Memory

Memory• When we receive some instruction or information

we retain them in our memory. Similarly a computer stores the instructions for solving a problem , the data to be processed , the intermediate results and the final results until they are displayed.

• This storage unit s known as computer memory or store.

• It consists of chips either on the motherboard or on a small circuit board attached to the motherboard.

• This electronic memory allows the CPU to store and retrieve data quickly.

Memory• Two types of built-in-memory:• Permanent memory.Some memory chips retain the data they hold, even

when the computer is turned off. This type of permanent memory is called nonvolatile.

• Non-Permanent memory.Other chips – in fact, most of the memory in a

microcomputer-lose their contents when the computers power is shut down. This type of nonpermanent memory is called volatile.

Nonvolatile Memory• Nonvolatile chips hold data even when the computer is

unplugged. • During normal use, the data in these chips is only read and

used- not changed- so the memory is called read only memory (ROM).

• Specifically, chips that cannot be changed are called programmable read only memory (PROM).

• PROM chips are often found on hard drives and printers.• They contain the instructions that power the devices. These

instructions, once set, never need to be changed.

Nonvolatile Memory• When a computer is turned on, it must know how to start.• ROM contains a set of start-up instructions called the basic

input output system (BIOS) for a computer.

FLASH MEMORY • Flash memory is a special type of nonvolatile memory. • It is often used in portable digital devices for storage.• Digital cameras, portable MP3 players, USB “keychain”

storage devices all use flash memory.

Volatile Memory• Volatile memory requires to store data.• The volatile memory in a computer is called random

access memory (RAM).• RAM’s job is to hold programs and data while they are in

use.• Physically, RAM consists of chips on a small circuit board.• Single inline memory modules (SIMMs) and dual inline

memory module (DIMM) chips are found in desktop computers.

• While the smaller, small outline DIMM (SO-DIMM) chips are found in laptop computers.

RAMRandom Access Memory

• It is also called main memory or direct access memory or read/write memory.

• A program must be loaded into RAM before execution.• Data and instructions can be read and modified.• It play an important role in processing of computer.

Types of RAMSRAM (Static Random Access Memory)• Retains contents as long as power is maintained.• Faster than DRAM.• SRAM is more expensive. • CPU does not wait to access data.• It is usually used to build a very fast memory

known as cache.

Types of RAM

DRAM (Dynamic RAM)• Must be constantly refreshed.• Requires an electric current to maintain its

electrical state.• The electrical charge of DRAM will decrease with

time that may result in lose of data.• Used for most PC memory because of size and

cost.

Read Only Memory (ROM)

• Programs and data that are permanently recorded at the factory.

• The instructions can only be read and cannot be deleted.

• Cannot be changed by the user.• When the computer is switched on ,the instructions

in the ROM are automatically loaded into the memory of the computer.

Types of ROMPROM

• Programmable ROM• This form of ROM is initially blank.• The user can write data and programs on it.• The user can write data and instructions on it only

once.

EPROM

• Stands for Erasable Programmable Read Only Memory

• It is initially blank• The user can write data and programs on it.• The data and program written on it can be

erased using ultraviolet rays.

EEPROM

• Stands for Electronically Erasable Programmable Read Only Memory

• It is initially blank• The user can write data and programs on it.• The content of EEPROM can be modified easily.• The data and program written on it can be erased

using electrical pulses.

Cache Memory

• Small block of very fast temporary memory• Speed up data transfer• It is located inside or close to CPU chip.• It is faster than RAM• Instructions and data used most frequently or

most recently.

Cache

Step 1Processor requests data or instructions

Step 2Go to address in main memory and read

Step 3Transfer to main CPU and cache

Next processor request• Look first at cache• Go to memory

PROCESSOR

R

A

M

Cache

Types of Cache

• Internal cache– Level 1 (L1)– Built into microprocessor– Up to 128KB or 256 KB

• External cache– Level 2 (L2)– Separate chips– 256KB or 512 KB– SRAM technology– Cheaper and slower than L1– Faster and more expensive than memory

Measuring Storage Capacity

KB – kilobyte• 1024 bytes• Some diskettes• Cache memory

MB – megabyte• Million bytes• RAM

GB – gigabyte• Billion bytes• Hard disks• CDs and DVDs

TB – terabytes• Trillion bytes• Large hard disks