Processor Organization and Architecture Module III

Addressing Modes How to specify operands The different modes are: – Immediate – Direct – Indirect – Register – Register Indirect – Displacement (Indexed) – Stack

Immediate Addressing Operand is part of instruction e.g. ADD 5 – Add 5 to contents of accumulator – 5 is operand Adv: – No memory reference to fetch data – Fast Disadv: – Limited range

Immediate Addressing OperandOpcode Instruction

Direct Addressing Address field contains effective address of operand e.g. ADD A – Look in memory at address A for operand Adv: – Single memory reference to access data – No additional calculations to work out effective address Disadv: Limited address space

Direct Addressing Address AOpcode Instruction Memory Operand

Indirect Addressing 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 operand e.g. ADD (A) – Add contents of cell pointed to by contents of A to accumulator

Indirect Addressing Adv: – Large address space – 2 n where n = word length – May be nested, multilevel, cascaded e.g. EA = (((A))) Disadv: Multiple memory accesses to find operand and hence slower

Indirect Addressing Address AOpcode Instruction Memory Operand Pointer to operand

Register Addressing Address field refers to a register EA = R Adv: – Only a small address field is needed in the instruction – Less access time Disadv: – Limited number of registers – Limited address space

Register Addressing Register Address ROpcode Instruction Registers Operand

Register Indirect Addressing Similar to indirect addressing EA = (R) Operand is in memory cell pointed to by contents of register R One fewer memory access than indirect addressing

Register Indirect Addressing Register Address ROpcode Instruction Memory Operand Pointer to Operand Registers

Displacement Addressing Combines direct and register indirect addressing EA = A + (R) Address field hold two values – A = base value – R = register that holds displacement – or vice versa

Displacement Addressing Register ROpcode Instruction Memory Operand Pointer to Operand Registers Address A +

Uses of Displacement Addressing Relative Addressing Base-Register Addressing Indexed Addressing

Relative Addressing R = Program counter, PC EA = A + (PC) i.e. get operand from A cells from current location pointed to by PC Exploits locality of reference If memory references are near to the instruction being executed, this saves address bits in the instruction

Base-Register Addressing The referenced register (R) may contain – A main memory address – Displacement from that address R may be explicit or implicit Exploits locality of reference

Indexed Addressing just opposite of base register addressing Address field contains main memory address A = base Register contains displacement R = displacement EA = A + R Good for accessing arrays

Stack Addressing Stack is a reserved block of locations Stack pointer holds the address of the top of the stack. It is a form of implied addressing: as no memory reference is required but operate on the top of the stack

Instruction Formats Defines the layout of bits in an instruction Includes opcode & (implicit or explicit) operand(s) Usually more than one instruction format in an instruction set

Common Instruction Formats Four of them are a)Zero-address instruction b)One-address instruction c)Two-address instruction d)Three-address instruction

Zero-address instruction 0 (zero) addresses – All addresses implicit – Example: stack push a push b pop c

One-address instruction 1 address – Implicit second address : Usually a register (accumulator) – E.g. Add C // C=C+A(accumulator)

Two-address instruction 2 addresses – One address doubles as operand and result – a = a + b – Reduces length of instruction – Requires temporary storage to hold some results

Three-address instruction 3 addresses – Operand 1, Operand 2, Result – a = b + c; – May be a forth - usually implicit – Not common – Needs very long words to hold everything