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

• 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

Immediate Addressing Diagram

Operand

Opcode

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

Direct Addressing Diagram

Address A

Opcode

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 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

Indirect Addressing Diagram

Address A

Opcode

Instruction

Memory

Operand

Pointer to operand

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

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Register Addressing Diagram

Register Address R

Opcode

Instruction

Registers

Operand

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

Register Address R

Opcode

Instruction

Memory

Operand

Pointer to Operand

Registers

Displacement Addressing

• EA = A + (R)
• Address field hold two values
• A = base value
• R = register that holds displacement
• or vice versa

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Displacement Addressing Diagram

Register R

Opcode

Instruction

Memory

Operand

Pointer to 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

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Base-Register Addressing

• A holds displacement
• R holds pointer to base address
• R may be explicit or implicit
• e.g. segment registers in 80x86

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Indexed Addressing

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

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Combinations

• Postindex
• EA = (A) + (R)

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• Preindex
• EA = (A+(R))

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• (Draw the diagrams)

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Stack Addressing

• Operand is (implicitly) on top of stack
• e.g.
• ADD Pop top two items from stack and add

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Pentium Addressing Modes

• Virtual or effective address is offset into segment
• Starting address plus offset gives linear address
• This goes through page translation if paging enabled
• 12 addressing modes available
• Immediate
• Register operand
• Displacement
• Base
• Base with displacement
• Scaled index with displacement
• Base with index and displacement
• Base scaled index with displacement
• Relative

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Pentium Addressing Mode Calculation

PowerPC Addressing Modes

• Load/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 address
• Absolute
• Relative
• Indirect
• Arithmetic
• Operands in registers or part of instruction
• Floating point is register only

PowerPC Memory Operand �Addressing Modes

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

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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

Allocation of Bits

• Number of addressing modes
• Number of operands
• Register versus memory
• Number of register sets
• Address range
• Address granularity

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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)

Foreground Reading

• Stallings chapter 11
• Intel and PowerPC Web sites

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• Calculate the time cost for the simple code shown below (Memory time = 1 second,

Arithmetic operation execution time = 5 seconds,

Logical operation execution time = 3 seconds,

Data transfer operation execution time = 2 seconds,

decoding time = 3 seconds)

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• PUSH A
• PUSH B
• MUL
• SUB ((100))

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• What is the number of bits required to design an instruction format according to the following
• The number of instructions is 21 instructions
• We want to support 5 addressing modes (immediate, register, direct , indirect, displacement)
• The number of registers is 9 registers
• The memory size is 4 kilo byte

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