# Computer Memory Terms and its Definitions

Computer Memory Terms and its Definitions:

(a) Memory cell : A part of the digital memory which can store a single bit (1 or 0). e.g. flip flops, a single spot on magnetic tape or disc, a charged capacitor etc.

(b) Memory word : A group of memory cells which can be used to store some instruction or data. One memory word may have 4 or 8 or 16 or 32 or 64 memory cells. e.g., a group to 16 flip flops can store a 16 bit word.

(c) Byte : A group of 8 bits (each bit may be 0 or 1).

(d) Memory capacity : A method of specifying the extent of information which can to stored in a memory, e.g., memory having 1024 eight bit words. This memory can store 1024 words each having 8 bits. Thus the total number of bits in this memory is 1024 x 8 = 8192 bits. This memory is written as 1024 x 8 memory (also written as 1 K x 8 memory). 1 M x 8 memory means 1048576 (=220) x 8 bits. Thus1 K = 1024 and 1 M = 1048576.

(e) Memory density : The extent of memory stored per unit area of memory surface. When we say that memory A has higher density than memory B, we mean that Memory A can store more bits of memory in the same area than memory B.

(f) Address : It is a number which specifies the location of a word in the memory. Each location on the memory has a unique address. If a memory has M locations, we need p inputs so that each of the M locations has a unique address (and 2P = M). The address of the memory is used to locate it.

(g) Word length : The number of bits that can be stored in one memory location of computer memory is called its word length.

(h) Read operation : The process of locating a binary word in the memory, fetching it and transferring it to another device. Read operation is by specifying the address at which that word is stored.

(i) Write operation : The process of storing a digital word into memory so that it can be used (by reading) as and when required. When we write a new word at a particular memory location, this new word replaces the word already stored in that memory location.

(j) Access time : The time needed for read operation. It is the time from the application of a valid address code on the input (i.e., key board) until the appearance of that information on the output.

(k) Volatile memory : A memory which remains available only as long as electric power supply available. If electric supply is switched off, the volatile memory is lost.

(l) ROM (Read only memory) : A memory which can only be read but whose contents cannot be changed. Generally the ROM is written at the manufacturer's premises. There after, it can only be read (and not changed). It is a permanent or semi-permanent memory. Some examples of ROM are conversion tables, preprogrammed instructions etc.

(m) Static memory : A memory in which the data remains stored as long as electric supply is applied. There is no need to re-write the data periodically. Semiconductor memory devices belong to this category.

(n) Dynamic memory : A memory in which the data gets lost even if electric supply is kept switched on. Thus the data needs to be refreshed periodically. This memory generally has capacitors. Since charge on the capacitor leaks out, the memory gets lost and needs refreshing. However, this memory can have very high density.

(o) Auxiliary memory : Mass storage devices like magnetic type and disc are called auxiliary memory. These devices can store massive amounts of data. This memory is non-volatile but slower in speed than internal memory of the computer.

(p) Internal memory : The computer's main memory is called internal memory. It is the working memory of the computer and has high speed. The data and instructions being used at any time are stored in this memory. It is always semiconductor memory.

(q) RAM (Random access memory) : It is read and write memory. The data can be written in to this memory, read from this memory and changed as per the requirement. It is called random access memory because the access time is the same for all memory locations.

(r) SAM (Sequential access memory) : In this memory, the required word can be found only by sequencing through the memory locations from the beginning till the required word is found. Thus the access time depends on the memory location. Magnetic tape, magnetic disc and magnetic bubble memories are all sequential access memories.

(s) Read-Write memory : In this memory any part of the memory can be read or written or changed at any time.

(t) Non-volatile memory : This memory does not owe its existence to the electric power supply. It remains intact even when electric supply is switched off.