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William Stallings

Computer Organizationand Architecture

Chapter 9Instruction Sets:Characteristics

and Functions

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What is an instruction set?

The complete collection of instructions that areunderstood by a CPU

Machine CodeBinaryUsually represented by assembly codes

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Elements of an Instruction

Operation code (Op code)Do this

Source Operand referenceTo this

Result Operand referencePut the answer here

Next Instruction ReferenceWhen you have done that, do this...

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Where have all the Operandsgone?

Long time passing. (If you dont understand, youre too young!)

Main memory (or virtual memory or cache)CPU registerI/O device

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Instruction Representation

In machine code each instruction has a uniquebit pattern

For human consumption (well, programmersanyway) a symbolic representation is usede.g. ADD, SUB, LOAD

Operands can also be represented in this way ADD A,B

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Instruction Types

Data processingData storage (main memory)

Data movement (I/O)Program flow control

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Number of Addresses (a)

3 addressesOperand 1, Operand 2, Result

a = b + c;May be a forth - next instruction (usually implicit)Not commonNeeds very long words to hold everything

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Number of Addresses (b)

2 addressesOne address doubles as operand and result

a = a + bReduces length of instructionRequires some extra work

Temporary storage to hold some results

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Number of Addresses (c)

1 addressImplicit second address

Usually a register (accumulator)Common on early machines

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Number of Addresses (d)

0 (zero) addresses All addresses implicit

Uses a stack e.g. push apush badd

pop c

c = a + b

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How Many Addresses

More addressesMore complex (powerful?) instructions

More registersInter-register operations are quickerFewer instructions per program

Fewer addresses

Less complex (powerful?) instructionsMore instructions per programFaster fetch/execution of instructions

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Design Decisions (1)

Operation repertoireHow many ops?

What can they do?How complex are they?

Data typesInstruction formats

Length of op code fieldNumber of addresses

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Design Decisions (2)

RegistersNumber of CPU registers available

Which operations can be performed on whichregisters?

Addressing modes (later)

RISC v CISC

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Types of Operand

AddressesNumbers

Integer/floating pointCharacters

ASCII etc.

Logical DataBits or flags

(Aside: Is there any difference between numbers and characters? Ask a C programmer!)

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Pentium Data Types

8 bit Byte16 bit word

32 bit double word64 bit quad word

Addressing is by 8 bit unit

A 32 bit double word is read at addressesdivisible by 4

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Specific Data Types

General - arbitrary binary contentsInteger - single binary valueOrdinal - unsigned integerUnpacked BCD - One digit per bytePacked BCD - 2 BCD digits per byteNear Pointer - 32 bit offset within segment

Bit fieldByte StringFloating Point

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Pentium Floating Point DataTypes

See Stallings p324

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Types of Operation

Data Transfer Arithmetic

LogicalConversionI/O

System ControlTransfer of Control

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Data Transfer

SpecifySourceDestination

Amount of data

May be different instructions for differentmovements

e.g. IBM 370Or one instruction and different addresses

e.g. VAX

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Arithmetic

Add, Subtract, Multiply, DivideSigned Integer

Floating point ?May include

Increment (a++)Decrement (a--)Negate (-a)

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Logical

Bitwise operations AND, OR, NOT

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Conversion

E.g. Binary to Decimal

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Input/Output

May be specific instructionsMay be done using data movement instructions

(memory mapped)May be done by a separate controller (DMA)

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Systems Control

Privileged instructionsCPU needs to be in specific state

Ring 0 on 80386+Kernel mode

For operating systems use

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Transfer of Control

Branche.g. branch to x if result is zero

Skipe.g. increment and skip if zeroISZ Register1Branch xxxx

ADD A Subroutine call

c.f. interrupt call

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Foreground Reading

Pentium and PowerPC operation typesStallings p338 et. Seq.

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Byte Order (A portion of chips?)

What order do we read numbers that occupymore than one byte

e.g. (numbers in hex to make it easy to read)12345678 can be stored in 4x8bit locations asfollows

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Byte Order (example)

Address Value (1) Value(2)184 12 78

185 34 56186 56 34186 78 12

i.e. read top down or bottom up?

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Byte Order Names

The problem is called EndianThe system on the left has the least significant

byte in the lowest addressThis is called big-endianThe system on the right has the leastsignificant byte in the highest addressThis is called little-endian

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StandardWhat Standard?

Pentium (80x86), VAX are little-endianIBM 370, Moterola 680x0 (Mac), and most RISC

are big-endianInternet is big-endianMakes writing Internet programs on PC moreawkward!

WinSock provides htoi and itoh (Host to Internet & Internet to Host) functions to convert