UNIT I EC2303 Computer Architecture

8/9/2019 UNIT I EC2303 Computer Architecture

1/29

UNIT I

INTRODUCTION

1. Give the organization of IAS computer with it Intruction et.

The IAS machine was the first electronic computer built at the Institute for Advanced

Study (IAS), in Princeton, New Jersey, SA! It is sometimes called the von Neumann machine,

The IAS computer has the followin" features,

• Stored#Pro"ram concept

• $ain memory stores both data and instructions

• Pro"ram %ontrol unit (P%), interprets the instructions in memory

• &ata Processin" unit (&P) is responsible for e'ecutin" binary instructions!

This is also nown as data#path or e'ecution unit

• Input and output (I*) e+uipment operated by the control unit

!emor"#

The basic unit of information in the IAS computer is a -#bit word, which is the standardunit of information stored in a memory location or transferred in one step between the %P and

the main memory $! The si.e of the main memory is / - bit words and a secondary stora"e of

01/ words based on electromechanical ma"netic drum technolo"y was provided for bul stora"e! 2ach location in $ can be used to store either a sin"le -#bit number or else a pair of

3-#bit instructions!


2/29

Data $ormat#

IAS Num%er $ormat

The IAS4s number format is fi'ed#point, meanin" that it contains an implicit binary pointin some fi'ed position! Numbers are usually treated as si"ned binary fractions lyin" between #0

and 50, but they can also be interpreted as inte"ers!

2'amples of IAS4s binary format are

-00-0----- ---------- ---------- ---------- 6 5 -!7038

0--00----- ---------- ---------- ---------- 6 # -!7038

Intruction $ormat#

IAS Intruction $ormat

2ach memory location is capable of storin" two 3- bit instructions! If $(9) denotes the

-#bit memory word with address 9, then $(9, -:0;) denotes the half word consistin" of bits -throu"h 0; of $(9) , also called as left instruction and $(9, 3-:= 'Intruction %uffer regiter() A= 'A**reregiter( and P% '&rogram Counter(!


3/29

The &P has the necessary component, the A? which contains the circuits that perform

addition, multiplication, etc@ The &P has the &= 'Data regiter(, A% 'Accumu+ator( and $(mu+tip+ier ,uotient( re"isters!

Regiter#

Intruction regiter 'IR(: It is one of the maBor components of P% and contains the 7#bitopcode instruction bein" e'ecuted!

Intruction %uffer regiter 'I-R(: The IAS fetches two instructions at a time from memory, so

it contains a second re"ister, the instruction buffer re"ister for holdin" a second instruction!

A**re regiter 'AR(: It holds the address of a data operand to be fetched from or sent to main

memory!

&rogram Counter '&C(: contains the address of the ne't instruction to be fetched from memory!

Data regiter 'DR(: &ata re"ister normally contains a word to be stored in memory, or issued to

receive a word from memory!

Accumu+ator 'AC( an* mu+tip+ier ,uotient '!(: The accumulator is a special re"ister

normally used for arithmetic operations and holds the end result of the operation! The $ is a

special re"ister used for multiplication and division operation!

Intruction Set#

The IAS computer has around


4/29

Intruction Set of IAS

Intruction /0ecution#

The IAS fetches and e'ecutes instructions in several steps that form an instructioncycle! The IAS fetches two instructions in an instruction cycle! 2ach instruction cycle consists of

two sub cycles!

$etch c"c+e# The opcode of the ne't instruction is loaded into the I= and the address portion is

loaded into the A=! This instruction may be taen from the I>=, or it can be obtained frommemory by loadin" a word into the &=, and then down to the I>=, I=, and A=!

/0ecute c"c+e: The control circuitry interprets the opcode and e'ecutes the instruction bysendin" out the appropriate control si"nals to cause data to be moved or an operation to be

performed by the A?!


5/29

. 2ow wi++ "ou *etect an* correct error *uring *ata tranmiion3

2rrors are common in computation due to various factors lie manufacturin"

defects and environmental effects! Such errors fre+uently appear when information is bein"

transmitted between two relatively distant points within a computer or is bein" stored in a

memory unit! CNoiseD in the communication lin can corrupt a bit ' that is bein" sent from A to

> so that > receives´ x instead of '! To "uard a"ainst errors of this type, the information can

be encoded so that special lo"ic circuits can detect, and possibly even correct, the errors!

A "eneral way to detect or correct errors is to append special chec bits to every word!

*ne popular techni+ue employs a sin"le chec bit c- called parity bit! The parity bit is appended

to an n#bit word 9 6 ('-, '0,!!!, 'n#0) to form the (n 5 l)#bit Eord 9F6 ('-, '0,!!!, 'n#0, c-)! >it c- is

assi"ned the value - or l that maes the number of ones in 9 F even, in the case of even#parity

codes, or odd, in the case of odd#parity codes! In the even#parity case, c - is defined by the lo"ic

e+uation

c-6 '-⊕ '0

⊕ ,!!!, ⊕ 'n#0 '1(

where ⊕ denotes 29%?SIG2#*=, while in the odd#parity case

Ć 0 6 '-⊕ '0

⊕ ,!!!, ⊕ 'n#0

Suppose that the information 9 is to be transmitted from A to >! The value of c - is

"enerated at the source point A usin", say, (0), and 9F is sent to >! ?et > receive the word 94

6 ('4-, '40,!!!, '

4n#0, c

4-)! > then determines the parity of the received word by re#computin" the

parity bit accordin" to (0) thus:

cF- 6 '4-

⊕ '40⊕ !!! ⊕ '4n#0

/rror *etection an* correction +ogic


6/29

The received parity bit c4- and the reconstituted parity bit cF

- are then compared! If c4-

≠ cF-, the received information contains an error! In particular, if e'actly 0 bit of 9

F has been

inverted durin" the transmission process (a sin"le#bit error), then c 4- ≠

cF-! If c4- 6 c

F-, it can be

concluded that no sin"le#bit error occurred!

The possibility of multiple#bit errors is difficult to detect! Hor e'ample, if a - chan"es to 0

and a 0 chan"es to - (a double error), then the parity of 94 is the same as that of 9F and the error

will "o undetected! The parity bit c- therefore provides sin"le#error detection! It does not detect

all multiple errors@ much less provide any information about the location of the erroneous bits!

The parity#checin" concept can be e'tended to the detection of multiple errors or to the

location of sin"le or multiple errors! These "oals are achieved by providin" additional parity bits

of which checs the parity of some subset of the bits in the word 9 F! >y appropriately

overlappin" these subsets, the correctness of every bit can be determined!

4. /0p+ain *ifferent 5in* of a**reing mo*e with an e0amp+e.

The purpose of an address field is to point to the current value G(9) of some operand 9 used by

an instruction! This value can be specified in various ways, which are termed as addressin"

modes! The addressin" modes of 9 affect the followin" issues:

• The speed with which G(9) can be accessed by the %P

• The ease with which G(9) can be specified and altered

Access speed is influenced by the physical location of G(9)#normally the %P

re"isters or the e'ternal memory $! The most commonly used addressin" modes are

0! Absolute (or) &irect Addressin"

3! Indirect Addressin"

a. PC relativeb. Base relative

c. Indexed

8! Auto#inde'in"

a. Pre-Decrement Modeb. Post-Increment Mode

1! =e"ister $ode

a! Register-direct b. Register Indirect Mode

c! Register Indirect with offset Mode

1. Absolute (or) Direct Addressing:


7/29

The correspondin" operand field contains the address 9 of the stora"e location

containin" the re+uired value as part of the instruction

/0# !O6/ A) 7

The instruction copies the contents of memory location 9 to the A re"ister!

2. Indirect Addressing:The instruction contains the address E of a stora"e location which in turn contains

the address 9 of the desired operand

/0# !O6/ A) 87

3. Immediate Addressing:

Ehen data 9 is constant, its value can be placed in the operand field! There is no

memory fetch re+uired for an immediate addressin" mode!

/0# !O6/ A) 9:

4. Relative Addressing:

In absolute addressin", it is re+uired for the complete operand address to appear inthe instruction operand field! In relative addressin", the operand fields contain a relative

address, also called an offset or displacement &! 2ach operand is associated with a sin"le

address re"ister = from a set of "eneral#purpose re"isters, and effective address A is

computed by addin" & to the contents of = that is

A #; R'A4( appearin" in an assembly lan"ua"e

instruction indicates that the contents of the desi"nated address re"ister A< should be

decremented automatically before the instruction is e'ecuted! This process is called pre=

*ecrementing!


8/29

Similarly 'A4(


9/29

② Arithmetic intruction# It performs operations on numerical data!

③ Bogica+ intruction# It includes >oolean and other non#numerical operations!

④ &rogram=contro+ intruction: It includes branch instructions, which chan"e the

se+uence in which pro"rams are e'ecuted!

⑤ Input=Output 'IO( intruction# It causes information to be transferred between the

processor or its main memory and e'ternal I* devices!


10/29

Bit of common intruction t"pe


11/29

. @rite hort note on evo+ution of computer.

!echanica+ /ra#

The Difference /ngine# In the 0;th century %harles >abba"e desi"ned the first

computers to perform multistep operations automatically, that is, without a human intervenin" in

every step! The technolo"y used was entirely mechanical! >abba"e4s first computin" machine,

which he called the &ifference 2n"ine, was intended to compute and print mathematical tablesautomatically, thereby avoidin" the many errors occurrin" in tables that are computed and

typeset by hand! The &ifference 2n"ine performed only one arithmetic operation: addition!

Kowever, the method of (finite) differences embodied in the &ifference 2n"ine can calculate

many comple' and useful functions by means of addition alone!

The Ana+"tica+ /ngine: >abba"e conceived a much more powerful computin" machine

that he called the Analytical 2n"ine! This machine is considered to be the first "eneral#purpose

pro"rammable computer ever desi"ned! The overall or"ani.ation of the Analytical 2n"ine is

outlined in the followin" fi"ure! It contains in rudimentary form many of the basic features found

in all subse+uent computers! The main components of the Analytical 2n"ine are a memory called

the store and an A? called the mill@ the latter was desi"ned to perform the four basic arithmetic

operations! To control the operation of the machine, >abba"e proposed to use punched cards of a

type developed earlier for controllin" the Jac+uard loom! A pro"ram for the Analytical 2n"ine

was composed of two se+uences of punched cards: operation cards used to select the operation to

be performed by the mill, and variable cards to specify the locations in the store from which

inputs were to be taen or results sent!

/+ectromechanica+ /ra#

A later innovation was the use of electric motors to drive the mechanical components,

thus main" calculators CelectromechanicalD and "reatly increasin" their speed! Another

important development was the use of punched cards to sort and tabulate lar"e amounts of data!


12/29

The punched#card tabulatin" machine was invented by Kerman Kollerith and used to process the

data collected in the 077- nited States census! In 07;1 Kollerith formed a company to

manufacture his electromechanical e+uipment! This company subse+uently mer"ed with several

others and in 0;3 was renamed the International >usiness $achines %orp! (I>$)! In Lermany,

the 1 was built in 0;abba"eMs wor! nlie previous computers, the

0 used binary, instead of decimal, arithmetic! The 2arvar* !ar5 I was constructed by I>$! in

0;abba"e4s machines, the $ar I employed decimal

counter wheels for its main memory! It could store seventy#two 3abba"e4s dream!

/+ectronic Computer#

A mechanical computer has two serious drawbacs: Its computin" speed is limited by

the inertia of its movin" parts, and the transmission of di"ital information by mechanical means

is +uite unreliable! In an electronic computer, on the other hand, the Cmovin" partsD are

electrons, which can be transmitted and processed reliably at speeds approachin" that of li"ht

(


13/29

placed in the 2&GA%4s main memory! The instructions were then transferred one at a time from

the main memory to the %P for e'ecution! 2ach instruction had a well#defined structure of the

form,

A1 A A4 A O&

The above instruction means, Perform the operation *P (addition, multiplication, etc!) on thecontents of main memory locations or CaddressesD A0 and A3 and then place the result in memory

location A

variant of this instruction format implements conditional branchin", where the ne't instruction

address is either A$, which had earlier constructed the Karvard $ar I,

introduced its first electronic stored#pro"ram computer, the E:1, in 0;8esides their use of

vacuum tubes in the %P, first#"eneration computers e'perimented with various technolo"ies for

main and secondary memory! The @hir+win* introduced the ferrite#core memory in which a bit

of information was stored in ma"netic form on a tiny rin" of ma"netic material! Herrite cores

remained the principal technolo"y for main memories until the 0;O-s!

The earliest computers had their instructions written in a binary code nown as

machine +anguage that could be e'ecuted directly! $achine#lan"ua"e pro"rams are e'tremely

difficult for humans to write and so are very error#prone! A substantial improvement is obtained

by allowin" operations and operand addresses to be e'pressed in an easily understood symbolic

form called Aem%+" +anguage! Assemblers were used to convert Assembly lan"ua"e to

machine lan"ua"e before it can be e'ecuted!

The econ* generation#

%omputer hardware and software evolved rapidly after the introduction of the first

commercial computers around 0;8-! The vacuum tube +uicly "ave way to the transistor, which

was invented at >ell ?aboratories in 0;O, and a second "eneration of computers based on

transistors superseded the first "eneration of vacuum tube based machines! ?ie a vacuum tube, a

transistor serves as a hi"h#speed electronic switch for binary si"nals, but it is smaller, cheaper,

sturdier, and re+uires much less power than a vacuum tube! Similar pro"ress occurred in the field

of memory technolo"y, with ferrite cores becomin" the dominant technolo"y for main memories

until superseded by all#transistor memories in the 0;O-s! $a"netic diss became the principal


14/29

technolo"y for secondary memories, a position that they continue to hold! Hloatin" point number

formats and supportin" instructions were introduced in I-! E:? to facilitate numerical

processin"! It also introduced input#output processors (I*Ps), or channels, which are special#

purpose processin" units desi"ned e'clusively to control I* operations! Another important

development was the introduction of CKi"h levelD pro"rammin" lan"ua"es, which are far easier

to use than Assembly lan"ua"es! Ki"h level lan"ua"es such as %*>*?, H*=T=AN were

developed durin" second "eneration! In the early days, all pro"rams or Bobs were run separately,

and the computer had to be halted and prepared manually for each new pro"ram to be e'ecuted,

this mode of system mana"ement is termed batch processin"! ?ater operatin" systems were

desi"ned to enable a sin"le %P to process a set of independent user pro"rams concurrently, a

techni+ue called multipro"rammin"! It reco"ni.es that a typical pro"ram alternates between

pro"ram e'ecutions! $ultipro"rammin" attempts to eep a %P and any available I*Ps busy by

overlappin" %P and I* operations!

The thir* generation#

Third "eneration is traditionally associated with the introduction of inte"rated circuits

(I%s)! I%s allowed lar"e numbers of transistors and associated components to be combined on a

tiny piece of semiconductor material, usually silicon! Perhaps the most si"nificant event of the

third#"eneration period was reco"nition of the need to standardi.e computers in order to allow

software to be developed and used more efficiently! I>$ developed what was to be the most

influential third#"eneration computer, the System


15/29

typical#limited hardware and software facilities, and small physical si.e, $ost important, their

low cost made them suitable for many new applications, such as the industrial process control

where a computer is permanently assi"ned to one particular application! The &i"ital GA9 series

of minicomputers introduced in 0;O7 brou"ht "eneral#purpose computin" to many small

or"ani.ations that could not afford the hi"h cost of a mainframe computer!

6BSI /ra#

Since the 0;1-s the dominant technolo"y for manufacturin" computer lo"ic and

memory circuits has been the inte"rated circuit or I%! This technolo"y has evolved steadily from

I%s containin" Bust a few transistors to those containin" thousands or millions of transistors@ the

latter case is termed very lar"e#scale inte"ration or G?SI! G?SI allows manufacturers to fabricate

a %P, main memory, or even all the electronic circuits of a computer, on a sin"le I% that can be

mass#produced at very low cost! This has resulted in new classes of machines ran"in" from

portable personal computers to supercomputers that contain thousands of %Ps!

The inte"rated circuit was invented in 0;8; at Te'as Instruments and Hairchild

%orporations! It +uicly became the basic buildin" bloc for computers of the third and

subse+uent "enerations! An I% is an electronic circuit composed mainly of transistors that is

manufactured in a tiny rectan"le or chip of semiconductor material! The I% is mounted into a

protective plastic or ceramic paca"e, which provides electrical connection points called pins or

leads that allow the I% to be connected to other I%s, to input#output devices lie a eypad or

screen, or to a power supply! An inte"rated circuit is rou"hly characteri.ed by its density, defined

as theQ number of transistors contained in the chip! As manufacturin" techni+ues improved over

the years, the si.e of the transistors in an I% and their interconnectin" wires shran! The earliest

I%s contained fewer than 0-- transistors and employed small#scale inte"ration or SSI, The terms

medium#scale, lar"e#scale, and very#lar"e scale inte"ration ($SI, ?SI and G?SI) are applied to

I%s containin" hundreds, thousands, and millions of transistors, respectively! The boundaries

between these I% classes are loose, and G?SI often serves as a catchall term for very dense

circuits!

Around 0;O- it became possible to manufacture all the electronic circuits for a pocet

calculator on a sin"le I% chip! This development was +uicly followed by sin"le#chip &=A$S

and microprocessors! The first microprocessor, Intel4s --, which was introduced in 0;O0, was

desi"ned to process #bit words! >y 0;;- manufacturers could fabricate the entire %P of a

System


16/29

CPU

Main Memory

M

Instructions

Data

CPU

Main Memory

MM

Cache

Memory

CM

Instructions

Data

External Memory M

The Primary function of %P is to e'ecute se+uences of instructions, that is, pro"rams

which are stored in an e'ternal memory! The pro"ram e'ecution is therefore carried as follows:

0! The %P transfers the instructions and, their input data (*perands) from main memory to

re"isters in the %P!

3! The %P e'ecutes the instructions in their stored se+uence e'cept when the e'ecution se+uence is

e'plicitly altered by a branch instruction!

(=A$)! The %P is si"nificantly faster than $, that is, the read or write to %P re"isters is 8 to

0- times faster than it can read or write to a =A$!

$igure 1.1 &roceor memor" communication without Cache

To remedy this situation, many computers have a cache memory %$ positioned between

the %P and main memory as in figre !."!

$igure 1. &roceor memor" communication without Cache

The cache %$ is smaller and faster than main memory and may reside, wholly or in part,

on the same chip as the %P! The cache is desi"ned to be transparent to the %P4s instructions

and it permits the %P to perform a memory load or store operation in a sin"le cloc cycle!

The %P communicates with the I* devices in the same way as it communicates with the

main memory! The I* devices are associated with addressable re"isters to which the %P can

load or store a word! If the I* data transfers are implemented by memory#referencin"

instructions, it is called as memor#-ma$$ed I%! Some computers employ I* instructions that are

distinct from memory referencin" instructions! These instructions produce control si"nals to

which I* ports, but not memory locations, respond! The second approach is sometimes called as I%-ma$$ed I%

c( Uer an* Supervior mo*e#

The pro"rams e'ecuted by "eneral purpose computers fall into two broad "roups: user

pro"rams and supervisor pro"rams! A user or application pro"ram handles a specific application,

such as word processin"! A supervisor pro"ram, on the other hand, mana"es various routine

aspects of computer system on behalf of its users@ it is typically part of the computer4s operatin"

InstructiData


17/29

system! 2'amples of supervisory functions are transferrin" data between secondary and main

memory! In normal operation the %P continually switches between bac and forth between user

and supervisor pro"rams!

The %P is desi"ned such that it can receive re+uests for supervisor services directly

from secondary memory units and other I* devices! Such a re+uest is called an interr$t. Ehen

an Interrupt occurs, the %P suspends the current e'ecution and transfers to an interrupt handlin"

pro"ram called Interrupt Service =outine!

*( C&U operation#

$igure 1.4 Overview of C&U %ehavior

The flowchart in fi"ure 0!< summari.es the main functions of a %P! The se+uence of

operations performed by the %P in processin" an instruction constitutes an instrction c#cle! Ehile

the details of the instruction cycle may vary with the type of instruction, all instructions re+uire two

maBor steps: a fetch step durin" which a new instruction is read from the e'ternal memory $ and an

e'ecute step durin" which the operations specified by the instruction are e'ecuted!

e( Accumu+ator=%ae* C&U#

The %P or"ani.ation proposed by Gon Neumann and his collea"ues for the IAS computer is the

basis for most subse+uent desi"ns! It comprises a small set of re"isters and the circuits needed to

e'ecute a functionally complete set of instructions! In many early desi"ns, one of the %P re"isters,

the accumulator played a central role, bein" used to store an input or output operand in the e'ecution

of many instructions!

The instructions are fetched by the pro"ram control unit P%, whose main re"ister is

the pro"ram counter P%! They are e'ecuted in the data processin" unit &P, which contains an n#bit


18/29

arithmetic#lo"ic unit (A?) and two data re"isters A% and &=! $ost instructions perform the

operation of the form

90:6 f i (90, 93)

Ehere 90 and 93 denote %P re"isters (A%, &=, or P%) or an e'ternal memory

location $(adr)! The operation f i performed by the A? are addition, subtraction, shiftin" and lo"icaloperations! $emory addresses are stored in two address re"isters in the P%: the pro"ram counter

P%, which stores the address of the ne't instruction to be e'ecuted, and the "eneral purpose addressre"ister A=! The instruction fetched from memory is placed in Instruction =e"ister (I=)!

The two essential memory addressin" instructions are the load and store!

The load instruction for our sample %P is

AC#;!'a*r(

This transfers a word from the memory location with address adr to the accumulator!

The correspondin" store instruction is

!'a*r(#;AC

This transfers a word from the accumulator to a memory location! The correspondin"

store instruction is

f( Architecture e0tenion#

There are many ways in which the basic desi"n of accumulator based %P can beimproved! $ost %P4s contain the followin" e'tensions, which si"nificantly increases their

performance and ease of pro"rammin"!

i( !u+tipurpoe regiter et for toring *ata an* a**ree#

There are additional re"isters for storin" instructions and data! These are called as

"eneral purpose re"isters! The third "eneration I>$


19/29

$igure 1. A t"pica+ C&U with genera+ regiter organization

g( Intruction Set#

$igure 1. Intruction et for the genera+ C&U organization

The above fi"ure "ives the possible instruction set for our simple accumulator based %P!

The above 0- instructions have the flavor of the instruction sets of some recent =IS% machines,

which demonstrate that small instruction sets can be both complete and efficient!

Type

Program

DataProcess

DataTransf


20/29

E. Draw an* e0p+ain the *ifferent t"pe of *ata repreentation.

a( -aic $ormat#

The basic items of information handled by a computer are instructions and data! &ata can

be further subdivided into numerical and non#numerical! &ue to the importance of numericalcomputation, computer desi"ners have paid a "reat deal of attention to the representation of

numbers! Two main number formats have evolved fi'ed#point and floatin"#point! Non#numerical

data usually tae the form of variable#len"th character strin"s encoded in one of the severalcodes lie AS%II code!

The %aic information t"pe

%( @or* Bength#

Information is represented in a di"ital computer by means of binary words, where a

word is a unit of information of some fi'ed len"th n. An n- bit word allows up to 3n different

items to be represented! Hor e'ample, with n 6 ! Ee can encode the 0- decimal di"its asfollows:

: 6 ---- 1 6 ---0 6 --0- 4 6 --00 6 -0--

6 -0-0 F 6 -00- E 6 -000 6 0--- ? 6 0--0

To encode alphanumeric symbols or characters& 7#bit words called b#tes are commonlyused! As well as bein" able to encode all the standard eyboard symbols, a byte allows efficient

representation of decimal numbers that are encoded in binary! A byte can store two decimal di"its

with no wasted space! $ost computers have the 7#bit byte as the smallest addressable unit of information in their main memories! The %P also has a standard word si.e for the data it pro#

cesses. Eord si.e is typically a multiple of 7, common %P word si.es bein" 7, 01,


21/29

c( Storage or*er#

An important aspect of data representation is the way in which the bits of a word areinde'ed! Ee will usually follow the convention illustrated in the followin" Hi"ure, where the

ri"ht#most bit is assi"ned the inde' - and the bits are labeled in increasin" order from ri"ht to left!

The advanta"e of this convention is that when the word is interpreted as an unsi"ned binaryinte"er, the lower order inde'es correspond to the numerically less si"nificant bits and the hi"h#

order inde'es correspond to the numerically more si"nificant bits!

!S- BS-

Bitt+e=en*ian %"te format

The above word Ei can be written as > i,-, >i,0, >i,3, >i, i,- is assi"ned

the lowest address is called little-endian storage convention& it assi"ns the lowest address to byte-!The alternative stora"e scheme, in which most si"nificant byte > i,< of word Ei is assi"ned the

lowest address and the least si"nificant byte > i,- is assi"ned the hi"hest address is called big-

endian storage convention! In other words, the bi"#endian scheme assi"ns the hi"hest address to byte -!

!S- BS-

-ig=en*ian %"te format

*( Tag#

In the von Neumann computer, instruction and data words are stored

to"ether in main memory and are indistin"uishable from one anotherRthis is

called the stored pro"ram concept! An item pluced at random from memory cannot be identified as an instruction or data! &ifferent data types such as fi'ed#point and

floatin"#point numbers also cannot be distin"uished by inspection! To identify the

information types, a "roup of bits are associated with each information word is called a

ta"4 that identifies the word4s type! The ta" may be considered as a physicalimplementation of type information found in some hi"h#level pro"rammin" lan"ua"es!

*ne of the earliest machines to use ta"s was >urrou"hs >18--O8-- series, whichemployed a


22/29

Tagge* wor* format of -urrough -F::HE:: erie

In selectin" a number representation to be used in a computer, the followin" factors should be

taen into account:

• The number types to be represented@ for e'ample, inte"ers or real numbers!

•The ran"e of values (number ma"nitudes) liely to be encountered!

• The precision of the numbers, which refers to the ma'imum accuracy of the representation

• The costs of the hardware re+uired to store and process the numbers!

The two principal number formats are fi'ed point and floatin" point

$i0e*=&oint# Allow a limited ran"e of values and relatively have simple hardware

re+uirements!

$+oating &oint# Allow a much lar"er ran"e of values, but re+uires costly hardware!

$i0e* =&oint Num%er#

i( -inar" Num%er# The fi'ed#point format is derived directly from the ordinary decimal representation of

a number as a se+uence of di"its separated by a decimal! The di"its to the left of the decimal

point represent an inte"er@ the di"its to ri"ht represent a fraction! This is $ositional notation in

which each di"it has a weight accordin" to its position relative to the decimal point! If i' 0, the ith

di"it to the left (ri"ht) of the decimal point has wei"ht 0- i#0 (0-#i)! Thus the five#di"it decimal

number 0;3!O< is e+uivalent to

0 ' 0-35; '0-053 ' 0--5 O '0-#05< '0-#3

$ore "enerally, we can assi"n wei"hts of the form r i

& where r is the base or radix thenumber system, to each di"it! The most fundamental number representation used in computers

employs a base#two positional notation! A binary word of the form

b N U!b


23/29

=epresents the number ∑¿i= M

N

e bi"i

Suppose that an n#bit word is to contain a si"ned binary number, one bit is reserved to represent

the si"n of the number, while the remainin" indicates its ma"nitude!

ii( Signe* Num%er#

a( Signe* !agnitu*e Repreentation#

Suppose that both positive and ne"ative binary numbers are to be represented by an

n#bit word 'n#0 'n#3 'n#


24/29

desi"ned for unsi"ned numbers! $ultiplication and division are more difficult to

implement if twos#complement code is used instead of si"n ma"nitude!

/0ceptiona+ con*ition#

If the result of an arithmetic operation involvin" numbers is too lar"e or small to be

represented by n bits, overflow or nderflow is said to occur! It is "enerally necessary to

detect overflow and underflow, since they may indicate bad data or a pro"rammin" error!

*verflow can occur only when addin" two numbers of the same si"n!

A related issue in computer arithmetic is rond-off error& which results from the fact that

every number must be represented by a limited number of bits! An operation involvin" n#

bit numbers fre+uently produces a result of more than n bits! =etainin" the n most

si"nificant bits of the result without modification is called trncation. %learly the

resultin" number is in error by the amount of the discarded di"its! This error can be

reduced by a process called ronding. *ne way of roundin" is to add r B3 to the number

before truncation, where r * is the wei"ht of the least si"nificant retained di"it! Hor

instance, to round -!


25/29

iv( 2e0a*ecima+ Co*e#

The he'adecimal number format uses a base of radi' r 601 and the use of 01

di"its, consistin" of decimal di"its -U!; and numerical values 0-, 00, 03, 0, %, &, 2 and H!

Hor e'ample the unsi"ned he'adecimal number 3HA-% has the interpretation

63 ' 01 5 H ' 012

am'le:

0!- ' 0-07

0!-# $antissa

07#e'ponent

0-#baseHloatin" Point number is stored as a word ($,2) consistin" of a pair of si"ned

fi'ed point number a mantissa $, which is usually a fraction or an inte"er and 2

which is an inte"er!2'tra mantissa di"its are included in a floatin" point processin" circuits# These

are called "uard bits to reduce appro'imation error! Luard bits are removed

automatically from the end result!

%( Norma+ization an* %iaing#

Norma+ize*#The mantissa is said to be normali.ed, if the di"it to the ri"htof the radi' point is not .ero! Normali.ation restricts the ma"nitude W$W of a

fractional binary mantissa to the ran"e

X Y W$W Z 0


26/29

-ia# The floatin" point e'ponents should be coded in e'cess code where

the e'ponent field 2 contains an inte"er i!e!, desired e'ponent value plus ! The

+uantity / is called bias, and the e'ponent encoded in this way is called a biasede'ponent or characteristic!

c( Stan*ar*#

Institute of 2lectrical and 2lectronics 2n"ineers (I222) sponsored a

standard format for


27/29

Data and ontrol:

Sing+e function circuit

A simple re"ister#level system shown above forms a sin"le action, in this case, the add

operation :6 A 5 >!

$ore complicated systems perform several different operations! Such a multifunction

system is "enerally partitioned into a data#processin" part, called a datapath, and a controllin"

part, the control unit, which is responsible for selectin" and controllin" the actions of the

datapath!


28/29

!u+tifunction circuit

The e'ample shown in the above fi"ure, control unit % selects the operation (add, shift,

and so on) for the A? to perform in each cloc cycle! It also determines the input operands to

apply to the A? and the destination of its results! It is easy to see that this circuit has the

connection paths necessary to perform the followin" data#processin" operations, as well as manyothers!

:6 A5>@

> :6 A#>@

?ess obvious operations that can be performed are the simple data transfer :6 >, which

is implemented as :6 - 5 >@ the clear operation > :6 -, which is implemented as > :6 > # >@

and the ne"ation operation > :6 - # >!

2ach of the fore"oin" operations re+uires % to send specific control si"nals, indicated

by dashed lines in the above fi"ure, to various places in the datapath! Hor instance, to e'ecute the

subtraction :6 A # >, the controller % must send select si"nals to the A? to select its subtractfunction@ it must send select si"nals to the multiple'er that connects re"ister A to the A?4s left

port@ and it must send a Cload dataD control si"nal to the output re"ister !

An e'ample of a lar"e multifunction system is a computer4s %P! Its control unit, which

is responsible for the interpretation of instructions, is called the pro"ram control unit or I#unit!

The %P4s datapath is also called the 2#unit!

A descri'tion language:

2DB '2ar*ware Decription Banguage() provide both behavioral and structural

descriptions at the re"ister level! There is often a close correspondence between the elements of

an K&? description and hardware components and si"nals in the system bein" described! Hor

e'ample, the statement :6 A 5 > describes the circuit of a sin"le function! In this interpretation,

5 represents the adder! The input connections to the adder from re"isters A and > are inferred

from the fact that A and > are the ar"uments of 5, while the output connection from the adder to

re"ister is inferred from :6!

Design %ec#ni*ues:

Liven a set of al"orithms in instructions, to desi"n a circuit usin" a specified set of

re"ister#level components while satisfyin" certain cost and performance criteria uses a heuristic

approach due to lac of appropriate mathematical tools

The followin" "eneral approach can be followed to the re"ister#level desi"n problem,

Step1: &efine the desired behavior by a set of se+uences of re"ister#transfer operations, such that

each operation can be implemented directly usin" the available desi"n components! This

constitutes an algorithm +, to be e'ecuted


29/29

Step: Analy.e +, to determine the type of components and the member of each type re+uired

for the data path DP !

Step4: %onstruct a bloc dia"ram for DP usin" the components identified in step 3! $ae the

connections between the components so that all data paths implied by +, are present and the

"iven performance#cost constraints are met!

Step: Analy.e +, and DP to identify the control si"nals needed! Introduce into DP the lo"ic

or control points necessary to apply these si"nals!

Step: &esi"n the control unit C for DP that meets all the re+uirements of +,.

StepF: Gerify, typically by computer simulation, that the final desi"n operates correctly and

meets all performance#cost "oals

UNIT I EC2303 Computer Architecture

Documents

Transcript of UNIT I EC2303 Computer Architecture