Different Types of Memory

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Computer Memory

Somnath Chaudhuri

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9th April2011


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Introduction

Hardware refers to the physical equipment used for theinput, processing, output and storage activities of acomputer system.

Central processing unit (CPU) manipulates the data andcontrols the tasks performed by the other components.

Primary storage internal to the CPU; temporarily stores

data and program instructions during processing.

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How the CPU Works

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How the CPU Works (Continued)

Binary form: The form in which data and instructions can beread by the CPU only 0s and 1s.

Machine instruction cycle: The cycle of computer processing,whose speed is measured in terms of the number of instructionsa chip processes per second.

Clock speed: The preset speed of the computer clock that timesall chip activities, measured in megahertz and gigahertz.

Word length: The number of bits (0s and 1s) that can beprocessed by the CPU at any one time.

Bus width: The size of the physical paths down which the dataand instructions travel as electrical impulses on a computerchip.

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What is Memory

Main memory consists of a number ofstorage locations, each of which is

identified by a unique address

The ability of the CPU to identify

each location is known as its

addressability

Each location stores a word i.e. the

number of bits that can be processed

by the CPU in a single operation.

Word length may be typically 16, 24,

32 or as many as 64 bits.

A large word length improves system

performance, though may be less

efficient on occasions when the full word

length is not used

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Computer Memory

Two basic categories of computer memory:

Primary storage andsecondary storage.

Primary stores small amounts of data and information

that will be immediately used by the CPU.

Secondary stores much larger amounts of data and

information (an entire software program, for example)

for extended periods of time.

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Primary Storage

Primary storage ormainmemory stores three types ofinformation for very brief periods of time:

Data to be processed by the CPU;

Instructions for the CPU as to how to process the data;

Operating system programs that manage variousaspects of the computers operation.

Primary storage takes place in chips mounted on thecomputers main circuit board, called the motherboard.

Four main types of primary storage: register, random

access memory (RAM), cache memory and read-onlymemory (ROM).

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Types of Primary Storage

Registers: registers are part of the CPU with the leastcapacity, storing extremely limited amounts ofinstructions and data only immediately before andafter processing.

Random access memory (RAM): The part ofprimary storage that holds a software program andsmall amounts of data when they are brought fromsecondary storage.

Cache memory: A type of primary storage where thecomputer can temporarily store blocks of data usedmore often.

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Types Primary Storage (Continued)

Read-only memory (ROM): Type of primarystorage where certaincritical instructions aresafeguarded;the storage is nonvolatileand retainsthe instructions whenthe power to the computeris turned off.

Flash memory: A form ofrewritable read-onlymemory thatis compact, portable, and requireslittle energy.

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Random Access Memory (RAM)

holds its data as long as the computer is switched on

All data in RAM is lost when the computer is switched off

Described as being volatile

It is direct access as it can be both written to or read from in any order

Its purpose is to temporarily hold programs and data for processing.

In modern computers it also holds the operating system

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Types ofRAM

1. Dynamic Random Access Memory (DRAM)

Contents are constantly refreshed 1000 times per second

Access time 60 70 nanoseconds

2. Synchronous Dynamic Random Access Memory (SDRAM)

Quicker than DRAM Access time less than 60 nanoseconds

3. Direct Rambus Dynamic Random Access Memory (DRDRAM)

New type of RAM architecture

Access time 20 times faster than DRAM

More expensive

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4. Static Random Access Memory (SRAM)

Doesnt need refreshing

Retains contents as long as power applied to the chip

Access time around 10 nanoseconds

Used forcache memory

Also fordate and time settings as powered by small battery

Types ofRAM

5. Video Random Access memory

Holds data to be displayed on computer screen

Has two data paths allowing READ and WRITE to occur at the same time

A systems amount of VRAM relates to the number of colours and resolution

A graphics card may have its own VRAM chip on board

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Read only memory (ROM)

ROM holds programs and data permanently even when computer isswitched off

Data can be read by the CPU in any order so ROM is also direct access

The contents of ROM are fixed at the time of manufacture

Stores a program called the bootstrap loaderthat helps start up the

computer

Access time of between 10 and 50 nanoseconds

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Types ofROM

3. Electrically Erasable Programmable Read Only Memory (EEPROM)

Reprogrammed electrically without using ultraviolet light

Must be removed from the computer and placed in a special machine to do this

Access times between 45 and 200 nanoseconds

4. Flash ROM

Similar to EEPROM

However, can be reprogrammed while still in the computer

Easier to upgrade programs stored in Flash ROM

Used to store programs in devices e.g. modems

Access time is around 45 90 nanoseconds

5. ROM cartridges

Commonly used in games machines

Prevents software from being easily copied15


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Caches

The rate of data fetching by the CPU from the main memory isabout 100 times faster than from secondary memory. But

there is also a mismatch between main memory and CPU.

CPU can process the data 10 times faster than the main

memory. Which limits the performance of the CPU due to

mismatch in CPU and main memory speed. So Cache memory

act as a buffer b/w main memory and CPU.

Cache: A smaller, high speed storage device used to increase

the speed of processing by making current programs and data

available to the CPU at a rapid rate. The basic characteristic ofcache memory is its fast access time.

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Small amount of memory typically 256 or 512 kilobytes

Temporary store for often used instructions

Level 1 cache is built within the CPU (internal)

Level 2 cache may be on chip or nearby (external)

Faster for CPU to access than main memory

Cache memory

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The operation of cache memory

1. Cache fetches data

from next to current

addresses in main

memory

2. CPU checks to see

whether the next

instruction it requires is in

cache

3. If it is, then the

instruction is fetched from

the cache a very fast

position

4. If not, the CPU has to

fetch next instructionfrom main memory - a

much slower process

Main

Memory

(DRAM)

CPU

Cache

Memory

(SRAM)

= Bus connections 18


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Virtual memory

Uses backing storage e.g. hard disk as a temporary location for

programs and data where insufficient RAM available

Swaps programs and data between the hard-disk and RAM as the

CPU requires them for processing

A cheap method of running large or many programs on a computer

system

Cost is speed: the CPU can access RAM in nanoseconds but hard-

disk in milliseconds

Virtual memory is much slower than RAM

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Secondary Storage

Memory capacity that can store very large amounts of

data for extended periodsof time.

It is nonvolatile.

It takes much more time to retrieve data because ofthe electromechanical nature.

It is cheaper than primary storage.

It can take place on a variety of media

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Secondary storage

Sequential access Direct access

Magnetic tape

Magnetic disk Optical disk

Floppy disk Hard disk CD-ROM WORM

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Secondary Storage (Continued)

Hard disk: A form of secondary storage that stores data onplatters divided into concentric tracks and sectors, which can be

read by a read/write head that pivots across the rotating disks.

Floppy disk: A form of easily portable secondary storage onflexible disks; also called floppy disks.


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Optical storage devices: A form of secondary storage inwhich a laser reads the surface of a reflective plastic platter.

Compact disk, read-only memory (CD-ROM): A form ofsecondary storage that can be only read and not written on.

Digital video disk (DVD): An optical storage device used tostore digital video or computer data.

Fluorescent multilayer disk (FMD-ROM): An opticalstorage device with much greater storage capacity than DVDs.

Secondary Storage (Continued)


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Memory Hierarchy

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An Example Memory Hierarchy

registers

on-chip L1

cache (SRAM)

main memory

(DRAM)

local secondary storage(local disks)

Larger,

slower,

and

cheaper

(per byte)

storage

devices

remote secondary storage

(distributed file systems, Web servers)

Local disks hold files

retrieved from disks on

remote network servers.

Main memory holds disk

blocks retrieved from local

disks.

off-chip L2

cache (SRAM)

L1 cache holds cache lines retrieved

from the L2 cache memory.

CPU registers hold words retrieved from

L1 cache.

L2 cache holds cache lines retrievedfrom main memory.

L0:

L1:

L2:

L3:

L4:

L5:

Smaller,faster,

and

costlier

(per byte)

storage

devices

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Different Types of Memory