Computer Systems Notes PDF

Computer Systems From the LCCS Specification – Computer Systems Overview Students learn about: Section 1: CPU in detail ALU, Registers, Program Counter Memory Section 2: Basic Electronics Voltage, current, reistors, c...

Computer Systems From the LCCS Specification – Computer Systems Overview Students learn about: Section 1: CPU in detail ALU, Registers, Program Counter Memory Section 2: Basic Electronics Voltage, current, reistors, capacitors, transistors Section 3: Operating systems layers Hardware, OS, Application, User Section 4: Web infrastructure Computer network protocols, HTTP, TCP, IP, VOIP Students should be able to: 1. Describe the components and the functions of the components within a computer 2. Describe logic gates and explain how they can be arranged to create large units 3. Describe the rationale for using the binary number systems in computing 4. Describe the difference between digital and analogue input. 5. Explain WWW, the internet, client server model, hardware components and communication protocols.  Computer Systems A computer system is a set of interconnected components that work together to process data and generate output. The main components of a computer system are the central processing unit (CPU), memory, storage, and input/output (I/O) devices. Von Neumann Architecture The Von Neumann architecture is a conceptual model for computer systems that was first proposed by John von Neumann in the 1940s. It is a four-stage process: 1. Fetch: The CPU fetches the next instruction from memory. 2. Decode: The CPU decodes the instruction to determine what operation to perform. 3. Execute: The CPU executes the instruction. 4. Store: The CPU stores the results of the operation back in memory. Figure 1. Von Neumann Architecture Section 1 -CPU CPU CPU stands for central processing unit. The CPU is the brain of the computer system. It is responsible for processing data and instructions. The CPU is made up of two main components: the control unit and the arithmetic logic unit (ALU). Arithmetic Logic Unit (ALU) The arithmetic logic unit (ALU) is the part of the CPU that performs arithmetic and logical operations. The ALU can perform operations such as addition, subtraction, multiplication, division, AND, OR, and NOT. The ALU is used by the CPU to execute instructions that require arithmetic or logical operations. Registers in a CPU  Registers are high-speed storage locations that are used by the CPU to store data and instructions. Registers are much faster than main memory, so the CPU uses them to store data and instructions that it needs to access frequently. There are many different types of registers, each with a specific purpose. Some common types of registers include: General-purpose registers: These registers can be used to store any type of data, such as integers, floating-point numbers, and addresses. Special-purpose registers: These registers are used for specific purposes, such as storing the program counter, the stack pointer, and the condition codes. Program Counter (PC) The program counter (PC) is a register that stores the address of the next instruction to be executed by the CPU. The PC is incremented by one after each instruction is executed. This ensures that the CPU always knows where to find the next instruction to execute. Memory in a CPU Memory in a CPU is used to store data and instructions that the CPU needs to access. Memory is typically much slower than registers, but it can store more data. When the CPU needs to access data or instructions that are stored in memory, it copies them into registers before processing them. How the ALU, Registers, PC, and Memory Work Together 1. The ALU, registers, PC, and memory work together to execute instructions. The following is a simplified example of how they work together to execute a simple addition instruction: 2. The CPU fetches the instruction from memory and stores it in a register. 3. The CPU decodes the instruction and determines that it is an addition instruction. 4. The CPU loads the operands for the addition operation into registers. 5. The CPU performs the addition operation in the ALU and stores the result in a register. 6. The CPU stores the result of the addition operation back in memory. 7. The CPU repeats this process over and over again until it encounters a halt instruction.  Section 2 Computer Components Memory in a computer Memory is where data and instructions are stored while they are being processed by the CPU. There are two main types of memory: random access memory (RAM) and read-only memory (ROM). RAM is volatile memory, meaning that it loses its contents when the computer is turned off. ROM is non-volatile memory, meaning that it retains its contents even when the computer is turned off. Input/Output devices [I/O Devices] I/O devices allow the computer to interact with the outside world. Input devices allow the user to enter data into the computer, such as a keyboard and mouse. Output devices allow the computer to display or produce data, such as a monitor and printer. Motherboard It’s the ‘heart’ of a computer The motherboard is the main circuit board in a computer. It connects all of the other components in the computer, such as the CPU, memory, storage, and I/O devices. The motherboard also contains the BIOS, which is a firmware program that starts the computer and loads the operating system. Functions of the Motherboard Provides a physical platform for connecting all of the other components in the computer. Provides electrical power to all of the other components in the computer. Provides communication pathways between all of the other components in the computer. Controls the startup process of the computer Secondary Storage Secondary storage is non-volatile memory that stores data even when the computer is turned off. It is used to store large amounts of data, such as operating systems, applications, and user files. Common types of secondary storage include hard drives, solid-state drives, optical discs, and magnetic tape. Differences Between Primary and Secondary Storage Fetch-Execute Cycle The fetch-execute cycle is the process by which the CPU executes instructions. It is a four-stage process: 1. Fetch: The CPU fetches the next instruction from memory. 2. Decode: The CPU decodes the instruction to determine what operation to perform. 3. Execute: The CPU executes the instruction. 4. Store: The CPU stores the results of the operation back in memory. The fetch-execute cycle is repeated continuously until the computer is turned off. Clock The clock is a device that generates a regular pulse signal. In a CPU, the clock is used to synchronize the execution of instructions. The CPU executes one instruction per clock cycle. Factors Determining the Speed of a CPU 1. Clock speed: The clock speed is the frequency of the clock signal. It is measured in hertz (Hz). A higher clock speed means that the CPU can execute more instructions per second. 2. Instruction pipeline: The instruction pipeline is a technique that allows the CPU to execute multiple instructions simultaneously. It is a complex topic, but in general, a more sophisticated instruction pipeline can lead to faster CPU performance. 3. Number of cores: A CPU core is a processing unit that can execute instructions independently. CPUs with multiple cores can execute multiple instructions simultaneously, which can improve performance for certain types of applications. 4. Cache: The cache is a small amount of high-speed memory that is used to store frequently accessed data and instructions. A larger cache can improve CPU performance by reducing the number of times that the CPU has to access slower main memory. The Bus A bus is a set of electrical wires that allows different components in a computer to communicate with each other. The bus that connects the CPU and memory is called the system bus.  The system bus transmits data, addresses, and control signals between the CPU and memory. The speed of the system bus is a major factor in determining the overall performance of a computer. The bus system bus is typically made up of three types of wires: 1. Data lines: Carry the data being transmitted between the CPU and memory. 2. Address lines: Carry the addresses of the memory locations that the CPU is accessing. 3. Control lines: Carry signals that control the operation of the CPU and memory.  Section 3 -The basic electronics of how a computer works 1. Binary code and electrical signals Binary code is a system of representing information using two symbols, 0 and 1. These symbols can be represented by electrical signals, such as a high voltage for 1 and a low voltage for 0. Computers use binary code to represent all types of information, including data, instructions, and text. When a computer processes information, it does so by manipulating the electrical signals that represent the binary code. For example, when you type a letter on a keyboard, the computer converts the letter to binary code and sends the code to the CPU. The CPU then manipulates the electrical signals that represent the binary code to execute the instruction that corresponds to the letter. 2. Electricity and electrons Electricity is the flow of electrons through a conductor. Electrons are negatively charged particles that are found in the atoms of all matter. Computers use electricity to power their components and to transport information. When a computer is turned on, electricity flows through the components, powering them up. Electricity is also used to transport information between the different components of a computer. For example, when the CPU needs to access data from memory, it sends an electrical signal to the memory controller. The memory controller then sends the data to the CPU over an electrical bus. 3. Electric current Electric current is the rate of flow of electric charge through a conductor. It is measured in amperes (amps). The electric current that flows through a computer's circuits is what powers the components and transports information. The amount of electric current that flows through a circuit depends on the voltage and the resistance of the circuit. Voltage is the electrical potential difference between two points in a circuit. Resistance is the opposition to the flow of electric current in a circuit. 4. Voltage Voltage is the electrical potential difference between two points in a circuit. It is measured in volts. The voltage of an electrical signal determines the strength of the signal. A higher voltage means a stronger signal. Computers use voltage to control the flow of electric current through their circuits. For example, the CPU uses voltage to control the flow of electric current through the transistors that make up the logic gates. Example of how a computers transport information: 1. When you type a letter on a keyboard, the keyboard sends an electrical signal to the computer. 2. The electrical signal is converted to binary code. 3. The binary code is sent to the CPU over an electrical bus. 4. The CPU decodes the binary code to determine what instruction to perform. 5. The CPU executes the instruction.  6. The results of the instruction are sent back to memory over an electrical bus. 7. The results of the instruction are stored in memory. 8. This process is repeated over and over again as the computer executes instructions and processes data. 9. Computers use binary code, electrical signals, electricity, electrons, electric current, and voltage to transport information between the different components of the computer and to store information in memory. This allows the computer to process data and execute instructions. 10. How a computer manipulates the flow of electricity using electronic devices 1. Transistors 2. Resistors 3. Capacitors They are used to manipulate electricity in a variety of ways, including: Controlling the flow of electricity through circuits Creating voltage dividers Filtering and smoothing out electrical signals Storing electrical energy These components allow computers to process data and execute instructions. 1. Transistors Transistors are semiconductor devices that can amplify or switch electronic signals. They are used in computers to control the flow of electricity through circuits. Transistors are made up of three layers of semiconductor material: the emitter, the base, and the collector. When a voltage is applied to the emitter and base, it creates a current that flows through the transistor to the collector. The amount of current that flows through the transistor can be controlled by the voltage applied to the base. This allows transistors to be used as switches or amplifiers. In computers, transistors are used in a variety of applications, including: Logic gates: Transistors are used to implement the logic gates that are used in the CPU to execute instructions. Memory: Transistors are used to store data in memory. I/O controllers: Transistors are used to control the input and output devices of the computer. 2. Resistors and Resistance Resistors are electrical components that resist the flow of electric current. Resistance is measured in ohms (Ω). The amount of resistance in a resistor determines how much current will flow through it. A higher resistance means that less current will flow. Resistors are used in computers to control the flow of electricity through circuits. For example, resistors are used to limit the amount of current that flows to the CPU to prevent it from overheating.  Resistors are also used to create voltage dividers, which are circuits that divide a voltage into two or more smaller voltages. Voltage dividers are used in computers to power different components of the computer with different voltages. 3. Capacitors Capacitors are electrical components that store electrical energy. Capacitors are made up of two metal plates that are separated by a dielectric material. When a voltage is applied to the capacitor, it creates an electric field between the plates. This electric field stores electrical energy. Capacitors are used in computers to filter and smooth out electrical signals. For example, capacitors are used to smooth out the voltage from the power supply to prevent it from fluctuating too much. Capacitors are also used to store energy for short periods of time. For example, capacitors are used to store the energy needed to power the CPU when it is in a low-power state. Section 3 (b)-Logic gates Logic gates are the basic building blocks of digital circuits. They are used to perform simple logical operations, such as AND, OR, and NOT. Logic gates are made up of transistors. The CPU uses logic gates to implement the fetch-execute cycle. The CPU uses logic gates to perform each of these steps. The CPU also uses logic gates to perform the arithmetic and logical operations. For example, if the instruction is to add two numbers, the CPU uses logic gates to add the two numbers together. Logic gates are also used in other parts of the computer system, such as the memory controller and the I/O controllers. 5 types of logic gate 1. And gate 2. Or gate 3. Not gate 4. Nand gate 5. Nor gate See page 136-137 in text book for a good explanation. Truth tables A truth table is a table that shows the output of a logical function for all possible inputs. It is a useful tool for understanding and designing digital circuits. A truth table has one column for each input and one column for the output. The input columns are labeled with the input variables and the output column is labeled with the output variable. Each row of the truth table represents a possible combination of input values. The output value for each row is determined by the logical function that the truth table is representing.  Section 4: Operating systems An operating system (OS) is a software that manages computer hardware and software resources and provides common services for computer programs. The primary functions of an operating system are: Booting the system: The OS loads the kernel and other essential system files into memory. Managing memory: The OS allocates and deallocates memory to programs. Managing the CPU: The OS schedules and executes programs on the CPU. Managing I/O devices: The OS provides a unified interface to I/O devices for programs. Providing system services: The OS provides common services to programs such as file systems, networking, and security. Hardware: The physical components of the computer, such as the CPU, memory, and storage. OS: The operating system, which is the software that manages the hardware and provides services to applications. Application: The software programs that users interact with, such as web browsers, word processors, and games. User: The person who is using the computer. How the Layers Work Together The layers of an operating system work together to provide a platform for users to run applications. The hardware layer provides the basic building blocks of the computer, the OS layer manages the hardware and provides services to applications, the application layer provides the software that users interact with, and the user layer is the person who uses the computer. Example The user interacts with the application layer, which in turn interacts with the OS layer. The OS layer interacts with the hardware layer to perform tasks such as reading and writing data to memory. Here is a simplified example of how the operating system layers work together to execute a simple instruction: The user clicks on a web browser icon to launch the web browser application. The application layer loads the web browser program into memory. The OS layer manages the memory allocation and schedules the web browser program to run. The web browser program requests a web page from a web server. The OS layer manages the network interface to send and receive data over the Internet. The web browser program receives the web page and renders it on the screen. The operating system layers work together seamlessly to provide the user with a smooth and efficient experience. Layer Model of a Computer System  The layer model of a computer system is a conceptual model that organizes the different components of a computer system into a hierarchy of layers. Each layer provides a set of services to the layer above it and uses the services of the layer below it. The most common layer model is the OSI seven-layer model 1. Application layer: Provides services to applications such as web browsers and email clients. 2. Presentation layer: Formats data for transmission and reception. 3. Session layer: Manages communication sessions between applications. 4. Transport layer: Provides reliable end-to-end communication between applications. 5. Network layer: Provides routing and addressing services. 6. Data link layer: Handles the physical transmission of data over a network. 7. Physical layer: Provides the physical interface between the computer and the network. Section 5: Web infrastructure Internet  The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite (TCP/IP) to link several billion devices worldwide. The Internet carries an extensive range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mail, telephony, and file sharing Internet Terminology IP address: A unique identifier for a device on the Internet. URL: A Uniform Resource Locator, also known as a web address, is a reference to a web page or other resource on the World Wide Web. HTTP: Hypertext Transfer Protocol is the protocol used to transfer data (such as HTML files and images) between web servers and clients. DNS: Domain Name System is a hierarchical decentralized naming system for computers, services, or any resource connected to the Internet or a private network. TCP: Transmission Control Protocol is a transport layer protocol used in the Internet protocol suite. It is one of the two core protocols of the Internet, along with the Internet Protocol (IP), and is used to ensure reliable, ordered, and error-checked delivery of data. UDP: User Datagram Protocol is a transport layer protocol used in the Internet protocol suite. It is a simple protocol that provides communication services for applications that do not require reliable data delivery. World Wide Web (WWW) The World Wide Web is a system of interlinked hypertext documents that are accessed via the Internet. The WWW is based on the Hypertext Transfer Protocol (HTTP), which is used to transfer data between web servers and clients. The WWW is the most popular service on the Internet, and it is used by billions of people around the world. Client-Server Model The client-server model is a system architecture that distributes tasks or workloads between the providers of a resource or service, generally called servers, and consumers of that resource or service, generally called clients. The client-server model is used in a wide variety of applications, including web servers, email servers, database servers, and file servers. Benefits of the Client-Server Model Scalability: The client-server model is scalable, meaning that it can be easily expanded to handle more users and data. Reliability: The client-server model is reliable, meaning that it can continue to operate even if some of the servers fail. Security: The client-server model can be more secure than other system architectures, because the server can be used to authenticate users and control access to resources. 1. Communication protocols Communication protocols are rules and guidelines that allow devices to communicate with each other. There are many different types of communication protocols, each  designed for a specific purpose. Some common examples of communication protocols include: 1. HTTP: Hypertext Transfer Protocol, used to transfer web pages and other resources on the World Wide Web. 2. HTTPS: Secure Hypertext Transfer Protocol, a secure version of HTTP that uses encryption to protect data in transit. 3. TCP: Transmission Control Protocol, used to ensure reliable, ordered, and error-checked delivery of data over the Internet. 4. UDP: User Datagram Protocol, a simpler transport layer protocol that does not guarantee reliable delivery of data. 5. TCP/IP: Transmission Control Protocol/Internet Protocol, a suite of communication protocols that is the foundation of the Internet. 2. HTTP and HTTPS HTTP (Hypertext Transfer Protocol) is the protocol used to transfer data (such as HTML files and images) between web servers and clients. HTTP is a text-based protocol, and the messages that are exchanged between web servers and clients are called HTTP requests and responses. HTTPS (Secure Hypertext Transfer Protocol) is a secure version of HTTP that uses encryption to protect data in transit. HTTPS is used for sensitive transactions, such as online banking and shopping. TCP TCP (Transmission Control Protocol) is a transport layer protocol used in the Internet protocol suite. TCP is responsible for ensuring reliable, ordered, and error-checked delivery of data over the Internet. TCP does this by using a three-way handshake to establish a connection between the sender and receiver, and by using sequence numbers and acknowledgments to track and retransmit lost or corrupted data. TCP/IP TCP/IP (Transmission Control Protocol/Internet Protocol) is a suite of communication protocols that is the foundation of the Internet. TCP/IP consists of a number of protocols, each of which is responsible for a different aspect of communication over the Internet. Some of the most important TCP/IP protocols include: IP: Internet Protocol, which is responsible for routing packets between devices on the Internet. TCP: Transmission Control Protocol, which is responsible for ensuring reliable, ordered, and error-checked delivery of data over the Internet. UDP: User Datagram Protocol, a simpler transport layer protocol that does not guarantee reliable delivery of data. ICMP: Internet Control Message Protocol, which is used to send error messages and other control messages between devices on the Internet. WiFi WiFi (Wireless Fidelity) is a wireless networking technology that uses radio waves to connect devices to the Internet. WiFi is the most popular wireless networking technology in the world, and it is used in billions of devices, including smartphones, laptops, and tablets. WiFi is a relatively fast and reliable wireless networking technology, and it can be used to connect to the Internet from anywhere with a WiFi signal. VoIP VoIP (Voice over Internet Protocol) is a technology that allows voice calls to be made over the Internet. VoIP works by converting voice calls into digital data packets, which are then transmitted over the Internet. VoIP calls can be made using a variety of devices, including smartphones, computers, and VoIP phones. VoIP is a relatively inexpensive and convenient way to make phone calls, and it is becoming increasingly popular around the world. I hope this brief overview is helpful. Please let Gateways A gateway is a network device that connects two networks that use different protocols. Gateways typically operate at the network layer of the OSI model. Gateways are used to connect different types of networks, such as local area networks (LANs), wide area networks (WANs), and the Internet. Routers A router is a network device that forwards data packets between networks based on their IP addresses. Routers typically operate at the network layer of the OSI model. Routers are used to connect different LANs, connect LANs to WANs, and connect to the Internet. Network bridges A network bridge is a network device that connects two LANs that use the same protocol. Network bridges typically operate at the data link layer of the OSI model. Network bridges are used to connect LANs that are too far apart to be connected by a single cable, or to segment a LAN into smaller subnetworks. Network switches A network switch is a network device that connects multiple devices on a LAN. Network switches typically operate at the data link layer of the OSI model. Network switches are used to improve the performance of a LAN by reducing traffic and collisions. Repeaters A repeater is a network device that amplifies and retransmits data signals. Repeaters typically operate at the physical layer of the OSI model.  Repeaters are used to extend the reach of a network or to improve the signal quality on a network. Cloud computing Cloud computing is the delivery of on-demand computing services over the Internet with pay-as-you-go pricing. Cloud computing services can be broadly divided into three categories: infrastructure as a service (IaaS), platform as a service (PaaS), and software as a service (SaaS). IaaS provides the underlying computing infrastructure, such as servers, storage, and networking. PaaS provides a platform for developing, deploying, and managing applications. SaaS provides software applications that are hosted and delivered over the Internet.