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GNDU Queson Paper 2024
Bachelor of Computer Applicaon (BCA)
3rd Semester
COMPUTER ARCHITECTURE
Time Allowed-3 Hours Maximum Marks-100
Note:- Aempt FIVE quesons in all, selecng at least ONE queson from each secon.
The h queson may be aempted from any secon. All quesons carry equal marks.
SECTION-A
1. (a) How informaon is represented using general registers? Explain the role of register
transfer language.
(b) Explain the use of logical micro-operaons in detail.
2. What is the need of ming signals and instrucon cycle 2 Explain by taking suitable
examples
SECTION-B
3. Explain the following concepts
(a) Types of instrucon formats.
(b) Role of control unit.
4 Discuss the characteriscs of the following
(a) Indirect and relave addressing modes,
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(b) Benets of RISC Architecture
SECTION-C
5. Write notes on the following:
(a) Memory Hierarchy
(b) Use of Auxiliary Memory.
6.(a) What is the concept of Virtual Memory? Explain.
(b) Why associave memory is used for execuon? Explain
SECTION-D
7.(a) How DMA is used for data transfer? Explain in detail
(b) Discuss programmed I/O for data transfer operaons
8.(a) Discuss the role of pipelining and its types
(b) How MISD and MIMD architectures are organised? Explain
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GNDU Answer Paper 2024
Bachelor of Computer Applicaon (BCA)
3rd Semester
COMPUTER ARCHITECTURE
Time Allowed-3 Hours Maximum Marks-100
Note:- Aempt FIVE quesons in all, selecng at least ONE queson from each secon.
The h queson may be aempted from any secon. All quesons carry equal marks.
SECTION-A
1. (a) How informaon is represented using general registers? Explain the role of register
transfer language.
(b) Explain the use of logical micro-operaons in detail.
Ans: (a) A Fresh Beginning
Imagine you’re in a busy post office. Letters and parcels are constantly arriving, being
sorted, stamped, and then sent out again. The post office has storage rooms (like memory),
workers’ desks (like registers), and a specific way of describing who passes what to whom
(like register transfer language).
Now, think of a computer in the same way. Data keeps coming in, being processed, and
going out. The “postmen” here are the general registers, and the language they use to
explain the movement of data is called register transfer language (RTL).
With that simple scene in your mind, let’s explore the two parts of your question:
1. How information is represented using general registers
2. The role of register transfer language (RTL)
Part 1: How Information is Represented Using General Registers
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First, let’s understand what a register is.
A register is a very small, very fast memory unit located inside the CPU. Think of it as a
pocket in the postman’s uniform—it’s not as big as the post office warehouse (RAM), but it’s
close to the postman and helps him deliver faster.
General Registers: The All-Rounders
Registers can be of different types, but general-purpose registers (GPRs) are like versatile
workers who can handle multiple tasks. They are not specialized for only one duty (like
“addressing” or “counting”), but can store:
• Numbers,
• Characters,
• Intermediate results of calculations,
• Addresses pointing to memory locations, etc.
For example, suppose the CPU needs to add two numbers (say, 5 and 10). Instead of going
back to the slow memory every time, the numbers are quickly placed inside general
registers. One register stores 5, another stores 10, and then the Arithmetic Logic Unit (ALU)
performs the addition.
So, information in general registers is represented as binary numbers (0s and 1s). Why
binary? Because at the hardware level, computers only understand ON (1) and OFF (0).
Every number, character, or instruction is encoded into binary and stored in these registers.
Representation in Action
Let’s visualize it:
• Suppose Register R1 = 00000101 (binary for 5)
• Suppose Register R2 = 00001010 (binary for 10)
When the CPU performs the operation R3 ← R1 + R2, the result is:
• R3 = 00001111 (binary for 15)
Here, R1, R2, R3 are general registers. Each stores data in binary form, and the ALU works
directly on these registers.
In simple words: General registers represent information as binary patterns that can be
numbers, addresses, or any other data.
Part 2: The Role of Register Transfer Language (RTL)
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Now comes the interesting part: how do we describe what’s happening inside these
registers? That’s where Register Transfer Language comes in.
Think of RTL as a shorthand or symbolic way of explaining the flow of data between
registers, memory, and the ALU. It’s like the “office notes” in our post office story that say:
• “Postman A hands letter to Postman B.”
• “Stamp this letter before moving.”
• “Deliver to destination X.”
In the computer world, RTL uses simple notations like arrows, register names, and
operations.
Basic Example of RTL
• R1 ← R2 means: transfer the contents of Register R2 into Register R1.
• R3 ← R1 + R2 means: add the contents of R1 and R2, and store the result in R3.
• PC ← PC + 1 means: increment the Program Counter by 1 (common in instruction
execution).
This is not a programming language like C or Python. Instead, RTL is a descriptive tool that
shows how data moves within the CPU and how operations are carried out.
Why is RTL Important?
1. Clarity for Designers: Computer engineers use RTL to design and explain how
hardware works internally.
2. Simplifies Complex Operations: Instead of describing a process in long sentences,
one short RTL statement does the job.
o Example: Instead of writing “Take the contents of R1 and R2, add them, and
put the sum in R3,” you just write R3 ← R1 + R2.
3. Foundation of Micro-Operations: Each RTL statement can be broken down into tiny
steps called micro-operations, which the CPU executes.
Connecting the Two: Registers + RTL
Let’s bring both concepts together with a mini story inside the CPU.
Suppose you want the computer to calculate (A + B) × C.
1. First, the numbers A, B, and C are stored in general registers:
o R1 ← A
o R2 ← B
o R3 ← C
2. Add A and B:
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o R4 ← R1 + R2
3. Multiply the result with C:
o R5 ← R4 × R3
Now R5 has the final answer.
Notice how registers hold the values (A, B, C, intermediate sums and products), while RTL
describes how these values travel and change step by step.
Another Analogy: Registers as Buckets
Think of registers as small water buckets. You scoop water (data) from a tank (memory) and
temporarily store it in buckets because they are closer to you. When you want to mix two
amounts of water (perform an addition), you pour them into a mixing pot (ALU).
The instructions written on a whiteboard nearby, telling which bucket to use and when to
pour, are the RTL statements. Without those notes, the workers (CPU circuits) wouldn’t
know which bucket to handle next.
Wrapping It All Up
• Information representation in general registers: Data inside the CPU is stored in
binary form within small, fast storage units called general registers. They can hold
numbers, addresses, or temporary results.
• Role of Register Transfer Language (RTL): RTL provides a neat, symbolic way to
describe how data moves between registers and how operations are performed.
Together, they form the “language of the CPU.” Registers hold the words (data), while RTL
forms the grammar and sentences (data transfers and operations).
(b) Explain the use of logical micro-operaons in detail.
Ans: A Different Beginning
Imagine you are holding a box of colored beads. Each bead can either glow bright
(representing 1) or stay dim (representing 0). Now, you want to create different patterns
with these beads—sometimes you want to compare two beads, sometimes combine them,
sometimes flip their glow. The simple rules you use to decide what happens to the beads
are like the logical micro-operations inside a computer.
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Just like beads help you make beautiful designs, logical micro-operations help the computer
“think logically” and manipulate information bit by bit. They don’t solve problems on their
own but are the building blocks of every big calculation and decision the computer makes.
What are Logical Micro-Operations?
To understand them better, let’s break the term into two parts:
1. Micro-operations → These are the smallest steps a computer performs on data
stored in registers (registers are tiny, super-fast memory cells inside the CPU).
2. Logical operations → These are operations that deal with logic, not arithmetic.
Instead of adding or subtracting numbers, they check relationships between bits (0s
and 1s).
So, logical micro-operations are the tiny logical steps that the CPU uses to manipulate data
at the bit level. They are extremely important because they allow the computer to:
• Compare data,
• Modify data,
• Make decisions, and
• Prepare information for further calculations.
Why Do We Need Them?
Think of logical micro-operations as the grammar rules of a language. Just as we need
grammar to form meaningful sentences, computers need logical micro-operations to form
meaningful decisions. Without them, a computer would only be able to do plain arithmetic
but not “decide” or “check” conditions.
For example:
• Before adding numbers, a computer may need to check if the data is valid.
• While searching, it may need to compare values.
• In security, it may need to mask certain bits of data.
All these become possible only because of logical micro-operations.
The Main Types of Logical Micro-Operations
Let’s explore the main logical micro-operations like characters in a story, each with their
own personality.
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1. AND Operation
Think of two friends who will only agree to go out if both are free. If even one says no, the
plan fails.
• In computer terms, AND checks if both bits are 1.
• Example:
o 1 AND 1 → 1
o 1 AND 0 → 0
Use case: Masking (hiding unwanted bits). For example, you may want to keep only certain
parts of data and remove the rest.
2. OR Operation
Imagine two friends again, but this time they will go out if at least one of them is free.
• In computer terms, OR checks if any bit is 1.
• Example:
o 1 OR 0 → 1
o 0 OR 0 → 0
Use case: Setting a particular bit. For example, turning on a feature flag in memory.
3. NOT Operation (Complement)
This one is the opposite thinker. If you say yes, it says no; if you say no, it says yes.
• In computer terms, NOT flips the bit.
• Example:
o NOT 1 → 0
o NOT 0 → 1
Use case: Creating bitwise complements. For example, if you want to highlight what’s
missing rather than what’s present.
4. XOR (Exclusive OR)
This is the quirky one in the group. It says: “I’ll give you 1 if the bits are different, but if they
are the same, I’ll give you 0.”
• Example:
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o 1 XOR 0 → 1
o 1 XOR 1 → 0
Use case: Checking for differences, or even performing bitwise addition without carrying
over.
5. XNOR (Equivalence)
This is XOR’s sibling. It says: “I’ll give you 1 if both bits are the same.”
• Example:
o 1 XNOR 1 → 1
o 1 XNOR 0 → 0
Use case: Equality checking.
How Logical Micro-Operations Work in Registers
Registers are like small bowls holding beads (bits). When a logical micro-operation is
performed, it is applied to all bits of the register simultaneously.
For example:
• Register A = 1010
• Register B = 1100
• If we perform AND → Result = 1000
• If we perform OR → Result = 1110
• If we perform XOR → Result = 0110
This shows how entire data chunks can be manipulated with one simple operation.
Real-Life Applications
1. Masking Data → Using AND to filter out unwanted parts of information.
2. Setting Bits → Using OR to ensure a particular bit is switched on.
3. Inverting Data → Using NOT to flip all bits, often used in error detection.
4. Checking Equality → Using XOR or XNOR to see if two registers are the same.
5. Data Security → Many encryption techniques rely heavily on logical micro-
operations.
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Wrapping it Up
Logical micro-operations may sound small, but they are the heartbeat of computer decision-
making. Without them, a computer would be like a calculator that only adds and subtracts,
without the power to make choices or control data.
So next time you see a computer running smoothly—opening apps, checking passwords, or
even showing you a website—remember that deep inside, millions of tiny logical micro-
operations are working like invisible gears, turning 0s and 1s into meaningful actions.
2. What is the need of ming signals and instrucon cycle ? Explain by taking suitable
examples
Ans: The Story of Timing Signals and Instruction Cycle
Imagine a classroom. There’s a teacher (the CPU) who has to teach lessons (the
instructions) to students. Now, in this classroom:
• The lesson plan decides what to teach (like an instruction set).
• The bell system rings at fixed intervals (like timing signals).
• The teaching process always happens in steps: calling attendance, explaining, and
then giving work (like the instruction cycle).
Without the bell, the class would be a mess. Without following the steps, the teacher and
students would get confused. This same logic applies to computers — they need timing
signals and instruction cycles to work in an organized way.
Let’s now explore this idea in detail.
What Are Timing Signals?
Timing signals are like the school bell that rings at specific intervals to signal what should
happen next.
• In a computer, timing signals are electrical pulses generated by the clock.
• They tell the CPU when to start and when to stop a particular step.
Example:
When you cook noodles, you don’t just dump everything at once. First, you boil water, then
add noodles, then add spices. You usually follow a timer to make sure each step happens in
sequence. The CPU works in exactly the same way — it cannot execute an instruction in one
go. It needs step-by-step control from timing signals.
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So, the need for timing signals:
1. To maintain synchronization between different parts of the CPU.
2. To make sure that one step finishes before the next begins.
3. To avoid chaos, just like how a school runs smoothly with bells.
What is an Instruction Cycle?
Now, let’s return to our classroom.
Every lesson (instruction) has to be taught in a certain cycle:
1. Fetch – The teacher calls out the lesson from the syllabus (CPU fetches the
instruction from memory).
2. Decode – The teacher reads and understands what has to be explained (CPU
decodes the instruction).
3. Execute – The teacher teaches it and students practice (CPU executes the
instruction).
This whole process is called the Instruction Cycle.
In computers:
• Each instruction (like add, subtract, load, store) passes through these steps.
• Timing signals make sure each step happens at the right time.
Example:
Suppose you want your computer to add 5 + 3.
• First, the CPU fetches the instruction “ADD” from memory.
• Then, it decodes it to understand it needs to add two numbers.
• Finally, it executes by actually adding 5 and 3 to give you 8.
All of this happens in microseconds, but the cycle is always followed.
The Relationship Between Timing Signals and Instruction Cycle
Think of timing signals as the beats of a drum and the instruction cycle as a dance routine.
• The dancer (CPU) cannot perform unless the drummer (clock) provides beats.
• Similarly, the CPU cannot carry out an instruction unless the timing signals guide
every step.
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So, timing signals divide the instruction cycle into small manageable parts called machine
cycles, and each machine cycle is further broken into T-states (time states).
Detailed Example
Let’s say the instruction is: LOAD A, 1050 (Load the value from memory location 1050 into
register A).
Step 1: Fetch
• The CPU receives a timing signal to fetch the instruction from memory.
• Memory sends back the instruction.
Step 2: Decode
• Next timing signal tells the CPU to decode.
• CPU understands it needs to bring data from memory 1050.
Step 3: Execute
• Timing signals now trigger execution.
• The CPU takes data from 1050 and puts it into register A.
Without these signals, the CPU might try to decode before fetching, or execute before
decoding — complete chaos.
Diagram of Instruction Cycle with Timing Signals
This shows how each part of the cycle is guided by specific timing signals (T1, T2, T3…).
Why Are They So Important?
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1. Accuracy
Just like you can’t solve math without clear steps, CPU can’t process instructions without
proper sequence. Timing signals give this accuracy.
2. Coordination
They ensure memory, input/output devices, and CPU work together in sync.
3. Efficiency
By dividing work into cycles, CPUs handle millions of instructions per second smoothly.
4. Error Prevention
No overlap or mix-up happens, reducing chances of wrong execution.
Real-Life Analogy
Imagine a relay race:
• The baton (instruction) must be passed step by step.
• Each runner (fetch, decode, execute) runs only when it’s their turn.
• The whistle (timing signal) decides when to start.
Without the whistle, runners might collide. Without baton-passing order, the race makes no
sense. This is exactly why timing signals and instruction cycles are essential in a CPU.
Conclusion
So, the need for timing signals and instruction cycles is just like the need for order and
rhythm in daily life.
• Timing signals act as the heartbeat or drumbeat of the computer.
• Instruction cycle acts as the step-by-step procedure for solving problems.
Together, they ensure that the CPU doesn’t just work fast, but also works correctly,
sequentially, and reliably.
In simple words:
Without timing signals, the CPU would be like a teacher without a bell system —
confused and disorganized.
Without instruction cycles, instructions would be like lessons taught without steps —
meaningless and incomplete.
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That’s why every modern computer you use today, from your smartphone to
supercomputers, relies on these two concepts as its foundation.
SECTION-B
3. Explain the following concepts
(a) Types of instrucon formats.
(b) Role of control unit.
Ans: A New Beginning – Let’s Step into a Computer’s Mind
Imagine you are the director of a huge orchestra. Each instrument in front of you is ready to
play its part, but none of them knows when to begin, how long to play, or which note to hit
unless you give them proper instructions.
A computer works in exactly the same way. It has many components: memory, processor,
input devices, output devices. But none of them can do anything unless someone tells them
what to do and when to do it.
That "someone" is the instruction format (the way commands are written) and the control
unit (the director who makes sure everything runs smoothly).
Today, let us go on this little journey where we explore:
• How computers understand instructions through different formats, and
• How the control unit acts as the boss of the CPU, making sure all instructions are
carried out properly.
So, grab your imagination, because we’re about to walk into the heart of a computer.
Part A: Types of Instruction Formats
When we humans write something, we use sentences with structure (subject, verb, object).
For example:
• Ravi eats an apple. (Subject = Ravi, Verb = eats, Object = apple)
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Similarly, computers also have a special way of structuring their sentences. These are called
Instruction Formats.
Definition:
Instruction format is the way an instruction is organized in binary form so that the
computer’s processor can understand it and perform the desired task.
An instruction generally has three important parts:
1. Opcode (Operation Code): Tells the CPU what action to perform (e.g., Add, Subtract,
Load, Store).
2. Operand(s): Tells the CPU on which data or where to perform the operation.
3. Addressing Mode / Additional bits: Provides extra information like location in
memory, type of data, etc.
Let’s now understand different types of instruction formats like we are exploring different
sentence patterns in a language.
1. Zero Address Instruction Format
• This format has no explicit operands in the instruction.
• Mostly used in stack-based architectures, where operations are performed on top of
the stack.
Example:
Instruction: ADD
Meaning: Add the top two elements of the stack.
Here, the CPU already knows where the data is (on top of the stack), so no need to mention
operands.
Diagram:
--------------------------------
| OPCODE (e.g., ADD) |
--------------------------------
Analogy:
It’s like saying to a waiter, “Bring the next dish”, without specifying which one, because he
already knows the sequence.
2. One Address Instruction Format
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• This format has one operand explicitly mentioned.
• The second operand is assumed to be in a special register called the Accumulator.
Example:
Instruction: ADD X
Meaning: Add the value stored in memory location X with the value in the Accumulator and
store the result back in the Accumulator.
Diagram:
------------------------------------------
| OPCODE | ADDRESS |
------------------------------------------
Analogy:
Imagine you have a piggy bank (Accumulator). The instruction says: “Add the amount in
Ravi’s wallet (X) to your piggy bank.” You don’t need to mention the piggy bank
separately—it’s always implied.
3. Two Address Instruction Format
• This format has two operands explicitly mentioned.
• The result is usually stored in one of the given operands.
Example:
Instruction: ADD A, B
Meaning: Add the contents of memory location A and memory location B, and store the
result in A.
Diagram:
------------------------------------------------------
| OPCODE | ADDRESS 1 | ADDRESS 2 |
------------------------------------------------------
Analogy:
It’s like saying: “Add sugar (B) into the cup (A).” After the action, the cup (A) contains the
mixture.
4. Three Address Instruction Format
• This format has three operands explicitly mentioned.
• Two operands are the source, and the third is the destination.
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Example:
Instruction: ADD A, B, C
Meaning: Add the contents of memory location B and C, and store the result in A.
Diagram:
-----------------------------------------------------------------
| OPCODE | ADDRESS 1 | ADDRESS 2 | ADDRESS 3 |
-----------------------------------------------------------------
Analogy:
It’s like saying: “Put the sum of Ravi’s money (B) and Rani’s money (C) into Mohan’s wallet
(A).”
5. Register Instruction Format
• In this format, operands are registers instead of memory addresses.
• Since registers are inside the CPU, these instructions are very fast.
Example:
Instruction: ADD R1, R2
Meaning: Add the contents of Register R1 and R2.
Diagram:
----------------------------------------
| OPCODE | REG 1 | REG 2 |
----------------------------------------
Analogy:
It’s like using your two hands directly to carry something instead of asking someone from
outside to fetch it for you.
6. Immediate Instruction Format
• One of the operands is given directly in the instruction itself (not in memory or
register).
Example:
Instruction: ADD R1, 5
Meaning: Add the number 5 directly to the contents of Register R1.
Diagram:
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----------------------------------------
| OPCODE | REGISTER | DATA |
----------------------------------------
Analogy:
It’s like saying: “Put 5 spoons of sugar directly into this cup.” No need to search sugar
elsewhere—it’s right in the instruction.
7. Hybrid/Complex Instruction Formats
Modern computers sometimes mix these formats to optimize speed and memory usage.
Summary of Instruction Formats:
Type
Operands Explicitly Mentioned
Example
Analogy
Zero Address
0
ADD
Bring the next dish
One Address
1
ADD X
Add X to piggy bank
Two Address
2
ADD A, B
Add sugar to cup
Three Address
3
ADD A, B, C
Put sum of B & C into A
Register Format
Registers only
ADD R1, R2
Using hands directly
Immediate Format
Operand inside instruction
ADD R1, 5
Add 5 directly
Part B: Role of the Control Unit
Now that we know how instructions are written, let’s see who executes them.
In our orchestra example, the control unit (CU) is the conductor.
It doesn’t play instruments itself, but without it, no instrument will know when to start or
stop.
Definition:
The Control Unit is a component of the CPU that directs and coordinates all the activities of
the computer. It fetches instructions from memory, decodes them, and then signals other
parts (ALU, memory, I/O devices) to perform the required task.
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Functions of Control Unit
1. Fetching Instructions: Gets the instruction from memory.
2. Decoding Instructions: Understands what the instruction means (which operation,
which operands).
3. Generating Control Signals: Sends timing and control signals to ALU, memory, and
I/O.
4. Executing Instructions: Coordinates the actual execution step by step.
5. Maintaining Order: Ensures that instructions are executed in correct sequence
unless told otherwise (like in branching).
Diagram of Control Unit’s Role:
Types of Control Units
1. Hardwired Control Unit
o Control signals are generated using fixed electronic circuits.
o Fast but not flexible (difficult to modify).
2. Microprogrammed Control Unit
o Control signals are generated by micro-instructions stored in control
memory.
o Easier to modify, slower than hardwired.
Real-Life Analogy
Think of the Control Unit as a school principal:
• He doesn’t teach every subject himself.
• But he makes the timetable, tells teachers when to enter the class, and ensures
everything is disciplined.
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Without the principal, the school would be chaotic. Similarly, without the CU, the CPU
would be useless.
Interconnection Between Instruction Format and Control Unit
Instruction formats are like the language in which we give commands.
The Control Unit is like the interpreter and manager that makes sure those commands are
followed correctly.
• Instruction Format = What to do (the recipe).
• Control Unit = How to do it (the chef managing the kitchen).
Final Summary
• Instruction Formats are the different ways of writing computer instructions (zero-
address, one-address, two-address, three-address, register, immediate).
• Each has its own structure, advantages, and use cases.
• Control Unit is the brain’s manager inside the CPU—it fetches, decodes, and directs
the execution of instructions.
• Together, they make computers run smoothly, just like a well-organized orchestra or
a disciplined school.
4 Discuss the characteriscs of the following
(a) Indirect and relave addressing modes,
(b) Benets of RISC Architecture
Ans: Discussing Addressing Modes & RISC Architecture – A Story-Like Explanation
A Fresh Beginning
Imagine you are entering a giant library. This isn’t just any library—it’s the Library of
Computers. Each book here represents instructions, numbers, and memory addresses. Now,
just like in a library, you need different ways to find the right book. Sometimes you go
straight to the shelf (direct addressing), sometimes you ask the librarian to check the catalog
for you (indirect addressing), and sometimes you move a few steps from where you are
(relative addressing).
At the same time, in another corner of this library, there’s a heated debate going on:
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• One group says, “Let’s make things simple and fast—a few clear instructions that
can run really quickly!” (This is the RISC team).
• The other group says, “Why not have complex instructions that do more in one step,
even if they take longer?” (This is the CISC team).
Today, we’re going to sit in this library, observe both these discussions, and understand two
things:
1. Indirect & Relative Addressing Modes – how a computer decides where to fetch
data from.
2. Benefits of RISC Architecture – why keeping things simple can actually make a
processor smarter and faster.
Let’s start with the first topic—addressing modes.
Part (a): Characteristics of Indirect & Relative Addressing Modes
1. What Are Addressing Modes?
Before diving deep, let’s clarify the concept.
When a computer program runs, it constantly needs to fetch data (numbers, characters,
instructions) from memory. But the big question is:
How does the CPU know where in memory the data is located?
That’s where addressing modes come in.
Think of addressing modes as different methods of finding the right house in a city:
• Sometimes you get the exact address written on paper (direct mode).
• Sometimes you are given the phone number of a friend who will guide you to the
house (indirect mode).
• Sometimes you are told “Go 5 blocks from your current location” (relative mode).
Each mode changes how flexible and efficient the CPU becomes.
2. Indirect Addressing Mode
The Story of Indirect Addressing
Imagine you want to visit a friend’s home. Instead of giving you the house number directly,
your friend says:
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“Go to this apartment, and inside you’ll find a note. That note will tell you the real house
address.”
This is exactly what indirect addressing does. The instruction doesn’t give you the data
itself. Instead, it gives you the address of another address.
So, the CPU has to take two steps:
1. First, go to the memory location mentioned in the instruction.
2. Then, read that memory location to find the real address of the data.
It’s like a two-step treasure hunt.
Technical Definition
• In indirect addressing mode, the operand field of an instruction specifies a memory
location, which itself contains the address of the real operand (data).
• It requires two memory accesses: one to fetch the effective address and another to
fetch the actual data.
Characteristics of Indirect Addressing
1. Double Memory Access – One to get the effective address, second to get the
operand.
2. Flexible – You can access data stored anywhere in memory.
3. Pointer-Based – Commonly used when working with pointers in high-level languages
like C or C++.
4. Slower – Because it needs two trips to memory.
5. Useful for Dynamic Data – Ideal for structures like linked lists and trees.
Diagram: Indirect Addressing
Instruction → [ 100 ] → Memory Location → [ 500 ] → Actual Data → [ 25 ]
Explanation:
• The instruction says: Go to address 100.
• At memory address 100, you find the number 500.
• Now go to address 500, and you’ll find the actual data 25.
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Example
Suppose:
• Instruction stored: LOAD [100]
• At address 100 → value 500
• At address 500 → value 25
So the CPU loads 25 into the register.
3. Relative Addressing Mode
The Story of Relative Addressing
Now imagine you’re in the same library. A friend tells you:
“From where you’re standing right now, walk forward 10 steps, and you’ll reach the book
you want.”
This is relative addressing.
Instead of giving the full address, the instruction says:
• “Take your current location (the Program Counter, PC), and add an offset to it.”
So the data or instruction you need is found relative to your current position.
Technical Definition
• In relative addressing mode, the effective address is determined by adding a
constant value (called offset or displacement) to the content of the Program
Counter (PC).
• It is mainly used in branching instructions (like jumps, loops, and conditional
statements).
Characteristics of Relative Addressing
1. PC-Based – Always calculated from the program counter.
2. Efficient for Branching – Commonly used in jumps, loops, and conditional branches.
3. Compact Instructions – Saves memory, since you don’t need to write full addresses.
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4. Position Independent – Makes it easier to move code blocks without rewriting
addresses.
5. Fast – Only one memory access is required.
Diagram: Relative Addressing
PC = 200
Instruction = Jump +10
Effective Address = 200 + 10 = 210
The CPU will jump to instruction stored at address 210.
Example
Suppose the Program Counter (PC) is currently pointing at 300.
• Instruction: JUMP +50
• Effective Address = 300 + 50 = 350
So, the CPU jumps to instruction at memory location 350.
Comparing Indirect vs Relative Addressing
Feature
Indirect Addressing Mode
Relative Addressing Mode
Method
Operand points to address of address
Operand found relative to PC
Speed
Slower (two memory accesses)
Faster (single calculation)
Flexibility
Very flexible, can access anywhere
Limited to nearby addresses
Use Cases
Pointers, linked data structures
Branching, loops, jumps
Complexity
Higher
Lower
Part (b): Benefits of RISC Architecture
1. What is RISC?
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RISC stands for Reduced Instruction Set Computer.
Imagine two chefs in a kitchen:
• Chef CISC: “I’ll make a fancy dish in one step, but it will take me a lot of time because
the recipe is very complicated.”
• Chef RISC: “I’ll break the recipe into small, simple steps. Each step will be quick, and
I’ll finish faster overall.”
That’s the philosophy of RISC: fewer, simpler instructions that execute very quickly.
2. Characteristics of RISC
1. Small set of simple instructions – Each instruction does one simple task.
2. Single-cycle execution – Most instructions execute in one CPU cycle.
3. Load/Store Architecture – Only load and store instructions access memory; all other
instructions operate on registers.
4. Large number of registers – To reduce memory access.
5. Pipelining – Multiple instructions executed in parallel stages.
3. Benefits of RISC Architecture
1. Simplicity = Speed
• RISC processors focus on fewer instructions, so the CPU can execute them much
faster.
• Example: Instead of one complex instruction taking 10 cycles, RISC uses 5 small
instructions, each taking 1 cycle → total 5 cycles (faster overall).
2. Pipelining Advantage
• Think of an assembly line in a factory. One worker paints, another adds wheels,
another does polishing.
• Similarly, in RISC, while one instruction is being executed, another can be decoded,
and yet another can be fetched.
• This pipelining makes RISC processors super fast.
3. Efficient Use of Registers
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• Since RISC has many registers, data can be stored and reused without going back to
slower memory repeatedly.
• This reduces memory bottlenecks.
4. Reduced Complexity in Hardware
• Simpler instructions mean simpler hardware design.
• This lowers manufacturing costs and increases reliability.
5. Easier for Compiler Optimization
• High-level language compilers (like C or Java compilers) can easily translate code into
RISC instructions.
• This improves performance and efficiency.
6. Scalability
• Because of the simplicity, RISC architecture can adapt well to new technologies like
smartphones, IoT devices, and embedded systems.
Diagram: RISC vs CISC
Real-Life Examples of RISC
• ARM Processors (used in almost every smartphone today).
• RISC-V (an open-source modern RISC architecture).
• MIPS architecture (used in many embedded systems).
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Wrapping It All Together
• Indirect Addressing Mode: Like following a two-step treasure map. Slower, but
flexible and powerful for handling pointers and linked structures.
• Relative Addressing Mode: Like walking a few steps from where you stand. Fast,
compact, and best for jumps and loops.
• RISC Architecture: A philosophy of “keep it simple.” Fewer instructions, faster
execution, pipelining advantage, and heavy use of registers make it one of the most
widely used processor designs today.
Final Thought
When we look at computers, it’s tempting to think of them as just cold machines. But once
you peek inside, you realize they have strategies, shortcuts, and philosophies—like
humans.
Indirect addressing is like asking a friend of a friend for help, relative addressing is like
moving a few steps from where you are, and RISC is like living by the mantra “simplify to
multiply speed.”
That’s why modern technology—from your phone to satellites—works faster and smarter,
all thanks to these clever ideas.
SECTION-C
5. Write notes on the following:
(a) Memory Hierarchy
(b) Use of Auxiliary Memory.
Ans: A Fresh Beginning
Imagine for a moment that your brain is like a computer.
When you are solving a math problem, you don’t keep running to the library to check books
every single second. Instead, you use your memory. Some information is right at your
fingertips (like remembering 2+2=4). Some information you can recall with a bit of thought
(like a poem you memorized last year). And some information is stored far away in your
school library (like rare history facts in a big book).
Computers work in exactly the same way. They have different types of memory, organized
in a hierarchy – some are fast but small, while others are huge but slow. The way these
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memories are arranged, and how computers use them, is what we call the Memory
Hierarchy.
And, just like how you depend on the library when your brain can’t hold all the books in the
world, computers depend on something called Auxiliary Memory (like hard drives, SSDs,
and external storage).
So, in this answer, let’s go step by step:
1. First, we’ll travel through the world of memory hierarchy (what it is, why it exists, its
levels, examples, and diagrams).
2. Then, we’ll move to the use of auxiliary memory (what it does, its importance,
examples, and everyday use).
3. Finally, we’ll tie everything together so it feels like a complete story.
Part (a): Memory Hierarchy
What is Memory Hierarchy?
Think of it as a pyramid where the top levels are small but super-fast (like instant reflexes
in your brain), and the lower levels are large but slower (like the books stored in the
library).
Formally:
The Memory Hierarchy in computer architecture refers to the arrangement of storage types
based on speed, cost, and size.
• Fastest & most expensive → Top (CPU registers, cache).
• Slowest & cheapest → Bottom (hard drives, magnetic tapes).
This design balances performance and cost, since making huge amounts of ultra-fast
memory is too expensive.
Why Do We Need Memory Hierarchy?
Let’s imagine:
• Your CPU (the brain of the computer) is extremely fast.
• But the main memory (RAM) can’t always keep up with its speed.
• If the CPU had to wait for every single piece of data to come from slow memory, the
computer would be painfully slow.
Thus, to solve this mismatch problem, computers are designed with a hierarchy:
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• Small, expensive, and fast memory keeps the CPU running smoothly.
• Large, cheaper, slower memory stores everything else.
This way, computers achieve both speed and large storage capacity.
Levels of Memory Hierarchy
Now let’s walk down the pyramid step by step.
1. CPU Registers (Top Level)
• These are like your short-term reflex memory.
• They are tiny storage locations inside the CPU itself.
• Very fast, but store only a few instructions or data values.
• Example: When adding two numbers, the CPU fetches them into registers first.
2. Cache Memory
• Imagine this as a notepad on your study table where you keep the most important
things you need again and again.
• Cache is faster than RAM but smaller in size.
• Divided into L1, L2, and L3 caches:
o L1: Smallest and fastest (inside the CPU core).
o L2: Larger but slower (may be inside or near CPU).
o L3: Shared among multiple cores, bigger but slower.
• Purpose: Reduce the time CPU spends waiting for RAM.
3. Main Memory (RAM)
• Think of this as your working desk where you spread out your books and notes while
studying.
• Random Access Memory (RAM) is faster than hard disks but slower than cache.
• Stores currently running programs and data.
• Volatile → Loses data when power is off.
4. Secondary Storage (Hard Disks, SSDs)
• This is like your home library, where you store a lot of books permanently.
• Non-volatile → Data stays even when power is switched off.
• Slower than RAM but much larger in capacity.
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• Examples: Hard Disk Drives (HDDs), Solid State Drives (SSDs).
5. Tertiary and Off-line Storage
• Imagine the city library or archives – rarely used but available when needed.
• Examples: Magnetic tapes, optical disks (CDs, DVDs), external hard drives.
• Used for backups and archival storage.
Memory Hierarchy Pyramid Diagram
Here’s a simple representation:
CPU Registers
(Fastest, Smallest, Most Expensive)
↓
Cache Memory
↓
Main Memory (RAM)
↓
Secondary Storage
↓
Tertiary Storage
(Slowest, Largest, Cheapest)
Characteristics of Memory at Each Level
Level
Speed
Cost per bit
Size
Volatile/Non-volatile
Registers
Fastest
Highest
Very Small
Volatile
Cache
Very Fast
High
Small
Volatile
Main Memory (RAM)
Fast
Medium
Medium
Volatile
Secondary (HDD/SSD)
Slower
Low
Large
Non-volatile
Tertiary
Slowest
Lowest
Very Large
Non-volatile
Key Idea
The memory hierarchy is all about trade-offs:
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• If we used only registers → Too costly.
• If we used only hard drives → Too slow.
• By combining all, computers achieve efficiency.
Part (b): Use of Auxiliary Memory
Now let’s shift gears. Imagine you are writing an exam. You can’t keep everything in your
head (CPU registers and RAM). So, you bring along your extra notebooks and files where
you have stored old notes. That’s exactly what Auxiliary Memory is for computers.
What is Auxiliary Memory?
Auxiliary memory is also known as secondary storage or external memory.
It refers to the devices used to store large amounts of data permanently, outside the main
memory.
Examples: Hard drives, SSDs, CDs, DVDs, Pen drives, Magnetic tapes, Cloud storage.
Characteristics of Auxiliary Memory
• Non-volatile → Data doesn’t disappear when power is off.
• Large capacity → Can store gigabytes, terabytes, even petabytes.
• Slower than RAM → Needs more time to access data.
• Cheaper per bit → More affordable for large storage.
Uses of Auxiliary Memory
Let’s list them, but with real-life analogies to make them fun:
1. Permanent Storage of Data
o Just like how you keep your certificates and old photos safely in an album,
computers keep files permanently in auxiliary memory.
o Programs, operating systems, movies, songs – all live here.
2. Backup and Recovery
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o Imagine you photocopy your class notes in case your notebook gets lost.
Similarly, auxiliary memory provides backup (on CDs, external drives, or
cloud).
3. Portability
o Pen drives, CDs, and external hard disks allow you to carry data from one
computer to another – like carrying a pendrive of songs to your friend’s
home.
4. Handling Big Data
o RAM can’t hold everything. But auxiliary memory easily stores huge
databases, software, and multimedia files.
5. Virtual Memory Support
o When RAM is full, part of the hard disk is used as virtual memory, allowing
the system to run larger programs than RAM alone could handle.
6. Archival Storage
o Think of old school records kept for decades. Auxiliary memory like magnetic
tapes and DVDs are used for archiving rarely used but important data.
Examples of Auxiliary Memory Devices
• Magnetic Storage → HDDs, Tapes.
• Optical Storage → CDs, DVDs, Blu-ray.
• Solid State Storage → Pen drives, SSDs, Memory cards.
• Cloud Storage → Google Drive, OneDrive, Dropbox (modern auxiliary memory).
Diagram of Primary vs Auxiliary Memory
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Putting It All Together
The Memory Hierarchy is like the structure of your learning process:
• Quick recall (registers, cache).
• Working desk (RAM).
• Home library (hard drives).
• City archives (tapes, cloud).
The Auxiliary Memory acts as your reliable storage friend, keeping all the knowledge safe
even when you’re asleep (computer is turned off).
Without memory hierarchy, computers would either be too costly or too slow.
Without auxiliary memory, we couldn’t save our files, photos, projects, or movies.
So, both concepts are like the heart and soul of computer memory design.
6.(a) What is the concept of Virtual Memory? Explain.
(b) Why associave memory is used for execuon? Explain
Ans: A Tale of Two Minds: Virtual Memory & Associative Memory
Let’s begin not with a definition, but with a scene.
Imagine you’re in a grand library. This isn’t just any library—it’s the Library of Computation,
where every book represents a program, every shelf a memory block, and every librarian a
processor. But this library has a twist: it’s powered by two brilliant minds—Virtual Memory,
the illusionist, and Associative Memory, the detective.
These two minds work behind the scenes to make computing seamless, efficient, and
intelligent. Let’s meet them one by one.
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Part A: Virtual Memory — The Illusionist of the Library
The Problem: Limited Shelf Space
In our grand library, there’s a problem. The shelves (RAM) are limited. But the number of
books (programs) keeps growing. How can we fit more books than the shelves allow?
Enter Virtual Memory, the illusionist.
What Is Virtual Memory?
Virtual Memory is a clever trick used by operating systems to make it seem like the
computer has more memory than it actually does. It creates an illusion of a large,
continuous memory space, even when the physical memory (RAM) is small.
It’s like having a tiny bookshelf but a massive catalog. You don’t need all the books at
once—just the ones you’re reading. So, the librarian swaps books in and out from a storage
room (hard disk), keeping only the active ones on the shelf.
How Does Virtual Memory Work?
Let’s break it down:
• Logical Address Space: The addresses generated by programs.
• Physical Address Space: The actual locations in RAM.
• Memory Management Unit (MMU): A hardware wizard that maps logical addresses
to physical ones.
When a program runs, it uses virtual addresses. The MMU translates these into physical
addresses. If the required data isn’t in RAM, the system fetches it from the hard disk (called
page file or swap space).
Techniques Used in Virtual Memory
1. Paging
Memory is divided into fixed-size blocks called pages. When RAM is full, inactive pages are
moved to disk. When needed again, they’re swapped back.
Analogy: Like rotating books on a shelf based on what you’re reading.
2. Segmentation
Memory is divided into segments based on logical divisions—code, data, stack. Each
segment can vary in size.
Analogy: Like organizing books by genre—fiction, science, history.
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Why Is Virtual Memory Important?
• Multitasking: Run multiple programs simultaneously.
• Security: Isolates processes to prevent interference.
• Efficiency: Loads only necessary parts of programs.
• Scalability: Supports large applications on small systems.
Real-Life Analogy
Imagine your phone has limited storage, but you use cloud storage. You only download the
files you need. Virtual Memory works the same way—keeping the system light and fast.
Advantages vs. Disadvantages
Advantages
Disadvantages
Efficient memory usage
Slower access due to disk swapping
Supports large programs
Increased complexity in memory management
Enhances security
Requires more hardware (MMU)
Enables multitasking
Can cause thrashing if poorly managed
Part B: Associative Memory — The Detective of the Library
Now let’s meet the second mind: Associative Memory, the detective.
What Is Associative Memory?
Associative Memory, also known as Content Addressable Memory (CAM), is a special type
of memory that retrieves data based on content, not location.
Analogy: Instead of asking “What’s on shelf 42?”, you ask “Where’s the book titled
‘Quantum Physics’?” And the librarian instantly finds it.
Why Is It Used for Execution?
During execution, speed is everything. Associative Memory allows the CPU to search and
retrieve data in a single step, making it ideal for tasks like:
• Cache memory lookup
• Translation Lookaside Buffer (TLB)
• Pattern matching
• Networking and AI applications
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How Does Associative Memory Work?
Let’s walk through the process:
1. Input Register (I): Holds the search key.
2. Mask Register (M): Specifies which bits to compare.
3. Select Register (S): Flags matching entries.
4. Output Register (Y): Retrieves the matched data.
The memory compares the input with all stored data in parallel. If a match is found, it’s
retrieved instantly.
Types of Associative Memory
Auto-Associative Memory
• Recalls a pattern from partial input.
• Used in speech and image recognition.
Hetero-Associative Memory
• Maps one pattern to another.
• Used in data compression and retrieval.
Why Is It So Fast?
Unlike traditional memory that searches sequentially, associative memory uses parallel
processing. It checks all entries at once, making it lightning-fast.
Real-Life Analogy
Imagine a detective with a photographic memory. You show them a clue, and they instantly
recall the matching case file. That’s associative memory in action.
Advantages vs. Disadvantages
Advantages
Disadvantages
Super-fast data retrieval
Expensive to implement
Ideal for pattern matching
Complex hardware design
Supports parallel search
Limited scalability
Enhances CPU performance
Higher power consumption
Wrapping It All Together
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Let’s return to our library.
• Virtual Memory is the illusionist who makes the shelves seem endless.
• Associative Memory is the detective who finds any book instantly, no matter where
it’s stored.
Together, they ensure that the library runs smoothly—handling more books than it can
hold, and finding them faster than ever.
SECTION-D
7.(a) How DMA is used for data transfer? Explain in detail
(b) Discuss programmed I/O for data transfer operaons
Ans: Imagine you are sitting in a classroom. The teacher has given you a big pile of answer
sheets to check. At the same time, your friend comes and asks you to write down a long
essay for him. Now, if you try to check answer sheets (your main work) and also keep
writing your friend’s essay (a side task) at the same time, you will get tired, waste time, and
might even make mistakes.
Wouldn’t it be better if you assign your younger brother to write the essay while you
continue checking the answer sheets? Later, your brother will give you the completed essay
without disturbing your main task.
This simple story is exactly what happens inside a computer when it has to transfer data
between devices like memory, hard disk, or input-output devices. The CPU is like you (the
main brain of the system), and the DMA controller is like your younger brother who handles
data transfer. On the other hand, sometimes the CPU itself has to stop its work and handle
the transfer directly. This is called Programmed I/O.
So, in this story, we will learn:
1. How DMA (Direct Memory Access) works for data transfer
2. How Programmed I/O works for data transfer operations
Both are methods of transferring data between CPU, memory, and I/O devices — but they
work in very different ways. Let us carefully explore them step by step.
Part (a) How DMA is used for data transfer?
1. The Problem Before DMA
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In the early days of computers, whenever data had to be moved (say from memory to disk
or from keyboard to memory), the CPU had to directly control every single step. This was
like a teacher writing all student names on the register by herself — slow and time-
consuming.
But soon, people realized that this was a waste of the CPU’s time. The CPU is meant for
processing data (calculations, logic, decision-making), not for shifting data like a coolie at a
railway station. That’s when DMA (Direct Memory Access) was invented.
2. What is DMA?
DMA stands for Direct Memory Access.
It is a technique in which a special hardware unit called DMA Controller is used to transfer
data directly between memory and I/O devices without continuously involving the CPU.
• CPU only initiates the transfer (like giving permission).
• DMA Controller handles the transfer independently.
• CPU is free to do other tasks while data transfer is going on.
This makes the system faster, more efficient, and less CPU-dependent.
3. How DMA Works – Step by Step
Let’s imagine a scenario: You want to copy a big movie file from a hard disk into RAM. How
does DMA do it?
Step 1: CPU Requests Transfer
The CPU tells the DMA Controller:
• From where to pick the data (Source address in memory/I/O).
• Where to place it (Destination address in memory).
• How much data to transfer (Number of bytes/words).
This is like a teacher telling her assistant: “Take these 200 notebooks from the cupboard and
put them on my table.”
Step 2: DMA Takes Control
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The DMA Controller takes over the system bus (address bus, data bus, control bus) from the
CPU for a short time. This is called Cycle Stealing because DMA steals a few memory cycles
from CPU.
Step 3: Data Transfer Happens
DMA starts moving data directly between memory and I/O device:
• It reads one word/byte from the source.
• It places it in the destination.
• It reduces the counter by one.
• It repeats until all data is transferred.
During this time, the CPU is not disturbed. It continues its own separate tasks.
Step 4: Completion and Interrupt
Once the transfer is complete, the DMA Controller sends an interrupt signal to the CPU. This
is like saying: “Sir, I have finished the job.”
Now the CPU knows that the requested data is ready, and it can proceed.
4. Modes of DMA Transfer
There are several modes in which DMA can work:
1. Burst Mode (Block Transfer Mode)
o DMA takes complete control of the system bus.
o It transfers the entire block of data in one go.
o CPU is kept idle during this time.
o Fast, but CPU has to wait.
2. Cycle Stealing Mode
o DMA steals memory cycles one at a time.
o CPU and DMA work alternately.
o CPU performance slightly decreases but does not stop completely.
3. Transparent Mode
o DMA only uses the bus when CPU is not using it.
o No CPU performance loss, but transfer speed may be slower.
5. Advantages of DMA
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• Faster: Transfers data at high speed.
• Efficient: Frees CPU from unnecessary work.
• Parallelism: CPU and DMA can work simultaneously.
• Better for large data: Useful in transferring huge blocks like disk data, multimedia
files, etc.
6. Diagram of DMA Working
Here’s a simple diagram:
This shows how CPU gives the job to DMA Controller, and DMA transfers data between
memory and I/O directly.
Part (b) Programmed I/O for Data Transfer
1. What is Programmed I/O?
Now let us move to another method: Programmed I/O.
In this method, the CPU is fully responsible for handling all data transfers between I/O
devices and memory.
• CPU keeps checking whether the device is ready (using status registers).
• Once the device is ready, CPU reads/writes the data.
• This process is controlled by software programs written by the user/OS.
That’s why it is called “Programmed I/O”.
2. How Programmed I/O Works
Let us take an example of reading characters typed on a keyboard:
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Step 1: CPU Checks Device Status
The CPU repeatedly checks the status of the keyboard: “Is any key pressed?”
Step 2: Device Becomes Ready
When a key is pressed, the keyboard sends a signal.
Step 3: CPU Reads Data
The CPU immediately stops its main work and reads the pressed key into memory.
Step 4: Repeat
This process is repeated again and again for every input/output operation.
3. Characteristics of Programmed I/O
• CPU is always involved in transfer.
• CPU waits until the device is ready.
• CPU directly reads/writes every byte or word.
• Works well for small data transfers.
4. Disadvantages of Programmed I/O
• CPU Wastage: Most of the CPU’s time is wasted in waiting.
• Slow: I/O devices are usually slower than CPU, so CPU sits idle.
• Not Efficient: For large data transfers, it is very inefficient.
5. Diagram of Programmed I/O
Here, CPU keeps checking device status and then transfers data directly.
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Difference Between DMA and Programmed I/O
Feature
Programmed I/O
DMA
CPU Involvement
CPU does everything
CPU only initiates, DMA does the rest
Speed
Slow
Fast
Efficiency
Low
High
Best For
Small transfers
Large transfers
CPU Utilization
Wasted
Saved
Conclusion
So, we can conclude the story like this:
• Programmed I/O is like a teacher who tries to do everything by herself — checking
copies, writing essays, distributing notebooks — she becomes slow and overworked.
• DMA is like a smart teacher who assigns an assistant (DMA Controller) to do the
heavy lifting, while she continues her main job smoothly.
Thus, both techniques are important: Programmed I/O for small, simple jobs and DMA for
big, fast transfers.
8.(a) Discuss the role of pipelining and its types
(b) How MISD and MIMD architectures are organised? Explain
Ans: A Fresh Beginning: A Journey Through a Busy City
Imagine you are standing on the balcony of a tall building, looking down at a busy city. Cars
are rushing on the roads, buses are carrying people, trains are moving on their tracks, and
signals are guiding traffic so that everyone reaches their destination smoothly.
Now, think of a computer processor (CPU) as this city. Inside the CPU, billions of instructions
are moving around like vehicles. Some instructions are simple like "add two numbers," while
others are more complex like "fetch data from memory" or "display something on screen."
If all these instructions tried to move one at a time without coordination, the whole system
would be as slow as a city where only one car is allowed to move at a time! That would
make our computers painfully sluggish.
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This is where the brilliant idea of pipelining and different computer architectures like MISD
and MIMD come into play. They act like smart city planners, making sure traffic flows
smoothly, and different vehicles (instructions) can move in parallel, without colliding.
Let’s go on this journey together and understand these two parts of the question step by
step—like an interesting story.
Part (a) – The Role of Pipelining and Its Types
What is Pipelining?
To understand pipelining, let’s use a simple analogy. Imagine you are in a kitchen making
sandwiches. If you work alone and follow these steps for each sandwich—
1. Take bread
2. Spread butter
3. Add vegetables
4. Add salt and spices
5. Pack the sandwich
—you will make one sandwich at a time. The process is slow because you wait until the first
sandwich is completely done before starting the second one.
But what if you have five people in the kitchen working together like an assembly line?
• Person 1 always takes bread,
• Person 2 spreads butter,
• Person 3 adds vegetables,
• Person 4 adds spices,
• Person 5 packs the sandwich.
Now, while person 2 is buttering the bread of the first sandwich, person 1 has already
started preparing bread for the second sandwich. Everyone is working in parallel, and
sandwiches come out much faster.
This is exactly what pipelining does in a CPU.
Instead of processing one instruction completely before moving to the next, the CPU
overlaps different stages of instruction execution so that multiple instructions are in
progress at the same time.
Role of Pipelining in Computers
Pipelining is the backbone of modern processors. Its role can be understood in these ways:
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1. Speed Improvement (Throughput):
o Without pipelining → only one instruction is executed at a time.
o With pipelining → multiple instructions are executed simultaneously, just like
multiple sandwiches being made in the kitchen.
o This increases the overall speed of the processor.
2. Efficient Use of Resources:
Every part of the CPU (fetching, decoding, execution, memory access, writing results)
can work simultaneously, instead of waiting for one another.
3. Better User Experience:
Thanks to pipelining, your computer feels fast—applications open quickly, games run
smoothly, and internet browsing feels instant.
4. Foundation for Modern Designs:
Advanced CPUs like Intel’s Core series or AMD’s Ryzen use super-pipelining and
parallel pipelines to reach blazing-fast speeds.
So, pipelining is like a traffic signal system in the city—it ensures vehicles (instructions)
move in an organized, overlapping manner without wasting time.
Types of Pipelining
Just like there are different ways to manage traffic in a city, pipelining also comes in various
types. Let’s explore them in a storytelling way.
1. Instruction Pipeline
Imagine a book printing press. One person types the text, another proofreads, another
prints pages, and another binds books. They all work at the same time.
Similarly, in instruction pipelining, different stages of an instruction (Fetch, Decode,
Execute, Memory Access, Write Back) are overlapped.
• Example: While one instruction is being decoded, another is being fetched, and
another is being executed.
This is the most common form of pipelining in CPUs.
2. Arithmetic Pipeline
Suppose you are solving a large multiplication problem by hand. You break it into smaller
steps (like multiplying digits, adding partial results) and keep passing results along.
In CPUs, arithmetic operations like multiplication, division, or floating-point calculations are
broken into pipeline stages. Each stage handles part of the calculation.
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This type is useful in scientific computers, where heavy mathematical operations are
frequent.
3. Instruction Prefetch Pipeline
Have you ever guessed what your friend is about to say before they actually say it?
Similarly, in instruction prefetch pipelines, the CPU tries to fetch upcoming instructions in
advance (before they are even needed). This saves time, as the CPU doesn’t have to wait
when it reaches the next instruction.
This is like Netflix preloading a few seconds of a movie so you can watch without
buffering.
4. Superscalar Pipeline
Imagine not one, but two kitchens making sandwiches at the same time. Both kitchens have
their own pipeline, so sandwiches are made twice as fast.
In CPUs, a superscalar pipeline means having multiple pipelines running in parallel. The
processor can execute more than one instruction per cycle.
Modern processors like Intel Core i7 and AMD Ryzen are superscalar.
5. Super Pipelining
This is like making each stage of the sandwich-making process smaller so that they finish
faster and more sandwiches flow through the line.
In CPUs, super pipelining means dividing the pipeline into more stages, so that work gets
done in finer steps, leading to higher instruction throughput.
6. Vector Pipelining
Imagine not making one sandwich at a time, but cutting vegetables for 10 sandwiches at
once, then buttering bread for 10 at once, and so on.
Vector pipelining is used in vector processors, where the same operation is applied to large
sets of data (like in graphics and scientific computing).
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Challenges in Pipelining (Hazards)
Just like traffic sometimes faces jams, pipelining also has hazards:
1. Data Hazard – When one instruction depends on the result of another (like waiting
for the vegetables before adding spices).
2. Control Hazard – When the CPU doesn’t know which instruction to execute next (like
waiting at a road signal when it’s unclear which way traffic will go).
3. Structural Hazard – When two instructions need the same hardware at the same
time (like two people fighting over one knife in the kitchen).
CPU designers solve these problems using techniques like branch prediction, forwarding,
and adding extra resources.
Part (b) – MISD and MIMD Architectures
Now let’s move to the second part of the question.
Computer architectures are classified based on Flynn’s Taxonomy, which groups them
according to the number of instruction streams and data streams.
• SISD – Single Instruction, Single Data (like old processors)
• SIMD – Single Instruction, Multiple Data (like GPUs)
• MISD – Multiple Instruction, Single Data
• MIMD – Multiple Instruction, Multiple Data
Let’s focus on MISD and MIMD since the question asks about them.
MISD (Multiple Instruction, Single Data)
Imagine a school exam where one student (data) writes a paper, and multiple teachers
(instructions) check it for different things: one for grammar, one for handwriting, one for
accuracy.
That’s MISD.
• Multiple processors execute different instructions but on the same data.
• It’s not very common in real life, but useful in fault-tolerant systems (like space
shuttles or nuclear reactors) where the same data is checked by multiple units to
ensure reliability.
Organisation of MISD:
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• All processors share the same input data stream.
• Each processor runs a different instruction on this same data.
• Final results are combined for safety or accuracy.
Example: Space shuttle control systems where one piece of sensor data is analyzed by
multiple processors to avoid errors.
MIMD (Multiple Instruction, Multiple Data)
Now, imagine a classroom where every student has their own question paper and each
solves it in their own way. Some are solving math problems, some are writing essays, some
are drawing diagrams.
That’s MIMD.
• Multiple processors execute different instructions on different sets of data.
• This is the most common architecture today, used in multicore processors,
supercomputers, and cloud computing.
Organisation of MIMD:
• Each processor has its own control unit and operates independently.
• They can work in loosely coupled systems (each with its own memory, connected via
a network) or tightly coupled systems (shared memory).
• This allows huge flexibility and speed in parallel computing.
Examples:
• Modern laptops and smartphones with quad-core or octa-core processors.
• Supercomputers like those used for weather forecasting, artificial intelligence, or
scientific simulations.
Bringing It All Together
So, our journey through this “city of computing” tells us:
• Pipelining is like an assembly line or traffic system, allowing instructions to overlap
and making the CPU much faster. It has different types—Instruction, Arithmetic,
Superscalar, Super, Vector, and Prefetch pipelines—each designed for specific needs.
• MISD is like many teachers checking the same student’s paper, ensuring safety and
reliability.
• MIMD is like many students solving different papers, which represents the real
world of modern computing, from your phone to the world’s fastest
supercomputers.
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Without these concepts, our computers would still be crawling like bullock carts on a village
road instead of zooming like high-speed trains.
“This paper has been carefully prepared for educaonal purposes. If you noce any mistakes or
have suggesons, feel free to share your feedback.”
