Computer Systems Notes
Systems Architecture and CPU Function
CPU Purpose: Fetch-Execute Cycle
Basic Computer Function
A computer:
Takes input.
Processes data.
Delivers output.
CPU's Role
The Central Processing Unit (CPU) fetches, decodes, and executes instructions through the fetch-execute cycle.
Fetch-Execute Cycle Steps
Fetch: Retrieves the next instruction from main memory (RAM) and brings it into the CPU.
Decode: Inspects the instruction to determine the required action.
Execute: Carries out the instruction; this may involve:
Fetching data from main memory.
Performing a calculation.
Storing information back into main memory.
CPU Components
Arithmetic Logic Unit (ALU): Performs calculations and logical decisions.
Control Unit (CU): Sends signals to manage data flow within the CPU.
Cache: Provides rapid access to frequently used instructions and data.
Registers: Very fast onboard memory with specific purposes.
Von Neumann Architecture
Based on the stored-program concept, the Von Neumann architecture includes:
Control Unit (CU)
Arithmetic and Logic Unit (ALU)
Memory Unit (typically RAM)
Inputs and Outputs
Stored-Program Concept
Both instruction data and program data are stored in the same memory in binary form. It's impossible to differentiate between instructions and data by merely examining the binary content in memory.
Key Registers
Program Counter (PC): Stores the address of the next instruction in memory.
Memory Address Register (MAR): Stores the address for fetching or storing data.
Memory Data Register (MDR): Holds data fetched from or to be written to memory.
Accumulator: Stores the results of calculations.
Fetch-Decode-Execute Cycle in Detail
Fetch Stage
The Program Counter (PC) is checked to get the address of the next instruction.
The Memory Address Register (MAR) uses this address to fetch the instruction from RAM.
The Program Counter (PC) is incremented to point to the subsequent instruction.
Decode Stage
The Control Unit (CU) decodes the instruction to determine the required action.
Execute Stage
The instruction is executed, which may involve:
Fetching data from RAM and adding it to the accumulator.
Jumping to another instruction.
Writing data from the accumulator back to RAM.
Factors Affecting CPU Performance
Number of Cores:
A core is a complete CPU copy that contains all the necessary components to execute instructions, independently of other cores on the same chip. These components include registers, an Arithmetic Logic Unit (ALU), an accumulator, and a control unit.
A quad-core processor has 4 processing units, which allows it to execute multiple instructions simultaneously, potentially leading to faster processing for multi-threaded applications and multitasking.
Limitations of Increasing Cores: Doubling cores does not double the speed due to several reasons:
Communication Overhead: Communication and synchronization between cores introduce overhead. The more cores, the more complex the coordination.
Program Design Limitations: Many programs are not designed to take full advantage of multiple cores. Some tasks cannot be easily split into parallel processes, limiting the performance gain from additional cores.
Amdahl's Law: This law states that the potential speedup of a program using multiple processors is limited by the fraction of the program that is sequential (non-parallelizable).
Embedded Systems
An embedded system is a specialized computer system designed to perform a dedicated function within a larger mechanical or electrical system. These systems are characterized by:
Low Power Consumption: Essential for battery-powered devices and to reduce heat generation.
Small Size: Allows integration into compact devices and environments.
Rugged Operating Ranges: Designed to withstand extreme temperatures, vibrations, and other harsh conditions.
Low Cost per Unit: Important for mass production and cost-sensitive applications.
Examples: Traffic lights, domestic appliances, factory equipment, engine management systems, and hospital equipment.
Primary Storage
Consists of:
Random Access Memory (RAM)
Read-Only Memory (ROM)
Registers & Cache
It holds data and instructions needed by the CPU while the computer is running. The CPU accesses primary storage faster than secondary storage.
RAM
RAM holds the operating system, currently running programs, and the data being used by those programs, providing quick access for the CPU.
Volatile: RAM loses its contents when power is off, meaning any unsaved data will be lost.
Read and Write Capable: Data can be both read from and written to RAM, allowing for dynamic modification during program execution.
Larger than ROM: Typically, RAM has a much larger capacity than ROM, accommodating more complex programs and data.
ROM
ROM holds initial startup instructions (bootstrap) that are executed when the computer is powered on. This includes the BIOS or UEFI, which initializes hardware components.
Use in Embedded Systems: May store entire programs in embedded systems where the software is fixed and does not need to be changed.
Non-Volatile: ROM retains its contents when power is off, ensuring that the startup instructions are always available.
Read-Only: Data in ROM cannot be easily modified, protecting it from accidental or malicious alteration.
Smaller than RAM: ROM usually has a smaller capacity compared to RAM, as it only needs to store essential startup code.
Virtual Memory
Virtual memory is a memory management technique used when physical RAM is insufficient to hold all running programs and data.
Location: It is located on the hard disk or SSD, acting as an extension of RAM.
Programs are moved between RAM and virtual memory as needed through a process called swapping. This allows more programs to run concurrently than could be accommodated by RAM alone, though it can slow down performance due to the slower access times of hard drives or SSDs compared to RAM.
Secondary Storage
Secondary storage is needed because ROM is read-only and RAM is volatile, providing a persistent storage solution.
Uses:
Storing programs and data when the power is off, ensuring long-term data retention.
Semi-permanent storage of changeable data, allowing users to modify and save files.
Backups, providing a copy of important data in case of primary storage failure.
Archiving, storing data that is no longer actively used but needs to be retained for future reference.
Types of Storage
Optical (CD-R/RW, DVD-R/RW, Blu-Ray):
Low capacity compared to other modern storage types.
Slow data access speeds.
Portable, making it easy to transfer data between devices.
Magnetic (Hard disk drive, tape):
High capacity, suitable for storing large amounts of data.
Relatively fast data access, though slower than solid-state drives.
Mechanical parts are subject to failure, making them less reliable than solid-state drives.
Requires defragmentation for optimal performance as data can become fragmented over time.
Solid-state (SSD, Memory sticks, Flash memory cards):
Medium capacity, balancing storage size with speed and reliability.
Very fast data access, significantly improving system responsiveness and application loading times.
No moving parts, making them more reliable and durable than magnetic drives.
Silent operation due to the absence of mechanical components.
Low power consumption, extending battery life in portable devices.
No need to defragment, as performance is not affected by data fragmentation.
Limited read/write cycles, although modern SSDs are designed to withstand many years of use.
More expensive per unit of storage compared to magnetic drives.
Choosing a Storage Device
Factors to consider:
Capacity: How much data needs to be stored, influencing the type of storage needed.
Speed: How quickly data can be read and transferred, impacting system performance.
Portability: Is the storage device easily transportable between different systems?
Durability: How well can the storage media withstand extreme conditions and physical stress?
Reliability: How often can the storage be used before failure, affecting data integrity?
Cost: How expensive is the media per byte, influencing overall budget considerations?
Storage Device Suitability
Optical:
Suitable for read-only data distribution on a large scale and small capacity situations, such as distributing software or music albums.
Magnetic:
Suitable when very high data capacity is required, such as storing large media libraries or server backups. Also appropriate for fast data access and low-cost cloud storage.
Solid State:
Suitable for low power, small embedded systems, rugged applications
Choosing a Storage Device
Factors to consider:
Capacity: How much data needs to be stored?
Speed: How quickly can data be read and transferred?
Portability: Is size and weight important?
Durability: How robust is the media in extreme conditions?
Reliability: How often can it be used without failing?
Cost: How expensive is the media per byte?
Optical Storage (CD-R/RW, DVD-R/RW, Blu-Ray):
Use Cases: Best suited for distributing read-only data on a large scale where data modification isn't required. Common examples include distributing software, music albums, and movies. They are also used for creating personal data backups, although less common now due to their lower capacity compared to newer storage options.
Advantages: Portable and easily shareable between devices. Standardized formats ensure compatibility across different systems.
Disadvantages: Limited storage capacity, relatively slow data access speeds, and susceptibility to physical damage (scratches).
Additional Information: Optical drives are becoming less common in modern computers due to the rise of faster and more convenient storage solutions like USB drives and cloud storage.
Magnetic Storage (Hard Disk Drive - HDD, Magnetic Tape):
Use Cases: Best suited for applications requiring very high data capacity at a relatively low cost. Ideal for storing large media libraries (videos, music, photos), server backups, and large databases. Also used in low-cost cloud storage solutions where capacity is more critical than speed.
Advantages: High storage capacities at lower costs compared to solid-state drives. Mature technology with well-established reliability.
Disadvantages: Slower data access speeds compared to SSDs. Mechanical components make them more susceptible to physical damage. Requires periodic defragmentation to maintain optimal performance. Higher power consumption compared to SSDs.
Additional Information: HDDs are sensitive to physical shock and vibration, making them less suitable for portable devices and rugged environments.
Solid State Storage (SSD, USB Flash Drives, Memory Cards):
Use Cases: Ideal for applications needing very fast data access, silent operation, and/or low power consumption. Well-suited for primary storage in laptops and desktops, embedded systems, rugged applications, and portable devices.
Advantages: Extremely fast data access speeds, improving system responsiveness and application loading times. No moving parts, resulting in greater durability and silent operation. Lower power consumption, extending battery life in portable devices. Not susceptible to fragmentation, eliminating the need for defragmentation.
Disadvantages: Higher cost per unit of storage compared to HDDs. Limited read/write cycles, although modern SSDs are designed to withstand many years of use. Capacity may be lower compared to HDDs for the same price point.
Additional Information: SSDs significantly enhance the overall user experience due to their speed. They are also less sensitive to physical shock and vibration, making them ideal for mobile devices.
Choosing a Storage Device
Factors to consider:
Capacity: How much data needs to be stored?
Speed: How quickly can data be read and transferred?
Portability: Is size and weight important?
Durability: How robust is the media in extreme conditions?
Reliability: How often can it be used without failing?
Cost: How expensive is the media per byte?
Storage Device Suitability
Optical:
Suitable for read-only data distribution on a large scale and small capacity situations.
Magnetic:
Suitable when very high data capacity is required, for fast data access, low-cost cloud storage.
Solid State:
Suitable for low power, small embedded systems, rugged applications, small to medium data capacity needs, silent operation, very fast data access, and lightweight devices.
Detailed Factors to Consider When Choosing a Storage Device:
Capacity: This is one of the primary considerations. If you need to store large amounts of data (e.g., video files, system backups), a high-capacity storage device is necessary. HDDs typically offer the highest capacities, making them suitable for large media libraries.
Speed: The speed of the storage device directly impacts performance. Faster storage devices result in quicker boot times, faster application loading, and improved overall system responsiveness. SSDs offer significantly faster read/write speeds compared to HDDs and optical drives.
Portability: If you need to transport data between different locations, portability is a key consideration. USB flash drives and external SSDs are highly portable due to their small size and lightweight design. Optical discs are also portable but have limitations in terms of capacity and durability.
Durability: The storage device’s ability to withstand physical stress and environmental conditions is important, especially for mobile and rugged applications. SSDs are more durable than HDDs due to the absence of moving parts.
Reliability: Reliability is a critical factor for ensuring data integrity and preventing data loss. SSDs generally have better reliability compared to HDDs, but it's crucial to consider the manufacturer's specifications and warranty.
Cost: The cost per byte of storage is an important factor when considering a storage solution. HDDs typically offer the lowest cost per byte, while SSDs are more expensive. Optical discs are relatively inexpensive but have limited capacities.
Power Consumption: Power consumption is an important consideration for portable devices like laptops and smartphones. SSDs consume less power compared to HDDs, leading to longer battery life.
Form Factor: The physical size and shape of the storage device may be a factor, particularly in space-constrained environments such as embedded systems and small form-factor computers.
Interface: The interface used to connect the storage device to the computer can affect performance. Common interfaces include SATA, NVMe, USB, and Thunderbolt. NVMe SSDs offer the highest performance due to their direct connection to the PCIe bus.
Units of Measurement
Unit | Symbol | Binary Value | Decimal Value | Approximation | Written |
|---|---|---|---|---|---|
Bit | b | 0 or 1 | |||
Nibble | 4 bits = 1/2 byte | ||||
Byte | B | 8 bits | 1 | 1 byte | |
Kilobyte | KB | bits | 1,000 bytes | Thousand bytes | |
Megabyte | MB | bits | 1,000,000 bytes | Million bytes | |
Gigabyte | GB | bits | 1,000,000,000 bytes | Billion bytes | |
Terabyte | TB | bits | 1,000,000,000,000 bytes | Trillion bytes | |
Petabyte | PB | bits | 1,000,000,000,000,000 bytes | Quadrillion bytes |
File Size Calculations
File Size Calculations
Text File Size:
Size=(Bits per character)×(Number of characters)Size=(Bits per character)×(Number of characters)
Image File Size:
Size=(Colour depth)×(Image height px)×(Image width px)Size=(Colour depth)×(Image height px)×(Image width px)
Sound File Size:
Size=(Sample rate)×(Duration in seconds)×(Bit depth)Size=(Sample rate)×(Duration in seconds)×(Bit depth)
Example Calculations
Image File Size Example
Image with height = 14 pixels, width = 17 pixels, and color depth = 3 bits:
Size=3×14×17=714 bits=89.25 bytesSize=3×14×17=714 bits=89.25 bytes
Sound File Size Example
Sound file with sample rate = 6 samples/second, duration = 3 seconds, and bit depth = 4 bits:
Size=6×3×4=72 bits=9 bytesSize=6×3×4=72 bits=9 bytes
Text File Size Example
Text file with 244 characters and 7 bits per character:
Size=7×244=1708 bits=213.5 bytesSize=7×244=1708 bits=213.5 bytes
Number Conversions (Denary/Binary/Hexadecimal)
Denary to Binary Conversion Methods
Divide by Two Method:
Divide the denary number by 2 and record the remainders.
Example: Convert 89 to binary.
89 / 2 = 44 R 1
44 / 2 = 22 R 0
22 / 2 = 11 R 0
11 / 2 = 5 R 1
5 / 2 = 2 R 1
2 / 2 = 1 R 0
1 / 2 = 0 R 1
Result: 01011001
Binary Number Line Method:
Use a binary number line (128, 64, 32, 16, 8, 4, 2, 1).
Example: Convert 89 to binary.
89=64+16+8+1 ⇒01011001 89=64+16+8+1 ⇒01011001
Binary to Denary Conversion
Use a binary number line.
Sum the denary values corresponding to the '1' bits.
Example: Convert 01100110 to denary.
01100110=64+32+4+2 ⇒102 01100110=64+32+4+2 ⇒102
Rules of Binary Addition
0+0=00+0=0
0+1=10+1=1
1+1=0, carry 11+1=0, carry 1
1+1+1=1, carry 11+1+1=1, carry 1
Binary Addition Examples (8-bit Integers)
Example 1:
85 + 170 = 255
01010101 + 10101010 = 11111111
Example 2:
43 + 74 = 117
00101011 + 01001010 = 01110101
Example 3:
59 + 124 = 183
00111011 + 01111100 = 10110111
Example 4 (Overflow):
95 + 222 = 317 (Overflow)
01011111 + 11011110 = 00111101
Hexadecimal
Denary, Binary, and Hexadecimal Equivalents
Denary | Binary | Hexadecimal |
|---|---|---|
0 | 0000 | 0 |
1 | 0001 | 1 |
2 | 0010 | 2 |
3 | 0011 | 3 |
4 | 0100 | 4 |
5 | 0101 | 5 |
6 | 0110 | 6 |
7 | 0111 | 7 |
8 | 1000 | 8 |
9 | 1001 | 9 |
10 | 1010 | A |
11 | 1011 | B |
12 | 1100 | C |
13 | 1101 | D |
14 | 1110 | E |
15 | 1111 | F |
Converting between Denary and 2-Digit Hexadecimal
Example 1: Convert 12 to hexadecimal: 0C
Example 2: Convert 24 to hexadecimal: 18
Example 3: Convert 11 to hexadecimal: 0B
Example 4: Convert 171 to hexadecimal: AB
Binary Shifts
Binary Left Shift
Shifts bits to the left, multiplying the number by 2.
Example: Left shift 22 by 1 bit.
00010110 (22) becomes 00101100 (44)
Binary Right Shift
Shifts bits to the right, dividing the number by 2.
Example: Right shift 20 by 1 bit.
00010100 (20) becomes 00001010 (10)
Storing Characters in Binary
ASCII
ASCII is an 8-bit character set.
28 combinations = 256 characters.
Character Sets
A character set is a list of characters recognized by a computer.
Each character is represented by a unique binary number.
Well-known character sets:
ASCII: 8 bits, 256 characters.
UNICODE: 24 bits, over 16 million characters.
Storing Images in Binary
Images are stored as bitmaps or vectors.
Bitmap images are made of pixels.
Vector images store mathematical descriptions of shapes.
Bitmaps
Each pixel is stored in binary.
The number of bits per pixel determines the number of colors.
1 bit = 2 colors (black or white)
2 bits = 4 colors
The number of colors is calculated as 2n2n, where nn is the number of bits per pixel (color depth).
Metadata stores additional information about the image (width, height, color depth, color palette).
Higher color depth and resolution result in larger file sizes.
Photographs are often stored in 24-bit color depth (over 16 million colors).
Bitmap Examples with Metadata
Image Height: 14 pixels
Image Width: 17 pixels
Color Depth: 2 bits per pixel (4 colors)
File Size (without metadata): Size=14×17×2=476 bits=59.5 bytesSize=14×17×2=476 bits=59.5 bytes
Metadata: Width = 17 pixels, Height = 14 pixels, Color depth = 2 bit
Image Height: 14 pixels
Image Width: 17 pixels
Color Depth: 3 bits per pixel
File Size (without metadata): Size=14×17×3=714 bits=89.25 bytesSize=14×17×3=714 bits=89.25 bytes
Metadata: Width = 17 pixels, Height = 14 pixels, Color depth = 3 bit
Storing Sound in Binary
Sample Rate
The frequency at which the amplitude of a sound wave is recorded.
Measured in Hertz (Hz).
A higher sample rate means smoother playback.
Example: CDs are typically recorded at 44,100 Hz (44,100 samples per second).
Bit Depth
Represents the number of different amplitude levels that can be represented.
Measured in bits.
A typical CD uses 16-bits, allowing for 216=65,536216=65,536 amplitude levels.
Sound File Size Calculation
Size=(Number of samples per second)×(Number of bits per sample)×(Length of sample in seconds)Size=(Number of samples per second)×(Number of bits per sample)×(Length of sample in seconds)
Sound File Size Example
Sound file with a sample rate of 6 samples/second, a duration of 3 seconds, and a bit depth of 4 bits:
Size=6×3×4=72 bits=9 bytesSize=6×3×4=72 bits=9 bytes
Data Compression
Reasons for Compression
Reduce file size.
Decrease storage capacity needed.
Quicker data transfer.
Lossy Compression
Some data is lost and cannot be recovered.
Greatly reduces file size.
Reduces quality.
Suitable for images, sound, and video.
Not suitable for text and executable files.
Lossless Compression
No data is lost; it is encoded differently.
Can be turned back into the original format.
Can be used on all types of data.
Usually less effective than lossy compression at reducing file size.
Most suitable for documents and executable files.
Networking
Advantages of Networking
Users can share files and peripherals.
Users can access files from any computer on the network.
Servers can control security, software updates, and data backup.
Communication with other people (email, social networking).
Local Area Network (LAN)
Covers a small geographical area on a single site.
Hardware owned by the organization.
Wired (UTP cable, fiber optic) or wireless (Wi-Fi) connections.
Wide Area Network (WAN)
Covers a large geographical area, connecting LANs together.
Infrastructure leased from telecommunication companies.
Connected with telephone lines, fiber optic cables, or satellite links.
Disadvantages of Networking
Increased security risks.
Malware and viruses spread easily.
Server failure can disrupt connected computers.
Network performance can slow down with high data traffic.
Network Performance Factors
Bandwidth
The amount of data that can be sent and received in a given time.
Measured in bits per second (bit rate).
Number of Users
Too many users can slow down the network if bandwidth is insufficient.
Transmission Media
Wired connections (fiber optic, copper) offer higher bandwidth than wireless.
Error Rate
Less reliable connections increase data re-transmission due to errors.
Signal quality depends on range from access point and environmental factors.
Latency
The delay from transmitting data to receiving it.
Caused by bottlenecks in network infrastructure.
Hardware components may operate at different speeds.
Client-Server Model
A server controls access and security to a shared file store and manages Internet access, printing jobs, email services, and data backups.
A client requests data and connections from the server.
Advantages
Easier to manage security files and backups.
Easier to install software updates.
Disadvantages
Can be expensive to set up and maintain.
Requires IT specialists.
The server is a single point of failure.
Peer-to-Peer Model
Each computer (peer) is equal to all others.
Peers serve their own files.
Each peer is responsible for its own security and backup.
Advantages
Easy to maintain.
No specialist staff needed.
No dependency on a single computer.
Cheaper setup.
Disadvantages
Less secure.
Users manage their own backups.
Difficult to maintain a well-ordered file store.
Network Hardware
Network Interface Controller (NIC): Connects a device to a network (wired or wireless) and uses a protocol for communication.
Wireless Access Points: Allow wireless devices to access the network, though wireless connections have lower bandwidth and greater security concerns than wired.
Switch: Sends data between computers on a LAN using NIC addresses.
Router: Connects a LAN to a WAN, using IP addresses to route traffic.
UTP Copper Cable: Used for connections between desktop computers and a switch, cheap and flexible, but has lower bandwidth. Fibre optic cable is used for longer distances and WANs due to higher bandwidth and less interference.
Web Servers and Clients
Web servers host websites and handle client requests (HTTP GET requests).
Web pages (HTML, CSS, JavaScript) are sent to a browser to be rendered.
The process:
A user enters a URL.
The browser sends the domain name to a Domain Name Server (DNS).
The DNS maps the domain name to an IP address and returns it to the browser.
A GET request is sent to the web server using the IP address.
The requested webpage is returned to the client's web browser.
Cloud Computing
Servers store data and programs that can be accessed over the Internet.
Advantages
Access any time, anywhere, from any device.
Large storage capacity.
Automatic backup.
Easy online collaboration.
The Internet
A global collection of interconnected networks.
Domain Name Servers (DNS) convert web addresses to IP addresses.
Websites are stored on dedicated servers (hosting).
Hosted solutions provide 24/7 access, multiple users, and greater security.
Network Topologies
Star
Central switch.
All devices connect to the switch.
The switch intelligently directs traffic.
A single cable break only affects one computer.
The switch is a potential point of failure.
Full Mesh
Every device is connected to every other device.
Multiple routes ensure continued traffic flow if a connection breaks.
Requires more cabling and switch hardware, increasing costs.
Partial Mesh
Multiple routes exist between devices, but not every device is connected to every other device.
A compromise between full mesh and other topologies.
Ethernet
A standard for wired LAN technologies, including protocols for communication.
Provides reliable, error-free, fast communication.
Data is transmitted in frames, including:
Preamble for synchronization
Start frame delimiter
Source and destination MAC addresses
Data
Error checking (cyclic redundancy check - CRC)
Limitations: physical cable connection, cost of hardware.
Wireless Networks (Wi-Fi and Bluetooth)
Bluetooth
Ideal for connecting personal devices (e.g., wireless keyboard, headphones).
Short range (10 meters).
Low bandwidth and power consumption.
Wi-Fi
Common standard for wireless networks.
Advantages: mobility, easier setup, convenience, supports large numbers of users.
Disadvantages: slower speeds, relies on signal strength, potential obstructions, less secure.
Wireless Security and Encryption
Wireless networks broadcast data, requiring encryption for security.
Data is scrambled into cyphertext prevent unauthorized access, ensuring that sensitive information remains protected as it travels over the network.