Computer Science

Computer Science

Von Neuman Architecture
1. A computer's main internal backing store is the hard drive. It stores the operating system, software applications or programs and majority of data files.
  1. Important files need to be backed up. Magnetic tape is small, robust and cheap, but very slow to read and write from. Hard disks have high capacity but are large and can be broken. Memory sticks can hold a lot of data, and are portable and durable, but can be lost or destroyed. Memory cards can hold a lot of data, are portable and durable, but can be lost or destroyed and is hard to read.
    1. Information needs to be backed up as a user may delete a file, the hard disk can fail or a virus or a fire can destroy the data. Archived data is not used often, but needs to be kept safe and secure.
  2. There are two types of optical disks. CD, compact disk, and DVD, digital versatile disk. ROM formats are read only, R formats are writable, RW formats are re-writable.
  3. Floppy and zip disks used to be used for transferring small files of data or backing up important small files.
2. Memory is where the computer stores data and provides the CPU with instructions. It is called primary memory and includes registers, cache, RAM and virtual memory
  1. MAR, memory address registers, hold the address of the next item of data. MDR, memory data registers, hold the data that is to be passed or is received from the data bus
  2. Virtual memory is volatile memory temporarily created on the storage drive. It is created when many processes are being run at once and RAM is running low. It is much slower, and swapping is the process of transferring data between virtual memory and RAM.
  3. RAM is random access memory and is the main place for storing data and instructions while a program is being executed. It is measured in gigabytes, and also has a word size in bits. It is volatile so data is lost when power turned off. It contains the operating system and applications.
    1. ROM is read only memory and is non-volatile. It contains the BIOS which is the firmware and has the bootstrap which is the process to load the operating system.
      1. Flash memory is a form of editable ROM. It is partitioned to hold a section for a specific purpose. Operating system, updates and applications are stored here.
3. Motherboard is a circuit board which connects the CPU to the memory and all the other hardware. They are connected by circuits called busses which move instructions and data around the system/
  1. A northbridge connects the CPU, RAM and GPU through the FSB. A southbridge connects to the northbridge, and all other hardware connects to the CPU through here.
    1. Bridges manage how data and instructions are transferred between the CPU, memory and other devices.
  2. Address busses send data from the CPU to the RAM. Data busses send data or recieve data from the memory
CPU
1. Performance is affected by cores, clock speed, cache size and processor type
  1. Cache is a small amount of memory used to temporarily hold instructions which the computer can reuse. The control unit automatically checks the cache for instructions before moving to the RAM. They are smaller, faster and more expensive.
  2. Clock speed is how fast the CPU can run. Overclocking increases CPU clock speed, but can corrupt data, execute instructions too quickly or overheat
  3. Cores are processing units which contain and ALU, control unit and registers
    1. Arithmetic logic unit performs arithmetic and logical operations
      1. Accumulator is a register where immediate calculations are stored.
    2. Registers, or immediate access store, is where all data and programs currently in use are held.
    3. Control unit controls the flow of data within the system. It monitors communications between hardware, controls the input and outputs, checks signals and monitors data
      1. Program counter is set to a memory location where instructions are stored and executed.
2. Executes programs and carries out processes using the fetch-decode-execute cycles
  1. The CPU fetches the instructions from the main memory and stores it in the registers. The CPU understands the instruction by decoding it. The CPU is designed to understand in instruction set and execute it.
Networks
1. Computers can connect to a network using an ethernet cable, or wirelessly if they have an NIC, network interface card, to a router using radio signals through the wireless access point.
  1. A wireless network is cheaper, not stuck in a location and can connect multiple devices compared to wired connections. However, there can be interference or loss of quality through walls and is slower and more open to hacking
    1. Networks are useful because sharing devices saves money, licenses are cheaper, files can be shared, security is good and there is communication. However, it can be expensive, it is complicated, viruses can spread and there is danger of hacking
  2. A modem enables a computer to connect to the internet through a telephone line by converting digital signals to analogue signals. A hub broadcasts data to all devices on a network. A bridge is used to connect LAN networks. A switch is like a hub but more efficient with high demand.
    1. A LAN, local network area, is a network of computers within the same building. A WAN, wide area network, is created by connecting LANS with media such as broadband cables.
      1. A VPN is a virtual private network used to securely host another network. A WPAN is a wireless personal area network which allows the connection of devices to machines.
  3. Wifi is run on two frequency bands, 2.4 and 5 GHz. 2.4 is more common and is split into 14 channels that overlap, with more isolated ones being faster. 5 does not have overlapping channels and is faster, but has a lower range.
  4. Bandwith is the maximum rate that information can be transferred. Throughput is the acutual rate it is transferred. Latency is the delay for processing. Jitter is the packet delay in recieving information. Error rate is the amount of corrupted information
2. A client-server model is the relationship between two computers where one is the client and makes a server request from the other which is the server. In a peer-to-peer network, each computer stores files and acts as a server, but there is less security, each computer needs to be backed up and a slow machine will slow down others.
  1. A star network has each device connected to a hub with its own cable and is the most popular way. It is very reliable and high performing, but expensive to install and requires extra hardware.
  2. A mesh network is where all the devices, nodes, cooperate to distribute data among each other. It is very robust, and a failure will only affect that node, but it is more expensive and complicated
3. The internet of things is the networking of lots of devices so they can collect and transmit data. To connect to the internet a computer needs an intenet service provider, a modem or router, a web browser and a fibre optic or copper cable.
  1. Fibre optic cabling is thin so lots can be used, and there is less interference of degeneration, but it is expensive to replace. Coaxial copper cables degenerate over long distances, unshielded twisted pair reduces degeneration and shielded twisted pair has less interference. They work with a loss of power and are cheaper, but degenerate over long distances.
  2. TCP/IP protocols organises how data packets are communicated over the internet. IP address is a unique number for each device given on the internet. MAC address is the unique number to identify a device. FTP transfers large files. HTTP transfers web pages. HTTPS is an encrytped web address.
    1. SMPT is used to send an email, POP will download the email and IMAP will read the email.
    2. Protocols interact with each other in layers. A layer is a group of protocols that serve a similar function, and are a way of categorising protocols. Layer 1 is application layer, layer 2 is transport layer, layer 3 is network layer and layer 4 is data link.
  3. Web servers host websites and handle requests. Application servers organise and run web applications. Print servers connect a device to a printer. Mail servers store email messages. Domain name server host and match website addresses to IP addresses.
System Security
1. Malware is malicious software in many forms. Spyware includes trojan horses which gather information about the user while pretending to be a useful program. Ransomware holds a computer captive by encrypting files. A virus copies itself and spreads to other files to damage to computer. Worms spread over networks and damage the computer.
  1. Phishing is pretending to be someone else to get personal information. A weak point in a secure system is a person with a weak password or no protection allowing a hacker in. Brute force attacks are programs run to guess a password with different combinations. DoS attacks prevent the use of a network by sending multiple requests to the server by exploiting weaknesses.
  2. Data packets sent in a network can be intercepted and stolen. SQL injection is a technique used to execute processes in an entry field. A poor network policies carry the risk of data loss.
2. Penetration testing is looking at a computer and making sure there are no weaknesses such as repeated passwords. Network forensics is the analysis of network events to determine the source of security attacks.
  1. Anti-malware software is a type of software designed to prevent, detect and remove malicious software. Firewall is a software which enforces a set of rules about what data packets are allowed to enter or leave a network.
  2. User access levels is the grouping of users to have different levels of control within a network. Secure passwords are important to prevent a person from accessing a restricted network. Encryption is the changing of the original characters, so information is intercepted, can't be accessed.

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	Created by Naill Kazmi about 6 years ago