Thus the main memory is a storage site inside the computer which overcomes these difficulties and possesses the following characteristics;
I. The storage capacity of this memory is much larger compared to registers in the CPU (which stores a byte of information!), so it can store a complete program other information.
II. The storage mechanism is electronic in nature, in such a way, that retrieval does not require physical movement of components. Thus the access time much faster in comparison to magnetic disk storage units.
III. Most part of this memory is volatile in nature, i.e. the stored information gets wiped out, the moment power supply is off. Some portions are non volatile — which retain the store information (all are in binary form even if the power supply is off.
IV. The most important characteristic this memory is that, each sit (location) is equally accessible, data retrieval time is independent its storage position. Such a facility’ often called as Random Access; because one can randomly access data from any location of the memo circuitry with equal ease.
V. Because of this random access property, the main memory is often called as Random Access Memory or RAM or simply the memory of a computer.
VI. When information is read from the memory locations, the contents of the locations remain intact-called as memory read operation. When some new information is written to these locations the previous data is over written-called as memory write operation.
Memory Organization and Capacity:
I. The main memory consists of several small storage areas, called as locations or cells.
II. Each of these locations can store a fixed number of bits, called as “word- length” of the memory. The word-length may be different for different computers. Usually the word-length is, 8-bit or 16-bit or 32-bit or 64-bit and so on. More is the number of bits per word, the more efficient the memory becomes.
III. Each location has a unique address, starting from O (zero) onwards.
IV. Each location can hold either a data item or an instruction. The CPU accesses the data or instruction from these locations just by referring to the address of the location.
The capacity of memory is defined as the number of bytes it can store. It is expressed in terms of kilobytes (kB = 210 = 1024 bytes), megabytes (MB = 220 =1048576 bytes) or gigabytes (GB = 230 = 1073741824 bytes). Remember kilo; mega and giga are decimal suffixes for 1000, 1000000 and 1000000000 times. Here we are using these suffixes, just because of the closeness of the corresponding binary values (1000:1024; 1000000:1048576; 1000000000:1073741824)!