Computer Organization: Instruction Sets and Addressing Modes, Slides of Architecture

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William Stallings Computer Organization and Architecture 7 th^ Edition

Chapter 11 Instruction Sets: Addressing Modes and Formats

Addressing Modes

• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Displacement (Indexed)
• Stack

Immediate Addressing Diagram

Opcode Operand

Instruction

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 (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 operand
• e.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 length
• May be nested, multilevel, cascaded —e.g. EA = (((A))) - Draw the diagram yourself
• Multiple memory accesses to find operand
• Hence slower

Register Addressing (1)

• 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 Addressing (2)

• No memory access
• Very fast execution
• Very limited address space
• Multiple registers helps performance —Requires good assembly programming or compiler writing —N.B. C programming - register int a;
• c.f. Direct addressing

Register Indirect Addressing

• C.f. 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

Register Indirect Addressing Diagram

Opcode Register Address R

Instruction Memory

Pointer to Operand Operand

Registers

Displacement Addressing Diagram

Opcode Register R

Instruction

Memory

Pointer to Operand Operand

Registers

Address A

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
• c.f locality of reference & cache usage

Indexed Addressing

• A = base
• R = displacement
• EA = A + R
• Good for accessing arrays —EA = A + R —R++

Combinations

• Postindex
• EA = (A) + (R)
• Preindex
• EA = (A+(R))
• (Draw the diagrams)