Memory system of a computer consists of primary and secondary memory units.
Primary Memory
Primary memory unit is used to store data ,instructions, and information before,during and after processing by Arithmetic Logic Unit(ALU) .It is actually a work area (physically,a collection of integrated circuits) within the computer ,where the CPU stores the data and instructions .It is mainly of the following three types.
1.Read Only Memory(ROM)
2.Random Access Memory (RAM)
3.Complementary Metal -Oxide-Semiconductor(CMOS)
Read Only Memory(ROM)
Read only memory is an essential component of the memory unit.We know that the computer ,being a machine ,itself has no intelligence or memory and requires instructions ,which are given by us.Whenever the computer is switched on ,it searches for the required instructions .The memory is permanent and is not erased when the system is switched off .So,ROM is non-volatile memory .
ROM contains a number of programs (set of instructions).The most important program of ROM is the Basic Input/Output System(BIOS) which activates the hardware (physical components of a computer ) such keyboard ,monitor ,floppy disk,etc. in communicating with the system and application software (set of instructions or programs).
Types of ROM: There are many types of ROM available for microprocessors like Mask ROM,PROM,EPROM,EEPROM AND EAPROM.
(a)Mask ROM: Mask ROM is the basic ROM chips.In this types of ROM ,the information is stored at the time of its manufacturing.So, it cannot be altered or erased later on.
(b)PROM:PROM stands for Programmable Read Only Memory.In this type of ROM ,the information is stored by programmers after its manufacturing .It also cannot be altered or erased later on.The information of PROM can be programmed only once after manufacturing .It cannot be altered or erased later on.
(c)EPROM:EPROM stands for Erasable Programmable Read Only Memory . It is similar to PROM,but its information can be erased later on by ultraviolet light and it can be reprogrammed.A window on the top of an EPROM chip allows the programmer to reprogram the chip using a chip burner .
(d)EEPROM:EEPROM stands for Electrically Erasable Programmable Read Only Memory.It is similar to EPROM but its information can be erased by using a high voltage current.The programmer flashes an Electric Charge through the EEPROM chip to reprogram its code.Most of the BIOSs used today have EEPROMs and can be flashed while they are still connected to the motherboard.A simple software utility can rewrite an entire BIOS having EPROMSs.
(e)EAPROM:EAPROM stands for Electrically Alterable Programmable Read Only Memory.As compared to EPROM and EEPROM ,the information stored in the EAPROM can be altered later.
Random Access Memory (RAM)
Random Access Memory (RAM) is another important component of the Memory Unit.It is used to store data and instructions during the execution of programs.Contary to ROM ,RAM is temporary and is erased when the computer is switched off.So , RAM is a volatile memory.RAM is a read/write type of memory ,and thus canbe read and written by the user /programmer.As it is possible to randomly use any location of this memory ,therefore ,this memory is known as Random Access Memory.
Types of RAM :There are two types of RAM used in a personal computer -Dynmic and Static.
(a)Dynamic RAM(DRAM):The information stored in Dynamic RAM has to be refreshed after every few milliseconds,otherwise it is erased . In other words DRAM must be continuously rewritten in order to maintain its data .This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second.DRAM is used for most system memory because it is cheap and small.DRAM has higher storage capacity and is cheaper than Static RAM.
(b)Static RAM(SRAM): The information stored in Static RAM need not be refreshed ,but it remains stable as long as power supply is provided .SRAM is costlier,but has higher speed than DRAM.
Complementary Metal-Oxide-Semiconductor(CMOS)
Complementary Metal-Oxide-Semiconductor(CMOS) is used to store the system configuration ,date,time, and other important data.When the computer is switched on ,BIOS matches the information of CMOS with the peripheral devices and displys error in case of mismatching.
Secondary Memory
Primary memory (especially RAM) stores the data , instruction and information temporarily during processing by CPU.When the computer is switched off ,this memory gets erased .Therefore , the data is saved on storage devices.The storage unit of the computer consists of different storage devices such as Magnetic Tape,Hard Disk Drive ,Floppy Disk Drive,Compact Disk Drive ,DVD(Digital Versatile Disc) and Pen Drive.The storage unit is also called the Secondary Memory Of Computer.
By Ashwani Kumar Sonu
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