11Instruction Sets:

Addressing Modes and Formats

Computer Organization

Addressing ModesImmediateDirectIndirectRegisterRegister IndirectDisplacement (Indexed) Stack

Immediate AddressingOperand is part of instructionOperand = address fielde.g. ADD 5

Add 5 to contents of accumulator5 is operand

No memory reference to fetch dataFastLimited range

Immediate Addressing Diagram

OperandOpcode

Instruction

Direct AddressingAddress field contains address of operandEffective address (EA) = address field (A)e.g. ADD A

Add contents of cell A to accumulatorLook in memory at address A for operand

Single memory reference to access dataNo additional calculations to work out

effective addressLimited address space

Direct Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Indirect Addressing (1)Memory cell pointed to by address field

contains the address of (pointer to) the operand

EA = (A)Look in A, find address (A) and look there for

operande.g. ADD (A)

Add contents of cell pointed to by contents of A to accumulator

Indirect Addressing (2)Large address space 2n where n = word lengthMay be nested, multilevel, cascaded

e.g. EA = (((A)))Draw the diagram yourself

Multiple memory accesses to find operandHence slower

Indirect Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Pointer to operand

Register Addressing (1)Operand is held in register named in

address filedEA = RLimited number of registersVery small address field needed

Shorter instructionsFaster instruction fetch

Register Addressing (2)No memory accessVery fast executionVery limited address spaceMultiple registers helps performance

Requires good assembly programming or compiler writing

N.B. C programming register int a;

c.f. Direct addressing

Register Addressing Diagram

Register Address ROpcode

Instruction

Registers

Operand

Register Indirect AddressingC.f. indirect addressingEA = (R)Operand is in memory cell pointed to by

contents of register RLarge address space (2n)One fewer memory access than indirect

addressing

Register Indirect Addressing Diagram

Register Address ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Displacement AddressingEA = A + (R)Address field hold two values

A = base valueR = register that holds displacementor vice versa

Displacement Addressing Diagram

Register ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Address A

+

Relative AddressingA version of displacement addressingR = Program counter, PCEA = A + (PC)i.e. get operand from A cells from current

location pointed to by PCc.f locality of reference & cache usage

Base-Register AddressingA holds displacementR holds pointer to base addressR may be explicit or implicite.g. segment registers in 80x86

Indexed AddressingA = baseR = displacementEA = A + RGood for accessing arrays

EA = A + RR++

CombinationsPostindexEA = (A) + (R)

PreindexEA = (A+(R))

(Draw the diagrams)

Stack AddressingOperand is (implicitly) on top of stacke.g.

ADD Pop top two items from stackand add

Addressing Modes in 8086 Direct (??)

Mov bx, [200] Register Indirect (??)

Add ax,[bx] Based/Indexed

Mov ax,[si+2] Sub cx,2[di] Base-indexed

Mov ax,[bx+si] add bx,[bp][di] Base-indexed with displacement

Mov ax,[bx+si+2]

Pentium Addressing ModesVirtual or effective address is offset into

segmentStarting address plus offset gives linear addressThis goes through page translation if paging enabled

12 addressing modes availableImmediateRegister operandDisplacementBaseBase with displacementScaled index with displacementBase with index and displacementBase scaled index with displacementRelative

Pentium Addressing Mode Calculation

PowerPC Addressing ModesLoad/store architecture

Indirect Instruction includes 16 bit displacement to be added to base

register (may be GP register) Can replace base register content with new address

Indirect indexed Instruction references base register and index register (both

may be GP) EA is sum of contents

Branch addressAbsoluteRelative Indirect

ArithmeticOperands in registers or part of instructionFloating point is register only

PowerPC Memory Operand Addressing Modes

Instruction FormatsLayout of bits in an instructionIncludes opcodeIncludes (implicit or explicit) operand(s)Usually more than one instruction format in

an instruction set

Instruction LengthAffected by and affects:

Memory sizeMemory organizationBus structureCPU complexityCPU speed

Trade off between powerful instruction repertoire and saving space

Allocation of BitsNumber of addressing modesNumber of operandsRegister versus memoryNumber of register setsAddress rangeAddress granularity

PDP-8 Instruction Format

PDP-10 Instruction Format

PDP-11 Instruction Format

VAX Instruction Examples

Pentium Instruction Format

PowerPC Instruction Formats (1)

PowerPC Instruction Formats (2)