The part of the digital computer which stores information is called storage or memory. The computer’s memory stores: the numbers to be operated on; intermediate results that are generated in the course of computation and final results.
There are two important factors about the memory unit:
1. an access time
2. and a capacity.
The time required to transmit one computer word out of the memory to where it will be used is called the memory access time. It usually amounts to a few millionths of a second or less in modern fast computers. The speed of modern computers is the speed of access to their memories. The capacity of a computer is the quantity of data that its memory unit can hold.
There are many ways of memorizing the information in memory cells of a digital computer. External memory or storage units may use magnetic drums, magnetic tapes, magnetic disks and floppy disks. The magnetic drum and the magnetic disk are called a Direct Access or Random Access Storage Device.
The magnetic disk is very similar to a magnetic drum but it is based upon the use of a flat disk with a series of concentric circles of magnetizable material, one read/write head for each track.
Information inside the computer is stored in registers, electronic units of hardware. Each register holds one machine word consisting usually of 32 bits or 4 bytes. Registers hold information temporarily during processing.
Most computers use floppy disks. Floppies conform to a standard and you can use them to carry data from one place to another. They are also very cheap, but they are slow and have a limited capacity.
Almost all desktop computers have hard disks. They are fast and can store much greater than floppies, but they are fixed inside the computer and you cannot use them to transfer data.
You can move data from place to place using removable hard disks. They are almost as fast as fixed hard disks and also have high capacities, but they are relatively expensive. They do not all conform to a standard and they are not very common.
CD-ROM disks are very common and conform to a standard. They are removable and can hold about 640 MB. They are also cheap to make. However, they are usually read-only. You can’t change the information on them. They are also slow compared to hard disks.
Magneto-optical disks are like CD-ROMs, but you can write data on to them. They are removable, have large capacities, and last for a long time, but they are expensive and do not all conform to one standard. For this reason they are not very common.
Magnetic tape is a cheap medium. You can use it to store very large amounts of data, but it doesn’t allow random access. Every time you read or write a piece of data, you start at the beginning of the tape. Tape drives are slow. Therefore, they are only suitable for doing backups.
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