1.3.1 What is Computer Architecture?
Computer architecture refers to the basic design and organization of a computer system. It explains how different parts of a computer are arranged and how they work together to execute instructions.
Simple Definition
Computer architecture is the conceptual design and functional organization of a computer system.
It mainly describes:
- CPU
- Memory
- Input/Output devices
- Data paths
- Control mechanisms
- Communication between components
Basic Computer Architecture
COMPUTER SYSTEM │ ┌───────────────┼───────────────┐ ↓ ↓ ↓ INPUT CPU OUTPUT DEVICES │ DEVICES │ ┌────────┴────────┐ ↓ ↓ ALU CU │ │ └────────┬────────┘ ↓ MEMORY │ ↓ STORAGE
1.3.2 Basic Components of a Computer
A computer system mainly consists of:
- Input Unit
- Central Processing Unit (CPU)
- Memory Unit
- Output Unit
- Storage Unit
1. Input Unit
Accepts data and instructions from the user.
Examples: Keyboard, mouse, scanner.
2. CPU
Processes instructions and controls the operation of the computer.
3. Memory
Stores data, instructions and intermediate results.
4. Output Unit
Provides processed results to the user.
Examples: Monitor, printer, speaker.
5. Storage
Stores data and programs for long-term use.
Examples: HDD, SSD, USB drive.
1.3.3 Von Neumann Architecture
One of the most important concepts in computer architecture is the Von Neumann architecture.
The Von Neumann model was proposed by John von Neumann and is based on the idea that instructions and data can be stored in the same memory.
Main Components
- Memory
- Control Unit
- ALU
- Input
- Output
┌───────────────┐ │ MEMORY │ │ Data + │ │ Instructions │ └───────┬───────┘ │ ↓ ┌─────────────────┐ │ CPU │ │ │ │ ┌─────┐ ┌────┐ │ │ │ ALU │ │ CU │ │ │ └─────┘ └────┘ │ └───────┬─────────┘ │ ┌─────────┴─────────┐ ↓ ↓ INPUT OUTPUT
Important Feature
Von Neumann architecture stores both data and instructions in the same memory.
1.3.4 Von Neumann Bottleneck
The Von Neumann architecture has an important limitation called the Von Neumann bottleneck.
The CPU and memory communicate through shared pathways. Since data and instructions must travel between memory and CPU, the speed of the overall system can be limited by this communication.
Simple Example
Imagine:
CPU ←────── Narrow Road ──────→ Memory
Even if the CPU is very fast, a limited communication path can reduce overall performance.
Exam Point
Von Neumann bottleneck = limitation caused by the limited data transfer between CPU and memory.
1.3.5 CPU — Central Processing Unit
The CPU (Central Processing Unit) is the main processing component of a computer.
It is often called the brain of the computer because it executes instructions and controls many operations.
Simple Definition
CPU is the component that executes instructions, performs calculations and controls the operation of the computer system.
Main Components of CPU
- ALU — Arithmetic Logic Unit
- CU — Control Unit
- Registers
CPU │ ┌──────────┼──────────┐ ↓ ↓ ↓ ALU CU Registers │ │ │ Arithmetic Control Temporary & Logic Signals Data
1.3.6 Arithmetic Logic Unit — ALU
ALU = Arithmetic Logic Unit
The ALU performs:
Arithmetic Operations
- Addition
- Subtraction
- Multiplication
- Division
Logical Operations
- AND
- OR
- NOT
- XOR
- Comparison operations
Example
If the CPU needs to calculate:
20 + 30
The operation is performed by the ALU.
Simple Definition
ALU is the part of CPU responsible for arithmetic and logical operations.
1.3.7 Control Unit — CU
The Control Unit (CU) controls and coordinates the activities of the computer.
It tells different components what to do and when to do it.
Main Functions
- Fetches instructions from memory
- Decodes instructions
- Generates control signals
- Controls data movement
- Coordinates CPU, memory and I/O operations
Simple Example
Suppose the instruction is:
ADD A, B
The Control Unit:
- Fetches the instruction.
- Decodes it.
- Tells the ALU to perform addition.
- Controls movement of the required data.
- Helps store the result.
Easy Definition
CU controls the operation of the computer but does not normally perform arithmetic calculations itself.
1.3.8 Registers
Registers are very small and very fast storage locations inside the CPU.
They temporarily hold:
- Data
- Instructions
- Addresses
- Intermediate results
Why are registers important?
The CPU can access registers much faster than main memory.
Common Registers
1. Accumulator (ACC)
Stores intermediate arithmetic and logical results.
2. Program Counter (PC)
Stores the address of the next instruction to be executed.
3. Instruction Register (IR)
Stores the current instruction being executed or decoded.
4. Memory Address Register (MAR)
Stores the address of the memory location to be accessed.
5. Memory Data Register (MDR)
Stores data being transferred to or from memory.
6. Status/Flag Register
Stores information about the result of operations.
Examples of flags:
- Zero flag
- Carry flag
- Sign flag
- Overflow flag
1.3.9 CPU Working Cycle
The CPU generally works through the following basic cycle:
Fetch → Decode → Execute → Store
This is known as the instruction cycle.
Step 1: Fetch
The CPU gets the next instruction from memory.
Step 2: Decode
The Control Unit interprets the instruction.
Step 3: Execute
The required operation is performed.
Step 4: Store
The result is stored in a register or memory as required.
Example
Suppose the computer needs to perform:
5 + 3
Instruction ↓ Fetch ↓ Decode ↓ ALU ↓ 5 + 3 = 8 ↓ Store
1.3.10 Instruction Cycle
The instruction cycle can be represented as:
┌─────────────┐ │ FETCH │ └──────┬──────┘ ↓ ┌─────────────┐ │ DECODE │ └──────┬──────┘ ↓ ┌─────────────┐ │ EXECUTE │ └──────┬──────┘ ↓ ┌─────────────┐ │ STORE │ └──────┬──────┘ │ └────→ Next Instruction
1.3.11 Control Unit Functions
The Control Unit performs several important functions.
1. Instruction Fetching
Gets instructions from memory.
2. Instruction Decoding
Determines what the instruction means.
3. Control Signal Generation
Generates signals to control different components.
4. Data Movement
Controls movement of data between:
- CPU
- Memory
- Input devices
- Output devices
5. Coordination
Coordinates the activities of ALU, registers, memory and I/O devices.
1.3.12 Types of Control Unit
There are two major types of Control Unit:
- Hardwired Control Unit
- Microprogrammed Control Unit
A. Hardwired Control Unit
A hardwired control unit uses fixed electronic logic circuits to generate control signals.
Advantages
- Very fast
- Efficient
Disadvantages
- Difficult to modify
- Complex for large instruction sets
B. Microprogrammed Control Unit
A microprogrammed control unit uses microinstructions stored in control memory to generate control signals.
Advantages
- Easier to modify
- Easier to design for complex instruction sets
Disadvantages
- Generally slower than hardwired control
Comparison
| Hardwired CU | Microprogrammed CU |
|---|---|
| Uses hardware logic | Uses microinstructions |
| Generally faster | Generally slower |
| Difficult to modify | Easier to modify |
| Commonly associated with simpler instruction control | Useful for complex instruction sets |
1.3.13 Computer Arithmetic
Computer arithmetic refers to the methods used by computers to perform arithmetic operations on binary numbers.
Computers internally use the binary number system.
The main arithmetic operations are:
- Addition
- Subtraction
- Multiplication
- Division
1.3.14 Binary Addition
Binary addition follows these basic rules:
| A | B | Sum | Carry |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
Example
1010 + 0011 ------ 1101
Therefore:
1010₂ + 0011₂ = 1101₂
1.3.15 Binary Subtraction
Basic rules:
| A | B | Difference | Borrow |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 0 |
| 0 | 1 | 1 | 1 |
Example
1101 - 0011 ------ 1010
Therefore:
1101₂ − 0011₂ = 1010₂
1.3.16 Binary Multiplication
Binary multiplication is similar to decimal multiplication.
Important rules:
0 × 0 = 0 0 × 1 = 0 1 × 0 = 0 1 × 1 = 1
Example
101 × 11 ----- 101 101 ----- 1111
Therefore:
101₂ × 11₂ = 1111₂
1.3.17 Binary Division
Binary division follows the same basic concept as decimal long division.
Example:
1100 ÷ 10 = 110
Therefore:
1100₂ ÷ 10₂ = 110₂
1.3.18 Signed Number Representation
Computers need a way to represent both positive and negative numbers.
Common methods include:
- Sign-Magnitude
- One's Complement
- Two's Complement
Sign-Magnitude Representation
The most significant bit represents the sign.
0 → Positive 1 → Negative
The remaining bits represent the magnitude.
Example
For an 8-bit number:
00000101 = +5 10000101 = -5
1.3.19 One's Complement
The one's complement of a binary number is obtained by changing:
0 → 1 1 → 0
Example
Original: 00000101 One's complement: 11111010
1.3.20 Two's Complement
Two's complement is widely used for representing signed integers.
Steps
- Find the one's complement.
- Add 1.
Example: Find two's complement of 00000101
Original: 00000101 One's complement: 11111010 Add 1: 00000001 --------- 11111011
Therefore:
Two's complement = 11111011
Important
Two's complement = One's complement + 1
1.3.21 Input/Output Organization
Input/Output (I/O) organization refers to how a computer communicates with external devices.
Examples of I/O devices:
Input
- Keyboard
- Mouse
- Scanner
- Microphone
Output
- Monitor
- Printer
- Speaker
- Projector
1.3.22 I/O Module
An I/O module is a hardware component that acts as an interface between the CPU/memory and peripheral devices.
Simple Structure
CPU / Memory │ ↓ I/O Module │ ├──── Keyboard ├──── Mouse ├──── Printer └──── Monitor
Functions of I/O Module
- Communicates with CPU
- Communicates with peripheral devices
- Provides buffering
- Controls data transfer
- Detects errors
- Provides status information
1.3.23 Methods of I/O Data Transfer
There are three important methods:
- Programmed I/O
- Interrupt-Driven I/O
- Direct Memory Access (DMA)
1.3.24 Programmed I/O
In Programmed I/O, the CPU continuously checks the status of an I/O device and controls the transfer.
Example
CPU repeatedly checks:
"Is the keyboard ready?"
If not:
"Check again."
This is also called polling.
Disadvantage
The CPU spends time waiting for the I/O device.
1.3.25 Interrupt-Driven I/O
In interrupt-driven I/O, the device sends an interrupt signal to the CPU when it needs attention or when an operation is ready.
Example
Instead of continuously checking the keyboard:
CPU → Continue other work Keyboard → "I have data!" CPU → Handles keyboard request
Advantage
CPU does not need to continuously wait for the device.
1.3.26 Direct Memory Access — DMA
DMA = Direct Memory Access
DMA allows an I/O device to transfer data directly to or from main memory with minimal CPU involvement.
Normal transfer
I/O Device → CPU → Memory
DMA transfer
I/O Device ─────────→ Memory DMA
The CPU mainly initializes and controls the DMA operation, while the DMA controller handles the actual data transfer.
Advantages
- Faster data transfer
- Reduces CPU workload
- Useful for large blocks of data
Examples
DMA is useful for:
- Disk transfers
- Network transfers
- Audio/video data
- High-speed peripherals
1.3.27 Programmed I/O vs Interrupt I/O vs DMA
| Feature | Programmed I/O | Interrupt I/O | DMA |
|---|---|---|---|
| CPU involvement | High | Moderate | Low during transfer |
| CPU waits/polls | Yes | No continuous polling | No |
| Speed | Relatively low | Better | High |
| Suitable for | Simple transfers | Event-based I/O | Large/high-speed transfers |
Easy Trick
Programmed I/O = CPU keeps checking
Interrupt I/O = Device calls CPU
DMA = Device transfers directly with memory
1.3.28 Memory Organization
Memory organization refers to how computer memory is arranged, addressed and accessed.
Memory stores:
- Instructions
- Data
- Intermediate results
- Programs
1.3.29 Memory Hierarchy
Computer memory is organized into levels according to:
- Speed
- Cost
- Capacity
Memory Hierarchy
Fastest ↑ Registers ↓ Cache ↓ RAM ↓ SSD / HDD ↓ Backup/Archive ↓ Slowest
General Rule
As we move down the hierarchy:
- Speed generally decreases
- Capacity generally increases
- Cost per bit generally decreases
1.3.30 Registers
Registers are the fastest storage locations inside the CPU.
Features
- Very fast
- Very small capacity
- Located inside CPU
- Hold temporary data and instructions
1.3.31 Cache Memory
Cache memory is a small, high-speed memory located close to or integrated with the CPU.
It stores frequently or recently used data and instructions so the CPU can access them quickly.
Why is cache needed?
CPU is much faster than main memory.
Cache helps reduce the effective time needed to access frequently used information.
Levels of Cache
- L1 Cache
- L2 Cache
- L3 Cache
Generally:
L1 is smaller and faster than L2, while L3 is generally larger and slower than L1/L2.
1.3.32 Main Memory
Main memory is the memory directly used by the computer during normal operation.
It mainly includes:
- RAM
- ROM
1.3.33 RAM
RAM = Random Access Memory
RAM stores programs and data that are currently being used by the CPU.
Features
- Fast
- Read/write
- Volatile
- Temporary storage
- Usually larger than cache
Example
When you open Microsoft Word:
SSD/HDD ↓ RAM ↓ CPU
The program is loaded into RAM for active use.
1.3.34 Types of RAM
Two major types are:
1. SRAM
SRAM = Static Random Access Memory
- Faster
- More expensive
- Does not require periodic refreshing in the same way DRAM does
- Commonly used for CPU cache
2. DRAM
DRAM = Dynamic Random Access Memory
- Slower than SRAM
- Less expensive
- Higher density
- Requires periodic refreshing
- Commonly used as main memory
SRAM vs DRAM
| SRAM | DRAM |
|---|---|
| Faster | Slower |
| More expensive | Less expensive |
| Lower density | Higher density |
| Used commonly for cache | Used commonly for main memory |
| Does not require refresh cycles like DRAM | Requires periodic refresh |
1.3.35 ROM
ROM = Read Only Memory
ROM is non-volatile memory used to store information that should remain available when power is turned off.
Traditionally, ROM referred to memory that was mainly read-only.
Modern systems commonly use forms of non-volatile memory such as flash memory for firmware.
Uses
- Firmware
- Boot-related programs
- Embedded systems
1.3.36 Types of ROM
Common types include:
1. PROM
Programmable Read Only Memory
Can be programmed once after manufacturing.
2. EPROM
Erasable Programmable Read Only Memory
Can be erased, traditionally using ultraviolet light, and programmed again.
3. EEPROM
Electrically Erasable Programmable Read Only Memory
Can be erased and programmed electrically.
4. Flash Memory
A widely used form of electrically erasable non-volatile memory.
Used in:
- SSDs
- USB drives
- Memory cards
- Firmware storage
1.3.37 RAM vs ROM
| RAM | ROM |
|---|---|
| Volatile | Non-volatile |
| Read/write during normal operation | Traditionally mainly read-oriented |
| Used for active programs/data | Used for persistent firmware/data |
| Contents normally lost when power is removed | Contents retained without power |
| Generally larger in modern computers | Usually smaller for firmware purposes |
1.3.38 Memory Address
Each memory location has a unique address.
Think of memory like a large collection of numbered boxes.
Address Data 1000 101010 1001 110011 1002 111000 1003 010101
The CPU uses the address to identify the location from which it wants to read or to which it wants to write.
1.3.39 Word
A word is the natural unit of data handled by a processor.
The word size depends on the architecture.
Examples:
- 8-bit
- 16-bit
- 32-bit
- 64-bit
Example
A 64-bit processor architecture generally has a 64-bit word size for many core operations, although specific implementation details can vary.
1.3.40 Memory Unit Conversion
Basic units:
1 Byte = 8 bits
Common decimal storage units:
1 KB = 1,000 Bytes 1 MB = 1,000 KB 1 GB = 1,000 MB 1 TB = 1,000 GB
Binary-based units use:
1 KiB = 1,024 Bytes 1 MiB = 1,024 KiB 1 GiB = 1,024 MiB 1 TiB = 1,024 GiB
Exam Note
Many traditional computer-fundamentals questions use 1 KB = 1024 bytes, but technically KiB is the correct term for 1024 bytes.
1.3.41 Buses
A bus is a communication pathway used to transfer data, addresses and control signals between computer components.
The three traditional categories are:
- Data Bus
- Address Bus
- Control Bus
1. Data Bus
Carries actual data between components.
CPU ↔ Memory
2. Address Bus
Carries the address of the memory or I/O location that the CPU wants to access.
CPU → Memory
3. Control Bus
Carries control signals.
Examples:
- Read
- Write
- Interrupt
- Clock-related control signals
1.3.42 Data Bus vs Address Bus vs Control Bus
| Bus | Main Function |
|---|---|
| Data Bus | Carries data |
| Address Bus | Carries addresses |
| Control Bus | Carries control signals |
Easy Trick
Data Bus = What?
Address Bus = Where?
Control Bus = How/When?
1.3.43 Computer Architecture — Complete Flow
A simple computer system works approximately like this:
INPUT ↓ ┌──────────────┐ │ MEMORY │ └──────┬───────┘ ↓ ┌──────────────┐ │ CPU │ │ │ │ ALU + CU + │ │ Registers │ └──────┬───────┘ ↓ OUTPUT
Storage provides long-term data retention:
CPU ↕ Memory ↕ SSD / HDD
⭐ 1.3.44 Important Comparisons
CPU vs ALU vs CU
| CPU | ALU | CU |
|---|---|---|
| Main processing unit | Part of CPU | Part of CPU |
| Contains ALU, CU, registers | Performs arithmetic/logic | Controls operations |
| Executes instructions | Calculates and compares | Fetches/decodes and generates control signals |
RAM vs Cache vs Register
| Register | Cache | RAM |
|---|---|---|
| Fastest | Very fast | Slower than cache/registers |
| Smallest | Small | Larger |
| Inside CPU | Close to/inside CPU | Main memory |
| Holds immediate values/instructions | Holds frequently used data/instructions | Holds active programs/data |
Easy Memory Hierarchy
Register → Cache → RAM → Secondary Storage
As you move right:
Capacity generally increases, speed generally decreases.
⭐ 1.3.45 Most Important Exam Questions
Long Questions
- What is computer architecture? Explain its major components.
- Explain Von Neumann architecture with a diagram.
- What is the Von Neumann bottleneck?
- What is CPU? Explain its major components.
- Explain ALU, Control Unit and registers.
- Explain the instruction cycle.
- Explain the functions of the Control Unit.
- Differentiate between hardwired and microprogrammed control units.
- What is computer arithmetic? Explain binary arithmetic.
- Explain binary addition and subtraction with examples.
- Explain sign-magnitude, one's complement and two's complement.
- What is I/O organization?
- Explain the functions of an I/O module.
- Explain programmed I/O, interrupt-driven I/O and DMA.
- What is DMA? Explain its advantages.
- What is memory organization?
- Explain memory hierarchy with a diagram.
- What is cache memory? Explain L1, L2 and L3 cache.
- Differentiate between SRAM and DRAM.
- Differentiate between RAM and ROM.
- Explain PROM, EPROM and EEPROM.
- What are data bus, address bus and control bus?
- Explain how CPU, memory and I/O devices communicate.
- Explain the complete instruction execution process.
🧠1.3.46 Quick Revision — One Page
Computer Architecture
Design and organization of a computer system
CPU
Main processing unit
CPU consists mainly of:
ALU + CU + Registers
ALU
Performs arithmetic and logical operations
CU
Controls and coordinates computer operations
Register
Very small and very fast storage inside CPU
Instruction Cycle
Fetch → Decode → Execute → Store
Von Neumann Architecture
Data and instructions stored in the same memory
Von Neumann Bottleneck
Limited CPU–memory data transfer can restrict performance
Computer Arithmetic
Arithmetic operations performed using binary numbers
Addition | Subtraction | Multiplication | Division
Two's Complement
One's Complement + 1
I/O Organization
Communication between computer and peripheral devices
I/O Transfer Methods
Programmed I/O → Interrupt I/O → DMA
Remember:
Programmed I/O = CPU checks
Interrupt I/O = Device alerts CPU
DMA = Direct data transfer between I/O and memory
Memory Hierarchy
Fastest ↓ Registers ↓ Cache ↓ RAM ↓ SSD/HDD ↓ Slowest
RAM
Volatile working memory
ROM
Non-volatile memory traditionally used for persistent firmware/data
SRAM
Fast + Expensive + Cache
DRAM
Less expensive + Higher density + Main Memory
Buses
Data Bus = Data
Address Bus = Location
Control Bus = Control Signals
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