Computer Memory

Subject: Computer Basics

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Overview

Based on their read write capability and their storage capability, computer memory can be classified into primary and secondary memory. This note explains about different types of computer memory.
Computer Memory

Based on their read write capability and their storage capability, computer memory can be one the following types.


Read Only Memory (ROM):

ROM is the computer memory on which data is prerecorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read. Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. ROM is referred to as being nonvolatile, whereas RAM is volatile. For e.g., ROM- BIOS (Basic Input and Output System). The function of BIOS is to load files from CD or Hard Disk into primary memory (ROM).

Masked ROM:

Here data and instruction are programmed during manufacturing process and if it malfunctions we have to replace it with new one, e.g. ROM- BIOS

Programmable ROM (PROM):

Programmable read-only memory (PROM) is read-only memory ( ROM ) that can be modified once by a user. PROM is a way of allowing a user to tailor a microcode program using a special machine called a PROM programmer .

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Erasable PROM (EPROM):

EPROM is reprogrammable ROM. We can erase the content written in EPROM and replace it with new one. The programs are erased using UV rays.

EEPROM (Electrically Erasable Programmable Read Only Memory):

EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., device configuration.

Flash Memory:

Like EEPROM, flash memory can be erased by electrical signal and erasing process is faster than EEPROM. It is possible to erase a particular block of data or instructions instead of entire data bits in flash memory.

Dynamic RAM (D- RAM):

It stores information in the form of charge; there is probability of leakage of charge, so it needs to be regularly refreshed. It can store more information than static RAM (S-RAM). Capacitor is the device that's used to store the charge, e.g. DDR, DDR2

Static RAM (S-RAM):

Unlike D- RAM, S- RAM stores information in the form of voltage. It is much faster than D-RAM and it is fully able to keep pace with modern processor. Here transistors are used instead of capacitor so refreshing is not required . The S-RAM chips are physically larger but stores less bits per area.

Things to remember
  • RAM is volatile memory.
  • ROM is non-volatile memory.
  • Data written in EEPROM can be erased electrically and rewritten.
  • The function of BIOS is to load files from CD or Hard Disk into primary memory (ROM).
  • It is possible to erase a particular block of data or instructions instead of entire data bits in flash memory.
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Videos for Computer Memory
Introduction to Computers - Lesson 2 - Memory Basics
What is computer memory? - Video
Questions and Answers
ROM RAM
i. It stands for Read only memory i. It stands for Random Access memory.
ii. It is non-volatile (permanent) memory. ii. It is volatile (temporary) memory.
iii. It's types are PROM, EPROM, EEPROM etc. iii. It's types are SRAM, DRAM etc.
SRAM DRAM
i. It stands for static Random Access
memory.
i. It stands for dynamic Random Access
memory.
ii. It does not need periodic
refreshing.
ii. It needs periodic refreshing.
iii. It stores data in the Flip-Flop
state.
iii. It stores data in transistor state.
 
Random Access Memory is called volatile memory because its data contents remain permanent and do not loss as soon as electricity in not supplied.
Read Only Memory is called non volatile memory because its data contents remain permanent and do not loss as soon as electricity is not supplied.

The functions of RAM are:

  1. It stores everything that processor chip needs to access.
The functions of ROM are:
  1. BIOS information in all modern computers is stored in EPROM (flash) memory.
  2. It is typically used to store small programs that never change.
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