Memory: A computer system has storage areas, referred to as memory. The memory can receive, hold and deliver data when instructed to do so. Data that are being processed are held in Primary Memory, which is capable of sending and receiving the data at very high speeds. Secondary memory stores data not currently being used and operates more slowly, but it is capable of storing large volumes of data.
Primary Memory: Primary Memory consists of semi-conductor memory chips and is used to store data and programs currently in use. Each storage element of memory is directly (randomly) accessible and can be examined and modified without affecting other cells. Main memory can be volatile or non-volatile. Primary memory is classified into two groups:
(A) RAM (Random Access Memory)
(B) ROM (Read Only Memory)
RAM is read/write memory. It is a volatile memory. It stores the information as long as power is switched on and the information is lost when the power supply is switched off. RAM is of two types:
DRAM: It needs constant refreshing in order for the stored data to be maintained.
SRAM: It does not need refreshing. It is faster and expensive than DRAM.
Features of RAM: The main features of RAM are as follows:
Data that needs to be processed and the instructions which are used for processing are held in the RAM. Each element of RAM is a memory location in which data can be stored. Each location has a unique address. Using this address, data can be directly retrieved or stored. Since RAM must hold the data to be processed and the instructions for processing, its size or capacity is one of the measures of the power of the computer.
Functions of RAM: The principal function of the main memory is to act as a buffer between the CPU and the rest of the computer system components. Main memory is used for the following purposes:
Storage of a copy of the main software program that controls the general operation of the computer. This copy is loaded into main memory when the computer is turned on. and it stays there as long as the computer is on.
The temporary storage of a copy of application programs and instructions to be retrieved by the CPU for interpretation and executed.
The temporary storage of data that has been input from the keyboard or another input device until instructions call for the data to be transferred into the CPU for processing.
Read-Only Memory (ROM): ROMs are the memories on which it is not possible to write the data when they are on-line to the computer. They can only be read. The ROMs can be used in storing programs provided by the manufacturer of computer for basic operations. ROMs are non-volatile in nature and need not be loaded in a secondary storage device.
All the different pair combinations from the factors of 70 above are the Factor Pairs of 70. ROMs can be written only at the time of manufacture. It is necessary, and also convenient, to have instructions stored in ROM. For example, if you are using a microcomputer with floppy disk drives, the more instructions in ROM. the fever diskettes you may have to handle.
The process of manufacturing ROM chips and recording data on them was more expensive than the process of producing RAM chips. As a result, manufacturers tended to record in ROM only those instructions that were crucial to the operation of the computer.
PROM, EPROM, EEPROM: In addition to ROM, three additional categories of non-volatile memory are used in some computer systems namely PROM, EPROM and EEPROM.
PROM: Programmable Read-Only Memory is a non-volatile memory which allows the user to program the chip with a PROM writer. The chip can be programmed once, thereafter it cannot be altered. Therefore, PROMs are more flexible than ROMs.
EPROM stands for Erasable Programmable Read-Only Memory. EPROM chips were developed as an improvement over PROM chips.
The EPROMs can be written electrically. It requires the erasure of whole storage cells by exposing the chip to ultraviolet light, thus brings them to the same initial state. This erasure is a time-consuming process. Once all the cells have been brought to the same initial state, then the EROM can be written electrically.
EEPROM stands for Electrically Erasable Programmable Read-Only Memory. EEPROMs are becoming increasingly popular as they do not require prior erasure of previous contents. It avoids the inconvenience of having to take chips out of the computer to change data and instructions. Instead, changes can be made electrically under software control. These chips are being used in point-of-sale terminals to record price-related data for products. The only disadvantage of EEPROM chips is that they cost more than regular ROM chips.
Secondary Memory: Secondary memory is also known as permanent memory as we can store data on it for future use. It is both read/write memory and consists of different storage devices known as secondary storage devices as follows:
(A) Magnetic tape: Magnetic tape allows large amount of data to be stored economically passes the devices write head, data is recorded by magnetising the iron oxide in different directions. While reading the iron oxide causes a current in the read head.
(B) Floppy disk: It is a plastic film coated with iron oxide metal and protected by a plastic jacket as cover that has an opening which allows the read/write head to retrieve or store data. The most common floppy disk now in use is the 3.5″ disk also known as micro floppy. Earlier floppies were of 5.25″ and 8″.
(C) Hard disk: It comprises at least one rigid disk protected by a strong and airtight casing. It contains disks, read/write head, access arms, servomotor and the electronic circuit board to control the disk operations. This is the most common secondary storage device having the highest storage capacity (2GB, 4GB, 10GB, 40GB….). It is fast and speeds of less than 10 microseconds are achievable.
(D) Optical disk: These are plastic disks handles more data in comparison to floppy disks. In Optical disk a light source is used to catch data patterns on the disks. Normally two laser lights are used; a weak to read data and stronger to write by burning the surface of the disk. Different types of optical disks are CD-R, CD-R/W, DVD. Free and easy to use online median calculator.
