L23-AddressingModes

8/6/2019 L23-AddressingModes

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Addressing mode

In an instruction opcode & operands ( no of addresses)are mentioned.

Once we have determined the number of addresses

contained in an instruction, the manner in which each

address field specifies memory location must be

determined i.e. how to specify operands in an

instruction should be determined.

So addressing modes have been devised. These are the set of syntaxes or methods used to specify

a memory address in an instruction.


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Addressing mode

Common addressing techniques are:

Immediate

Direct

Indirect

Register

Register indirect

Displacement

Relative Base register

Indexing

stack


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Addressing mode

All computer architecture provide more than one ofthese addressing modes.

Different opcode use different addressing modes.

One or more bits in the instruction format can be usedas a mode field.

Value of mode field determines which addressing mode

is to be used.


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Immediate Addressing

Simplest form of addressing

The operand doesnt specifies address, but operand

value is present in the instruction.

Operand =A

No memory reference to fetch data

Fast, no memory access to bring the operand

Size of the operand (thus its range of values) is

limited


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Direct Addressing

EA = A

Instruction includes the a memory address

No calculations are required. Requires only one additional

memory reference to fetch the operand

Address is a constant at run time but data itself can be changed

during program execution

Provides a limited address space

Address AOpcode

Instruction Memory

Operand


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Indirect Addressing

Starts like the direct mode, but

it makes an extra memory

access. The address specified

in the instruction is not theaddress of the operand, it is the

address of a memory location

that contains the address of the

operand.

Address AOpcode

Instruction Memory

Pointer to operand

operand


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Two memory accesses are required

The first to fetch the effective address

The second to fetch the operand itself

EA=(A)


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Register Addressing

EA=R

It specifies a register instead a memory address

No memory access

Very fast execution

Very limited address space

Register Address ROpcode

Instruction Registers

Operand


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Register indirect Addressing

EA = ( R )

Register contains the address of the operand in the memory Register R , contains value which represents the address of

the operand in the memory

One fewer memory access than indirect addressing

Register Address ROpcode

Instruction Memory

Operand

Registers

Pointer to

operand


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Displacement addressing

Effective Address = A + (content of R)

Address field hold two values

A = base value

R = register that holds displacement

or vice versa

Register ROpcode

InstructionMemory

OperandPointer to

Operand

Registers

Address A

+


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Relative addressing

Also called as PC-relative addressing, as implicitly referenced registeris Program Counter (PC)

It is a particular case of the displacement addressing, where the

register is the program counter

Effective Address = A + (PC)

Thus Effective address is a displacement relative to the address of the

instruction

If memory references are relatively ear to the instruction being

executed, then the use of this addressing saves address bits in the

instruction.

E.g. Consider that the address A is 5 and the next instruction is at

location 12, so the operand is actually located at (12 +5) 17; the

instruction loads the operand at address 17 and stores it in the CPUs

accumulator


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Indexed addressing

Address field references a main memory address &referenced register contains a positive displacement

from that address.

A acts as abase R acts as displacement

Use of indexing is to provide an efficient

mechanism for performing iterative operations. Good for accessing arrays


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Base-register addressing

R holds a main memory address (acts as Base)

A holds displacement which is usually an unsigned

integer representation Effective Address = A + (R)

Register reference may be explicit or implicit

Convenient means of implementing segmentation


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Instruction

Implicit

Top of stack

register

Stack Addressing

The machine instructions need not include a memory referencebut implicitly operate on the top of stack.

Stack pointer is maintained in a register, so references to stack

locations in memory are in fact register indirect addresses.

Stack is a linear array of

locations

Also referred asPushdown list

orlast-in-first-out queue


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Instruction formats (1)

An instruction is represented as a binary value with specificformat, called the instruction code

It is made out of different groups of bits, with different

significations:

Opcode represents the operation to be performed (it is theinstruction identifier)

Operands one, two or three represent the operands of the

operation to be performed

A microprocessor can have one format for all theinstructions or can have several different formats

An instruction is represented by a single instruction code


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Fewer operands translates into more instructions toaccomplish the same task

The hardware required to implement the microprocessor

becomes less complex with fewer operands;

microprocessors whose instructions specify a fewer numberof operands can execute instructions more quickly than

those that specify more operands

The example was simplified to show the difference between

three, two, one and zero operands instructions; in practice,

the instructions require many more bits than used in these

examples; an operand field may specify an arbitrary

memory address, rather than one of the four registers; this

could require 16, 32 or even more bits per operand

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