Machine Instruction CharacteristicsMachine Instructions- are instructions

executed by the processor.

Instruction set- a collection of instructions that the processor can execute

Elements of a machine instructionOpcode- uses a binary code which specifies

the operation to be preformed.Source operand reference- the input for the

operationResult operand reference- the result

produced by the operationNext instruction reference- tells the

processor where to get the next instruction

Source and result operands can be found inMain or virtual memoryProcessor register- contains registers that

can be used by machine instructions Immediate- the operand value is being

contained when the instruction is being executed

I/O devices- instruction specifies I/O module and device but if memory mapped then just another main or virtual memory address

Representation of instructionRepresentation of instruction

OPCODE (4 Bits)

Ref to Operand 1 (6 Bits)

Ref to Operand 2(6 Bits)

• An instruction format is used to define the layout of the bits allocated to these elements of instructions.• The instruction format explicitly or implicitly indicates the addressing Modes used for each operand in that instruction.

Instruction formatInstruction format

Instruction length: -Instruction length: - The longer instruction length gives: More Opcodes More operands More addressing modes Greater address range.

Factors considered for instruction lengthFactors considered for instruction length

Memory Size.Memory organization.Bus structureProcessor speedProcessor complexity.

Allocation of Bits for different fields in an Allocation of Bits for different fields in an instructioninstructionNumber of addressing modes.Number of operands.Register versus memory.Number of register sets.Address range.Address granularity.

Variable length instructionsVariable length instructions

Advantages: -Provides flexibility in addressing modes.Provides complex addressing modes.Reduces number of instruction fetch cycles.Reduces amount of space taken by a program.Disadvantages: -Disadvantages: -Requires more complex CPU.Decoding of instruction becomes difficult.

Instruction SetsInstruction Characteristics

Types of Operands

Types of Operations

Instruction Representation Are represented by a sequence of bits It is common to use “symbolic

representation” of machine instructions Op codes and operands are represented by

abbreviations called “mnemonics”

Mnemonics ADD addSUB subtractMUL multiplyDIV divideLOAD load data from memorySTOR store data to memory

Instruction typesData processing- arithmetic and logic

instructionsData storage- moving data into or out of

register or memory locationsData movement- I/O instructions Control- test and branch instructions

Number of Addresses• A traditional way of describing processor

architecture• What is the maximum number of addresses

one might need in an instruction?• An instruction could be required to contain

four addresses

Number of Addresses• 3 addresses

instructions• Operand 1,

Operand 2, Result• a = b + c;• May be a forth -

next instruction (usually implicit)

• Not common• Needs very long

words to hold everything

Number of Addresses• 2 addresses

instructions• One address doubles

as operand and result

• a = a + b;• Reduces length of

instruction• Requires some extra

work• Temporary storage to

hold some results

Number of Addresses• 1 address

instructions• Implicit second

address• Usually a register

(accumulator)• Common on early

machines• E.g. ADD B

Number of Addresses• Zero address instruction:

• Special memory organization

• Last in first out (Stack)

• Stack is a known location

• E.g. CMA

Number of AddressesUtilization of Instruction Addresses

(Nonbranching Instructions)

Number of Addresses1 address instructions 2 Or 3 address

instructions• One general purpose

registerMultiple general

purpose registers

Instruction Set DesignOperation Repertoire- How many and which

operations to provide, and how complex the operations should be.

Data Types- The various types of data upon which operations are performed.

Instruction Format- Instruction length (in bits), number of addresses and size of fields.

Instruction Set DesignRegisters- Number of processor registers

that can be referenced by instructions, and their use

Addressing- The mode or modes by which the address of an operand is specified.

TYPES OF OPERANDS Addresses Numbers Characters Logical Data Specialized Data Types Data Structures

Addresses Main or Virtual memory address Unsigned Integers

NumbersAll machine languages include numeric typesNon-numeric processing: counters, field

widths, etcNumbers in a machine are limited by

magnitude and precisionConsequences: rounding, overflow, underflow

Numerical DataBinary integer or Binary fixed pointBinary floating pointDecimal

DecimalMajority of users deal with decimalsNecessity to convert from decimal to binary

on input and binary to decimal on outputPacked decimal

Packed DecimalUsed when there is a great deal of I/O and

simple computationEach decimal digit is represented by a 4-bit

code. Ex: 0 = 0000, 1 = 0001Numbers are formed by a string of 4-bit

codes usually in multiples of 8-bit246 = 0000 0010 0100 0110Positive: 1100Negative: 1101

CharactersText or character stringsRepresented by a sequence of bitsMorse CodeInternational Reference Alphabet (IRA)American Standard Code for Information

Interchange (ASCII)Extended Binary Coded Decimal Interchange

Code (EBCDIC)

Logical DataConsider n-bit unit as consisting of n 1-bit

items of data, each having a value of 0 or 1Advantages:

Store array of Boolean or Binary data itemsManipulating bits of data items

Types of OperationsData TransferArithmeticLogicalConversionInput/outputSystem ControlTransfer of Control

Data TransferOperation Name DescriptionMove(Transfer)StoreLoad (Fetch)ExchangeClear ( Reset)Set PushPop

Transfer word from source to destination

From processor to memory From memory to processor Swap contents of source

and destination Transfer 0s to destination Transfer 1s to destination Word from source to top of

stack From top of stack to

destination

Arithmetic

Operation Name DescriptionAddSubtractMultiply DivideAbsoluteNegateIncrementDecrement

Compute sum of two operands

Difference of two operands

Product of two operands Quotient of two

operands Replace by its absolute

value Change sign Add 1 to operand Subtract 1 to operand

LogicalOperation Name DescriptionAND; OR; NOT; ORTestCompareSet Control VariablesShift Variables

Perform the specified logical operation bitwise

Test specified condition; set flags based on outcome

Make logical and arithmetic comparison of two operands

Class of instructions to set controls for protection purposes, interrupt handling, timer control.

Left/right operand introducing constants at end.

Left/right with wraparound end

Conversion

Operation Name DescriptionTranslate Convert

Translate values in a section of memory based on a table of correspondences

Convert the contents of a word from one form to another. E.g(packed decimal to binary

I/OOperation Name DescriptionInput (Read)Output ( Write)Start I/OEnd I/O

Transfer data from specified I/O port or device to destination

From specified source to I/O port or device

Transfer instructions to I/O processor to initiate I/O operation

Transfer status info from I/O system to specified destination

System Of Control Can be executed only while the processor is in a

certain privileged state or is executing a program in a special privileged area of memory.

These instructions, are reserved for the use of the operating system.

Examples: A system control instruction may read or alter a

control register An instruction to read or modify a storage

protection key Access to process control blocks in

multiprogramming system

Transfer Of Control

Operation Name

Description

Jump (Branch) Jump Conditional Jump to Subroutine Return Execute Skip Skip conditional Halt Wait (Hold) No Operation

Unconditional Transfer: load PC with specified address

Test condition: either load PC with specified address or do nothing.

Place current program control info in known location; jump to specified address

Replace contents of PC and other register from known location

Fetch operand from location and execute as instructed

Increment PC to skip next instruction

Either skip or do nothing based on condition

Stop program execution Test specified repeatedly and

resume condition No operation is performed; but

program execution is continued.

QUESTIONSHow many different address designs are there?Name two types of operations.What are the most general categories of data?What type of endian mode does x86 use?What are the most used Character Codes

nowadays?What are the elements of machine instruction?What are the advantages of Logical Data?