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 – 2n 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
OperandPointer 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
OperandPointer 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 area) Zero-address instructionb) One-address instructionc) Two-address instructiond) 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
Recommended
