James Bowes - Post 16 Computer Science

Files are compressed to reduce the size of them. This is done to save the amount of space they take up on a hard drive and also allow them to be downloaded over the internet quickly and without using up large amounts of 'data'. There are two types of compression: Lossy and Lossless. Lossy is where certain bits of information is removed from the file to make it smaller. Mainly used for video, images and sound. Lossless is where all the data is retained but it is compressed by recording 'patterns' of the data. Used to compress files that contain computer programs (they wouldn't run if anything was missing). Lossless leaves quite large files even after the compression whereas files compressed with lossy can be made very small. 

A virtual machine is any instance where software is used to take on the function of a machine, including executing intermediate code or running an operating system within another to emulate hardware. You can use them play around with different operating systems, run software your main operating system can’t, and try out apps in a safe, sandboxed environment.

A program that provides a software interface to a particular hardware device. This enables the OS to access hardware functions without needing to know the details of the hardware being used. Drivers are hardware dependent and operating system specific. A driver communicates with the device to which the hardware connects. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Examples of Device Drivers:

BIOS is the program stored in Read Only Memory (ROM) that gets your computer started when it’s turned on. The purpose of the BIOS is to initialise and test the system hardware components and to load the operating system from the hard disk into RAM. POST (Power on self test) When the computer power is turned on the POST runs a test sequence to check the computer's input/output systems (mouse, keyboard, monitor, RAM, disk drives etc) are working correctly. Boot loader (Bootstrap) A program that runs as soon as the computer is powered on which starts the Operating System (loads it from the hard drive to RAM and gets it running).

Distributed OS -  A single task is split up into a number of smaller tasks. Each of these is run on a separate computer. The results are then gathered together.   Multi-user system -  Multi - user  software is software that allows access by  multiple users  of a computer. Real-time OS - U sed in aircraft or other safety critical machines, r espond very quickly to Inputs/sensors and can deal with multiple inputs at once. Can quickly switch to backup systems if something goes wrong. Multitasking system - an OS that allows users to perform multiple tasks at the same time. Embedded system - An  embedded operating system  is a specialised  OS  for use in the computers built into larger  systems . Examples include washing machines and dishwashers. 

Input An  input device  is any hardware  device  that sends data to a computer, allowing you to interact with and control the computer. E.g: Mouse & Keyboard.  Output An output device is a hardware device that outputs data that the computer has processed  to the user. E.g: Computer Monitor and Speakers.  Storage Where data that has been processed, or waiting to be processed, is stored by the computer when not in use or switched off. E.g: USB Stick or Internal Hard Drive. 

CISC (Complex Instruction Set Computers) This is where single instructions can execute several low-level operations (such as a load from memory, an arithmetic operation, and a memory store all at once). For example: 'MULT A, B' could be used to multiple value A by B and then store the answer in A. RISC (Reduced Instruction Set Computers) This is where the opposite of CISC occurs. Multiple instructions (each requiring one clock cycle) and used to carry out tasks. For example: 'LDA R1, A / LDA R2, B / MULT R1, R2 / STO R1 A' would be used o multiple value A by B and then store the answer in A. CISC vs RISC Advantages and Disadvantages:

Data is transferred between the RAM and CPU via the system 'Buses'. Data and control signals travel in both directions between the processor, memory and I/O controller along the buses. Addresses on the other hand, only travel one way along the address bus. The processor sends all  instructions/addresses to the RAM to be stored/retrieved to either the memory or I/O controller. There are three types of 'bus': Address Bus Memory is divided into units called 'words' of a fixed size. Each 'word' has its own address.  The address bus transmits the address of these 'words' that are required to be used in the program so the data can be found and sent back to the processor.  All data is transmitted via the Data Bus.  Data Bus Consists of separate lines. Different processors have different amounts of lines. Either 6, 16, 32.....etc.  Data/instructions can travel in both directions on the data bus.  Control Bus This transfers information

We use Little Man Computer to understand how the Von Neumann Architecture works.  Syntax Used in LMC: To LOAD data from a variable and OUTPUT it: This program loads the contents of variable FIRST into the accumulator (LDA FIRST), outputs the contents of the accumulator to the OUTBOX (OUT) and then pauses the program (HLT).  Variable 'FIRST' is defined using 'FIRST DAT 42', this means the variable first contains the data 'DAT 42'. 

Scheduling is where a computer prioritises tasks to be carried out in a certain order to maximise the use of the computer and all of its peripherals. The algorithm must be robust so the computer does not freeze and also so every task/user gets a fair response time. Two main types of jobs are I/O bound jobs and processor bound jobs: I/O bound jobs (input/output) examples: Sending a document to the printer. Moving the mouse. Typing on the keyboard. Processor bound jobs examples: Playing Music Watching a video Calculating in Excel. There are different ways a processor can prioritise tasks: FCFS (first come first served) - tasks are started and completed in the order they are received. RR (round robin) – the time slice is 1 minute - each job has a minute spent on it before moving onto the next one. This occurs in a circuit until all jobs are complete. SJF – shortest job first - The job with the shortest completion time is carri

A cpu processes things one at a time. An interrupt is where a CPU compares multiple tasks to see which has the biggest priority. It then start the most important task before moving on to anything else. Interrupts are recorded in the interrupt register. An example of an interrupt would be: if music is playing but then the user opens PowerPoint, the computer will prioritise opening powerpoint and then quickly switch back to the music (all without the user knowing). Interrupts are used in a computer system to prevent the computer system for slowing down and any data loss. The main types of interrupts: