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: