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Friday, 22 November 2019

Types of Memories in Digital Electronics


Memories are classified as

(i) Registers, Main memory and Secondary memory.
(ii) Sequential Access Memory and Random Access Memory.
(iii) Static and Dynamic Memory.
(iv) Volatile and Non-Volatile Memory.
(v) Magnetic and Semiconductor Memory.

(i) Registers, Main Memory and Secondary Memory

Though memories are scattered throughout the computer, those based on the location and usage are called Registers, Main memory and Secondary memory. Registers are available within the CPU to store data temporarily during arithmetic and logical operations like addition, subtraction, AND, OR, etc. They have very low access time, as they are available inside the CPU. Main memories of a computer, usually of semiconductor type, are available external to the CPU to store program and data during the execution of a program. In the main memory, each memory location is identified by an unique address and is accessed for read/write operation in a lesser speed than registers. As the storage capacity of main memory is inadequate, secondary or auxiliary memories are added to enhance storage capabilities. This secondary memory operates at a lesser speed when compared to registers and main memory. Normally, secondary memories are of magnetic memory type (Magnetic tape, Magnetic drum, Floppy disk and Hard disk) that are used to store large quantities of data.

(ii) Sequential Access Memory and Random Access Memory

Based on the method of access, memory devices can be classified as Sequential Access and Random Access Memories (RAM). The access time of a sequential memory varies depending on the location to be accessed. An example of sequential access memory is the magnetic tape memory.
On the other hand, a random access memory is one in which any location can be accessed in a random manner and thus has equal access time for all memory locations.
An example of random access memory is the semiconductor RAM.

(iii) Static and Dynamic Memory:

In static memory, the content does not change with time, in dynamic memory, its content changes with time. Dynamic memory cells use the capacitance of a transistor as the storage device. Only one transistor is needed to store one bit of information. The capacitor must be refreshed periodically without being discharged in order to prevent loss of information. Static memory devices require no refreshing and hold data as long as d.c. power is applied. Examples for static memory are register and MOS cell; semiconductor dynamic RAM and circulating registers using Charge-coupled Devices (CCD) are examples of dynamic memory.

(iv) Volatile and Non-Volatile Memory :

Volatile memory loses its stored data when power to the memory circuit is removed; a non-volatile memory retains stored data permanently even after the power supply is turned OFF. Magnetic Core Memory and Read Only Memory (ROM) are examples of non-volatile memory devices.

(v) Magnetic and Semiconductor Memory:

These memories are classified based on the material used for construction. The magnetic memories are constructed using magnetic material, e.g. magnetic tape, floppy and compact disks. Magnetic recording is the process of storing data magnetically on the surface of a tape, disk or drum. Magnetic tape is a storage medium using the surface of a magnetic tape to hold data. Magnetic disk is a storage medium using the surfaces of a disk to hold magnetically stored data. Magnetic drum is a storage medium using the surface of a rotating magnetic drum to hold data.

Semiconductor memories are constructed out of semiconductor material using LSI and VLSI technologies. The examples of this type are Random Access Memory (RAM) and Read Only Memory (ROM).

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