Addressing Modes

• Immediate


• Direct


• Indirect


• Register


• Register Indirect


• Displacement (Indexed)


• Stack

Immediate Addressing

• Operand is part of instruction


• Operand = address field


• e.g. ADD 5

—   Add 5 to contents of accumulator

—   5 is operand

• No memory reference to fetch data


• Fast


• Limited range

Direct Addressing

• Address field contains address of operand


• Effective address (EA) = address field (A)


• e.g.  ADD A

—   Add contents of cell A to accumulator

—   Look in memory at address A for operand

• Single memory reference to access data


• No additional calculations to work out effective address


• Limited address space

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

Register Addressing

• Operand is held in register named in address filed


• EA = R


• Limited number of registers


• Very small address field needed

—   Shorter instructions

—   Faster instruction fetch

Register Indirect Addressing

• EA = (R)


• Operand is in memory cell pointed to by contents of register R


• Large address space (2n)


• One fewer memory access than indirect addressing

Displacement Addressing

• EA = A + (R)


• Address field hold two values

—   A = base value

—   R = register that holds displacement

Relative Addressing

• A version of displacement addressing


• R = Program counter, PC


• EA = A + (PC)


• i.e. get operand from A cells from current location pointed to by PC


• locality of reference & cache usage

Base-Register Addressing

• A holds displacement


• R holds pointer to base address


• R may be explicit or implicit


• e.g. segment registers in 80x86

Indexed Addressing

• A = base


• R = displacement


• EA = A + R


• Good for accessing arrays

—   EA = A + R

—   R++

Instruction Formats

• Layout of bits in an instruction


• Includes opcode


• Includes (implicit or explicit) operand(s)


• Usually more than one instruction format in an instruction set

Instruction Length

• Affected by and affects:

—   Memory size

—   Memory organization

—   Bus structure

—   CPU complexity

—   CPU speed

• Trade off between powerful instruction repertoire and saving space