Addressing Modes in Computer Architecture-

 

Before you go through this article, make sure that you have gone through the previous article on Addressing Modes.

 

We have discussed-

• Addressing modes are the different ways of specifying the location of an operand.
• Various kinds of addressing modes and their applications.

 

 

Also Read- Syntax Of Addressing Modes

 

In this article, we will discuss practice problems based on addressing modes.

 

PRACTICE PROBLEMS BASED ON ADDRESSING MODES-

 

Problem-01:

 

The most appropriate matching for the following pairs is-

 

Column-1:

X: Indirect addressing

Y: Immediate addressing

Z: Auto decrement addressing

 

Column-2:

1. Loops

2. Pointers

3. Constants

 

1. X-3, Y-2, Z-1
2. X-1, Y-3, Z-2
3. X-2, Y-3, Z-1
4. X-3, Y-1, Z-2

 

Solution-

 

Option (C) is correct.

 

Problem-02:

 

In the absolute addressing mode,

1. The operand is inside the instruction
2. The address of the operand is inside the instruction
3. The register containing the address of the operand is specified inside the instruction
4. The location of the operand is implicit

 

Solution-

 

Option (B) is correct.

 

Problem-03:

 

Which of the following addressing modes are suitable for program relocation at run time?

1. Absolute addressing
2. Base addressing
3. Relative addressing
4. Indirect addressing

 

1. 1 and 4
2. 1 and 2
3. 2 and 3
4. 1, 2 and 4

 

Solution-

 

Option (C) is correct.

 

Problem-04:

 

What is the most appropriate match for the items in the first column with the items in the second column-

 

Column-1:

X: Indirect addressing

Y: Indexed addressing

Z: Base register addressing

 

Column-2:

1. Array implementation

2. Writing relocatable code

3. Passing array as parameter

 

1. X-3, Y-1, Z-2
2. X-2, Y-3, Z-1
3. X-3, Y-2, Z-1
4. X-1, Y-3, Z-2

 

Solution-

 

Option (A) is correct.

 

Problem-05:

 

Which of the following addressing modes permits relocation without any change whatsoever in the code?

1. Indirect addressing
2. Indexed addressing
3. Base register addressing
4. PC relative addressing

 

Solution-

 

Option (C) is correct.

 

Problem-06:

 

Consider a three word machine instruction-

ADD A[R₀], @B

The first operand (destination) “A[R₀]” uses indexed addressing mode with R₀ as the index register. The second operand operand (source) “@B” uses indirect addressing mode. A and B are memory addresses residing at the second and the third words, respectively. The first word of the instruction specifies the opcode, the index register designation and the source and destination addressing modes. During execution of ADD instruction, the two operands are added and stored in the destination (first operand).

The number of memory cycles needed during the execution cycle of the instruction is-

1. 3
2. 4
3. 5
4. 6

 

Solution-

 

For the first operand,

• It uses indexed addressing mode.
• Thus, one memory cycle will be needed to fetch the operand.

 

For the second operand,

• It uses indirect addressing mode.
• Thus, two memory cycles will be needed to fetch the operand.

 

After fetching the two operands,

• The operands will be added and result is stored back in the memory.
• Thus, one memory cycle will be needed to store the result.

 

Total number of memory cycles needed

=  1 + 2 + 1

= 4

 

Thus, Option (B) is correct.

To watch video solution, click here.

 

Problem-07:

 

Consider a hypothetical processor with an instruction of type LW R₁, 20(R₂), which during execution reads a 32-bit word from memory and stores it in a 32-bit register R₁. The effective address of the memory location is obtained by the addition of a constant 20 and the contents of register R₂. Which of the following best reflects the addressing mode implemented by this instruction for operand in memory?

1. Immediate Addressing
2. Register Addressing
3. Register Indirect Scaled Addressing
4. Base Indexed Addressing

 

Solution-

 

Clearly, the instruction uses base indexed addressing mode.

Thus, Option (D) is correct.

 

Problem-08:

 

The memory locations 1000, 1001 and 1020 have data values 18, 1 and 16 respectively before the following program is executed.

 

MOVI	Rs, 1	Move immediate
LOAD	Rd, 1000(Rs)	Load from memory
ADDI	Rd, 1000	Add immediate
STOREI	0(Rd), 20	Store immediate

 

Which of the statements below is TRUE after the program is executed?

1. Memory location 1000 has value 20
2. Memory location 1020 has value 20
3. Memory location 1021 has value 20
4. Memory location 1001 has value 20

 

Solution-

 

Before the execution of program, the memory is-

 

 

Now, let us execute the program instructions one by one-

 

Instruction-01: MOVI R_s, 1

 

• This instruction uses immediate addressing mode.
• The instruction is interpreted as Rs ← 1.
• Thus, value = 1 is moved to the register R_s.

 

Instruction-02: LOAD R_d, 1000(R_s)

 

• This instruction uses displacement addressing mode.
• The instruction is interpreted as R_d ← [1000 + [R_s]].
• Value of the operand = [1000 + [R_s]] = [1000 + 1] = [1001] = 1.
• Thus, value = 1 is moved to the register R_d.

 

Instruction-03: ADDI R_d, 1000

 

• This instruction uses immediate addressing mode.
• The instruction is interpreted as R_d ← [R_d] + 1000.
• Value of the operand = [R_d] + 1000 = 1 + 1000 = 1001.
• Thus, value = 1001 is moved to the register R_d.

 

Instruction-04: STOREI 0(R_d), 20

 

• This instruction uses displacement addressing mode.
• The instruction is interpreted as 0 + [R_d] ← 20.
• Value of the destination address = 0 + [R_d] = 0 + 1001 = 1001.
• Thus, value = 20 is moved to the memory location 1001.

 

Thus,

• After the program execution is completed, memory location 1001 has value 20.
• Option (D) is correct.

 

To watch video solution, click here.

 

Problem-09:

 

Consider the following memory values and a one-address machine with an accumulator, what values do the following instructions load into accumulator?

• Word 20 contains 40
• Word 30 contains 50
• Word 40 contains 60
• Word 50 contains 70

Instructions are-

1. Load immediate 20
2. Load direct 20
3. Load indirect 20
4. Load immediate 30
5. Load direct 30
6. Load indirect 30

 

Solution-

 

Instruction-01: Load immediate 20

 

• This instruction uses immediate addressing mode.
• The instruction is interpreted as Accumulator ← 20.
• Thus, value 20 is loaded into the accumulator.

 

Instruction-02: Load direct 20

 

• This instruction uses direct addressing mode.
• The instruction is interpreted as Accumulator ← [20].
• It is given that word 20 contains 40.
• Thus, value 40 is loaded into the accumulator

 

Instruction-03: Load indirect 20

 

• This instruction uses indirect addressing mode.
• The instruction is interpreted as Accumulator ← [[20]].
• It is given that word 20 contains 40 and word 40 contains 60.
• Thus, value 60 is loaded into the accumulator.

 

Instruction-04: Load immediate 30

 

• This instruction uses immediate addressing mode.
• The instruction is interpreted as Accumulator ← 30.
• Thus, value 30 is loaded into the accumulator.

 

Instruction-02: Load direct 30

 

• This instruction uses direct addressing mode.
• The instruction is interpreted as Accumulator ← [30].
• It is given that word 30 contains 50.
• Thus, value 50 is loaded into the accumulator

 

Instruction-03: Load indirect 30

 

• This instruction uses indirect addressing mode.
• The instruction is interpreted as Accumulator ← [[30]].
• It is given that word 30 contains 50 and word 50 contains 70.
• Thus, value 70 is loaded into the accumulator.

 

To watch video solution, click here.

 

Next Article- System Bus in Computer Architecture

 

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Addressing Modes in Computer Architecture-

 

Before you go through this article, make sure that you have gone through the previous article on Addressing Modes.

 

We have discussed-

• Addressing modes are the different ways of specifying the location of an operand.
• Various kinds of addressing modes and their applications.

 

 

In this article, we will discuss syntax used for addressing modes.

 

Syntax Of Addressing Modes-

 

• The computer instructions may use different addressing modes.
• Different addressing modes use different syntax for their representation.

 

The syntax used for addressing modes are discussed below-

 

1. Immediate Addressing Mode-

 

Syntax-

# Expression

 

Interpretation-

Operand = Expression

 

Examples-

 

• Load R1, #1000 is interpreted as R1 ← 1000
• ADD R2, #3 is interpreted as R2 ← [R2] + 3

 

2. Direct Addressing Mode-

 

Syntax-

Constant

 

Interpretation-

 

Effective address of the operand = Constant

Operand = Content of the memory location ‘Constant’

Operand = [Constant]

 

Examples-

 

• Load R1, 1000 is interpreted as R1 ← [1000]
• ADD R2, 3 is interpreted as R2 ← [R2] + [3]

 

3. Register Direct Addressing Mode-

 

Syntax-

R_n or [R_n]

 

Interpretation-

 

Operand = Content of the register R_n

Operand = [R_n]

 

Examples-

 

• Load R1, R2 is interpreted as R1 ← [R2]
• ADD R1, R2 is interpreted as R1 ← [R1] + [R2]

 

4. Indirect Addressing Mode-

 

Syntax-

@Expression or @(Expression) or (Expression)

 

Interpretation-

 

Effective address of the operand = Content of the memory location ‘expression’

Operand = [[ Expression]]

 

Examples-

 

• Load R1, @1000 is interpreted as R1 ← [[1000]]
• ADD R1, @(1000) is interpreted as R1 ← [R1] + [[1000]]
• ADD R1, (1000) is interpreted as R1 ← [R1] + [[1000]]

 

5. Register Indirect Addressing Mode-

 

Syntax-

@R_n or @(R_n) or (R_n)

 

Interpretation-

 

Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

 

Examples-

 

• Load R1, @R2 is interpreted as R1 ← [[R1]]
• ADD R1, @(R2) is interpreted as R1 ← [R1] + [[R2]]
• ADD R1, (R2) is interpreted as R1 ← [R1] + [[R2]]

 

6. Displacement Addressing Mode-

 

This addressing mode may be-

• Relative addressing mode
• Index addressing mode
• Base register addressing mode

 

Syntax-

disp (R_n)

 

Interpretation-

 

Effective address of the operand = disp + Content of the register R_n

Operand = [disp + [ R_n ]]

 

Examples-

 

• Load R1, 100(R2) is interpreted as R1 ← [100 + [R1]]
• ADD R1, 100(R2) is interpreted as R1 ← [R1] + [100 + [R2]]

 

7. Auto-Increment Addressing Mode-

 

Syntax-

(R_n)+

 

Interpretation-

 

Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

After fetching the operand, increment [R_n] by step size ‘d’

 

Examples-

 

• Load R1, (R2)+ is interpreted as R1 ← [[R2]] followed by R2 ← [R2] + d
• ADD R1, (R2)+ is interpreted as R1 ← [R1] + [[R2]] followed by R2 ← [R2] + d

 

8. Auto-Decrement Addressing Mode-

 

Syntax-

-(R_n)

 

Interpretation-

 

Before fetching the operand, decrement [R_n] by step size ‘d’

Then, Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

 

Examples-

 

• Load R1, -(R2) is interpreted as R2 ← [R2] – d followed by R1 ← [[R2]]
• ADD R1, -(R2) is interpreted as R2 ← [R2] – d followed by R1 ← [R1] + [[R2]]

 

To gain better understanding about Syntax of Addressing Modes,

Watch this Video Lecture

 

Next Article- Practice Problems On Addressing Modes

 

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Addressing Modes-

 

The different ways of specifying the location of an operand in an instruction are called as addressing modes.

 

Types of Addressing Modes-

 

In computer architecture, there are following types of addressing modes-

 

 

1. Implied / Implicit Addressing Mode
2. Stack Addressing Mode
3. Immediate Addressing Mode
4. Direct Addressing Mode
5. Indirect Addressing Mode
6. Register Direct Addressing Mode
7. Register Indirect Addressing Mode
8. Relative Addressing Mode
9. Indexed Addressing Mode
10. Base Register Addressing Mode
11. Auto-Increment Addressing Mode
12. Auto-Decrement Addressing Mode

 

In this article, we will discuss about these addressing modes in detail.

 

1. Implied Addressing Mode-

 

In this addressing mode,

• The definition of the instruction itself specify the operands implicitly.
• It is also called as implicit addressing mode.

 

Examples-

 

• The instruction “Complement Accumulator” is an implied mode instruction.
• In a stack organized computer, Zero Address Instructions are implied mode instructions.

(since operands are always implied to be present on the top of the stack)

 

2. Stack Addressing Mode-

 

In this addressing mode,

• The operand is contained at the top of the stack.

 

Example-

 

ADD

• This instruction simply pops out two symbols contained at the top of the stack.
• The addition of those two operands is performed.
• The result so obtained after addition is pushed again at the top of the stack.

 

3. Immediate Addressing Mode-

 

In this addressing mode,

• The operand is specified in the instruction explicitly.
• Instead of address field, an operand field is present that contains the operand.

 

 

Examples-

 

• ADD 10 will increment the value stored in the accumulator by 10.
• MOV R #20 initializes register R to a constant value 20.

 

4. Direct Addressing Mode-

 

In this addressing mode,

• The address field of the instruction contains the effective address of the operand.
• Only one reference to memory is required to fetch the operand.
• It is also called as absolute addressing mode.

 

 

Example-

 

• ADD X will increment the value stored in the accumulator by the value stored at memory location X.

AC ← AC + [X]

 

5. Indirect Addressing Mode-

 

In this addressing mode,

• The address field of the instruction specifies the address of memory location that contains the effective address of the operand.
• Two references to memory are required to fetch the operand.

 

 

Example-

 

• ADD X will increment the value stored in the accumulator by the value stored at memory location specified by X.

AC ← AC + [[X]]

 

6. Register Direct Addressing Mode-

 

In this addressing mode,

• The operand is contained in a register set.
• The address field of the instruction refers to a CPU register that contains the operand.
• No reference to memory is required to fetch the operand.

 

 

Example-

 

• ADD R will increment the value stored in the accumulator by the content of register R.

AC ← AC + [R]

 

NOTE-

 

It is interesting to note-

• This addressing mode is similar to direct addressing mode.
• The only difference is address field of the instruction refers to a CPU register instead of main memory.

 

7. Register Indirect Addressing Mode-

 

In this addressing mode,

• The address field of the instruction refers to a CPU register that contains the effective address of the operand.
• Only one reference to memory is required to fetch the operand.

 

 

Example-

 

• ADD R will increment the value stored in the accumulator by the content of memory location specified in register R.

AC ← AC + [[R]]

 

NOTE-

 

It is interesting to note-

• This addressing mode is similar to indirect addressing mode.
• The only difference is address field of the instruction refers to a CPU register.

 

8. Relative Addressing Mode-

 

In this addressing mode,

• Effective address of the operand is obtained by adding the content of program counter with the address part of the instruction.

 

Effective Address = Content of Program Counter + Address part of the instruction

 

 

NOTE-

 

• Program counter (PC) always contains the address of the next instruction to be executed.
• After fetching the address of the instruction, the value of program counter immediately increases.
• The value increases irrespective of whether the fetched instruction has completely executed or not.

 

9. Indexed Addressing Mode-

 

In this addressing mode,

• Effective address of the operand is obtained by adding the content of index register with the address part of the instruction.

 

Effective Address = Content of Index Register + Address part of the instruction

 

 

10. Base Register Addressing Mode-

 

In this addressing mode,

• Effective address of the operand is obtained by adding the content of base register with the address part of the instruction.

 

Effective Address = Content of Base Register + Address part of the instruction

 

 

11. Auto-Increment Addressing Mode-

 

• This addressing mode is a special case of Register Indirect Addressing Mode where-

 

Effective Address of the Operand = Content of Register

 

In this addressing mode,

• After accessing the operand, the content of the register is automatically incremented by step size ‘d’.
• Step size ‘d’ depends on the size of operand accessed.
• Only one reference to memory is required to fetch the operand.

 

Example-

 

 

Assume operand size = 2 bytes.

Here,

• After fetching the operand 6B, the instruction register R_AUTO will be automatically incremented by 2.
• Then, updated value of R_AUTO will be 3300 + 2 = 3302.
• At memory address 3302, the next operand will be found.

 

NOTE-

 

In auto-increment addressing mode,

• First, the operand value is fetched.
• Then, the instruction register R_AUTO value is incremented by step size ‘d’.

 

12. Auto-Decrement Addressing Mode-

 

• This addressing mode is again a special case of Register Indirect Addressing Mode where-

 

Effective Address of the Operand = Content of Register – Step Size

 

In this addressing mode,

• First, the content of the register is decremented by step size ‘d’.
• Step size ‘d’ depends on the size of operand accessed.
• After decrementing, the operand is read.
• Only one reference to memory is required to fetch the operand.

 

Example-

 

 

Assume operand size = 2 bytes.

Here,

• First, the instruction register R_AUTO will be decremented by 2.
• Then, updated value of R_AUTO will be 3302 – 2 = 3300.
• At memory address 3300, the operand will be found.

 

NOTE-

 

In auto-decrement addressing mode,

• First, the instruction register R_AUTO value is decremented by step size ‘d’.
• Then, the operand value is fetched.

 

Also Read- Practice Problems On Addressing Modes

 

Applications of Addressing Modes-

 

Addressing Modes	Applications
Immediate Addressing Mode	To initialize registers to a constant value
Direct Addressing Mode and Register Direct Addressing Mode	To access static data To implement variables
Indirect Addressing Mode and Register Indirect Addressing Mode	To implement pointers because pointers are memory locations that store the address of another variable To pass array as a parameter because array name is the base address and pointer is needed to point the address
Relative Addressing Mode	For program relocation at run time i.e. for position independent code To change the normal sequence of execution of instructions For branch type instructions since it directly updates the program counter
Index Addressing Mode	For array implementation or array addressing For records implementation
Base Register Addressing Mode	For writing relocatable code i.e. for relocation of program in memory even at run time For handling recursive procedures
Auto-increment Addressing Mode and Auto-decrement Addressing Mode	For implementing loops For stepping through arrays in a loop For implementing a stack as push and pop

 

To gain better understanding about Addressing Modes,

Watch this Video Lecture

 

Next Article- Syntax Of Addressing Modes

 

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Watch video lectures by visiting our YouTube channel LearnVidFun.