
own address. These addresses are like the addresses of houses, in that is
they do not change. Because they are always fixed, the control unit
knows where to find them at a very high speed. When it finds them, it
puts into the compartments whatever must go there and wipes out
whatever was stored there. The information present in these
compartments is called the contentsof the memory. 20

[3] Most primary memory is costly, and therefore it is used transiently, which means that a program, or parts of it, is kept in internal storage while the program is being executed. This, however, is not true for mini and micro applications where the computer performs the same function, referred to as a dedicated function,all the time. But since computers 25 must process vast quantities of data and programs, a lot of storage space is required. For this reason various secondary memory technologies have been developed.

[4] Secondary memory devices fall into two categories: sequential devices

and random-accessdevices. Sequential devices permit information to 30 be written on to or read off some storage medium in a fixed sequence only. In order to get at a particular data item, it is necessary to pass over all the data preceding it. An example of such a device is the magnetic tape.Its cost is low, but access to specified data may take a considerable length of time. On the other hand, random-access devices are designed 35 to permit direct, or almost direct, access to specified data. These devices bypass large quantities of irrelevant data and therefore reduce access


time considerably. An example of this technology is the magnetic disk,
which is faster than the magnetic tape and also more expensive. When
disks are hooked up to the computer and used as an extension of 4o

internal storage in order to increase the capacity of primary memory,
this is called virtual storage.For example, a computer with 256K bytes
of real storage may seem to have 512K bytes of virtual storage by using
disks to provide additional storage. The memory size of computers is
increasing as memory chips become cheaper. 45




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