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Types of Memories: Magnetic and Solid state in Digital Electronics- Memory literally means to memorize (or to preserve something in mind). In digital electronics, memory means ability or capability of an electronic device (e.g. digital watches, calculators, computers etc.) to store binary data or information. The capacity to store information or data in a digital system happens to be one of its significant needs. The specific circuits or systems which are designed in order to store data, are referred to as memory. Thus, ability of a digital system or circuit or device (e.g. computer etc.) to store binary for a short or long period of time, is known as memory and the circuit or device having this ability, is called a memory unit or memory element. It has to be remembered that apart from storing problem solving related data, function of memory is also to store necessary instructions regarding the solution of this problem (set of instructions is called data, that’s it refers to those basic facts on the basis of which a computer operates or functions. Information stored on a certain memory location or register or contents of memory elements are also called data).
The most basic function of a digital computer is to store information and memory units existing on a digital system and perform data storage function. Thus, memory is one of the most active part of a computer system, which not only stores program and data, but also processes data (mechanism, wherein data is collected and converted to some kind of information, is called data processing. In other words, the process of collecting data, storing data, performing appropriate action on it and then transferring it to some desired location on the system, is called data processing). In its simple form, a memory may consist of just one flip – flop or multiple flip – flops. If numerous flip – flops are interconnected, it becomes a register. However, in a large system, memory consists of thousands of registers, wherein every register stores a binary word.
Memory can be distributed as operation memory, inner memory and auxiliary memory. Operation memory is the fastest memory and it generally consists of flip – flop registers. This type of memory is not used for huge storage purposes owing to the fact that flip – flops are expensive with respect to per stored bit. Inner or “main” or primary memory normally comprises passive elements. Price of this type of memory per stored bit is reasonable and its speed is also fast to some reasonable extents. Generally, an additional or secondary memory is used in computers. An important feature of this memory is that its price per stored bit data or information, is low and as compared to operation memory and inner memory, its access time is high (after address, time required to read contents of a memory, is called access time). It means that it has an extraordinary fast speed (the process of storing data in a computer is called access).
Types of Memories
Following two types of memories are used for the purpose of storing data in digital circuits;
(1). Magnetic or electro-mechanical memories
(2). Solid state or semi-conductor memories
Magnetic or Electromagnetic Memories
These memories are available in the form of magnetic layers. The source of these memories in a computer for catering of information to the system is a motor (generally a steeper or servo motor). VCR, tape recorder, computer disc drives are common examples of this type of memory, where in data is stored on cassettes or diskettes in the form of magnetic layers. A VCR head, tape recorder or disk drive is provided motion by means of a motor, as a result of which magnetic layers convert into electrical signals. In this way, process of data loading and data transmission takes place. Most of the magnetic memories being commonly used include floppy disks, hard disks, magnetic tape and magnetic drums etc. Hard disk drive and floppy disk drive are essential components of nearly all the microcomputers and mini computers. Remember that in this type of memories, data remains stored almost on constant basis, its per stored bit cost is low as compared to semi- conductor memories and its storing capacity (i.e. ability or capacity to store digital data) is high.
Solid State or Semi-Conductor Memories
Memories which consist of semi-conductors and also have to deal with current and voltage factors, are called semi-conductor memories. In other words, memories which have been designed with respect to voltage level, are called semi – conductor memories. These memories are normally available in an IC form and consist of bipolar transistors or metal oxide semi – conductors. These memories are cheap, requires small space, most famous and are easily available in market in IC models. All other memories available in the form of flip – flops, registers, integrated circuits are semi – conductor type memories.
A semi–conductor memory has following types with respect to operational characteristics;
(i). Volatile memories
(ii). Non–volatile memories
Memories, in which stored data is lost as a result power turns off, are called volatile memories. An important instance of a volatile memory is RAM (Random Access Memory), because it is not possible to store data on it permanently. In such memories, generally D type latches and registers are used in order to store binary information. There are further following two types of a volatile memory;
(a). Static RAM
(b). Dynamic RAM
Static RAM is a volatile type semi – conductor memory, which if once stored, can remain on its cell till an indefinite period, provided power supply does not turn off or no new bit is stored to replace this particular bit. Remember that storage cells of a static RAM (SRAM) consist of bipolar or MOS (metal oxide semiconductor type latches) and they are used in high speed memories. On the contrary, dynamic RAM (DRAM) is such a volatile type semi – conductor memory, where in stored data cannot remain stored or protected indefinitely, rather stored data is re- stored or refreshed periodically. The greatest disadvantage of a dynamic RAM is that if stored data is not refreshed, it is lost.
Both SRAMs and DRAMs are used for the purpose of building memory in a mini computer or microcomputer. Remember that DRAMs consist on just MOS and their storage capacity is quite high as compared to SRAMs.
Memories in which stored data does not lose in the event of power supply break down or in case power supply turns off, or memories, in which data remains stored permanently irrespective of power outages status, are called non – volatile memories. These memories are also known as non–volatile storage devices. Remember that a long – life durable lithium battery is usually fixed in such memories which ensures a continuous supply of power on it. As a result, data stored on it does not lose despite turning off an external supply. ROM, magnetic disk and magnetic cover etc. are non–volatile memories.
As set of information stored on a ROM remains stored permanently even if DC power turns off, therefore ROM is considered as an ideal storing element for the purpose of storing necessary instructions for a computer’s start – up and its operation. These instructions remain stored on a computer even if it is powered off. And every time when a computer is tuned on, these instructions immediately pop up on computer’s monitor. A non-volatile memory is of the following types;
ROM Read-only memory
PROM Programmable read-only memory
EPROM Erasable programmable read-only memory
UVPROM Ultra violet PROM
EEPROM Electrically erasable PROM
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