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Basic Organization of a

Microcomputer

MEMORY

INPUT

OUTPUT

ALU

CPU

CU

REGISTER

FILE

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Memory

• Part of thecomputersystem where

programs anddata are stored

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Memory

Central Processing Unit Main Memory

Bus

Current Instruction Being

Executed

Other

Program

Instructions

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• Bit

 – basic unit of the memory

 – A binary digit that can either be 0 or 1

Memory

http://isis.astrogeology.usgs.gov/IsisWorkshop/uploads/0/0d/Bit-byte-word.jpg

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• Composed of a number of cells whichcan hold an information

• A memory with m cells can hold atmost m information

• A memory with m cells has addressesfrom 0 to m-1

Memory

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Memory

• The addresses of an

8-bit memory is from

0 to 28-1 and can

hold 28

values

Memory Address Values

00000000

00000001

00000010

00000011

00000100

00000101

.

.

.

11111000

11111010

11111011

11111100

11111101

11111110

11111111

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• Data transfer between CPU andMemory takes place through the useof MAR and MDR

CPU and Memory

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CPU and Memory

MDRn bits long

MARk bits long

Values  Memory

Address

0

.

.

.

.

.

.

.

.

2k-1

CPUData bus

n bits of data

Address bus

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Random Access Memory (RAM)

http://image.made-in-china.com/2f0j00mvsTKMPFlBua/DDR-RAM-DDR2-DDR3-.jpg

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• Any location can be accessed for aread or write operation in some fixedamount of time that is independent of

its position or address

• Every location is directly accessible

• Static RAM and Dynamic RAM

RAM

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• Once the information is written, it doesnot need to be further addressed ormanipulated for the information to

stay in the place it is stored

• Composed of flip-flops that use a smallcurrent to maintain the logic level

Static RAM (SRAM)

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• Used by CPU registers and other high-speed storage devices

• Some computers use SRAM for cachesand main memory

• Fastest and most expensive of thesemiconductor memory circuits

Static RAM (SRAM)

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Dynamic RAM (DRAM)

• Stores one bit of

data in a capacitor

so that more bits

can be packedwithin an integrated

circuit

• Used in PC RAMs

and Video RAMs

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Dynamic RAM (DRAM)

• To store a 1, the

capacitor is filled

with electrons

• To store a 0, thecapacitor is

emptied

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Dynamic RAM (DRAM)

• Because of leaks,

capacitors become

empty in a matter

of milliseconds• To fix this problem,

the capacitors with

value 1 are

recharged

http://www.syntechsa.co.za/ram-memory-how-it-works/

http://www.syntechsa.co.za/ram-memory-how-it-works/http://www.syntechsa.co.za/ram-memory-how-it-works/http://www.syntechsa.co.za/ram-memory-how-it-works/

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Dynamic RAM (DRAM)

• Recharging is done

by the CPU or

memory controller 

• This is donethousands of times

per second

http://www.syntechsa.co.za/ram-memory-how-it-works/

http://www.syntechsa.co.za/ram-memory-how-it-works/http://www.syntechsa.co.za/ram-memory-how-it-works/http://www.syntechsa.co.za/ram-memory-how-it-works/

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• Dedicated for video display

• Used to display and manipulategraphic images

Video RAM

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• Contents stored in this memory arepermanently fixed and remain valideven without power.

Read-Only Memory

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• True ROM

 – Data are inserted during manufacture

• Programmable ROM (PROM)

 – Data can be written under computer

control, but only once

• Erasable PROM (EPROM)

 – Data can be rewritten

Read-Only Memory

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• Technique used access memory faster 

• Set of fast memory registers that holdthe locations that mare most likely tobe accessed in the main memory

• 95% of memory accesses are found inmemory cache

Memory Cache

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• The cache is checked first when CPUrequests for data

• The memory will be accessed onlywhen the data is not buffered in thecache

Memory Cache

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• Most popular high capacity secondarystorage

• Uses magnetic spots on a magnetizeddisk 

 – Presence of a spot: 1

 – Absence of a spot: 0

Magnetic Disks

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• Disk surface is divided into a number ofconcentric circles called tracks

• Tracks are further divided into sectors

• Example: Floppy disk, hard disk 

Magnetic Disks

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http://www.birmingham.ac.uk/Images/Research-and-teaching/Engineering-and-Physical-Sciences/Metallurgy-and-Materials/Magnets/recording2.gif 

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• Uses laser technology to create tinymicroscopic spots on the disk surface

• Laser is also used to read from the disk 

 – No reflection: 1

 – 0 otherwise

Optical Memory

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• Write Once and Read Many

• Data are recorded on the disk vendorand cannot be altered or erased.

• CD-ROM (Compact Disk – 

Read OnlyMemory)

• CD-R (CD  – Recordable)

•

CD-RW (Compact Disk – 

Rewritable)• DVD-ROM (Digital Video Disk  – Read Only

Memory)

WORM

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Magnetic Tape

• First kind of

secondary

memory

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• Variation of magnetic disks.

• Information are written magnetically

Magnetic Drum

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• Operates on fully electronic fashion but its

content is not erased when power is gone

• A type of erasable programmable read only

memory (EPROM)• Usually inserted in the Universal Serial Bus

(USB) port of the computer.

Flash Memory