1.2.1 Operating Systems

• Provides a user interface • Manages hardware (peripherals) • Manages CPU usage / Interrupts • Provides utilities • Manages Memory • Provide system security (user permissions) • Provides a platform for software to run

• Provides security as it does not let programs access memory reserved for other programs. • Ensures that RAM is used efficiently and not wasted. • Removes data not needed anymore (garbage collection) and allocates memory to applications. • Provides multitasking as it allows multiple programs to run at once

• Physical addressing with fixed-size divisions • Data can be held non-contiguously and makes the best use of storage. • Can cause internal fragmentation (space wasted inside page due to fixed size)

• Logical addressing with variable size divisions. • Can cause external fragmentation - Can leave gaps in RAM due to differing sizes of segments

• Uses secondary storage (HDD) • Pages not currently needed get moved to virtual memory to create room • Pages are swapped between virtual memory and RAM as nothing can be running from virtual memory.

A signal to the processor indicating a device/process needs attention Examples of interrupts • Software interrupt - Closing a piece of software • Hardware Interrupt - Printer out of paper

Higher Priority • CPU finishes its current FDE cycle. • The contents of the CPU’s registers are copied to a stack in memory. • The location of the appropriate interrupt service routine is loaded into the program counter. • When complete, the previous contents are popped from the stack and loaded back into the registers. Lower/Equal Priotiy * Current process, then the current process continues.

A theoretical computer that runs like software Shares the resources with the actual OS that it is running on

• Allows you to run the software on several different systems without needing multiple physical machines. • Protects the physical machine from a virus as the VM can quickly be reset to its original state. • Some VM are used to execute intermediate code (Example Java VM)

• Used to process as many jobs as possible in the least possible time • Ensures all jobs are processed fairly • Enables the most efficient use of the processor

• Round Robin (RR) • First come first served (FCFS) • Shortest job first (SJF) • Shortest remaining time (SRT) • Multi-level feedback queues (MLFQ)

• Each process is given a fixed equal time slice • If the process is not completed, then it joins the back of the queue and the next one starts.

• Jobs are processed to completion in the order in which they arrive (First in First Out) • The job will be completed before the next one starts • Each job has an equal priority.

• Picks the job that will take the shortest time and runs it until it finishes (unlike SRT which will check everytime a new job comes into the queue - even if it is in the middle of a task) • Needs to know the time each job will take in advance

The quickest job is completed first but stops and re-evaluates when a new job arrives (meaning the current shortest job could be interrupted by an even shorter job).

• Puts processes into different queues • Each queue has a different priority • The algorithm can move processes between these queues based on their priority • Designed to minimise Input/Output bottlenecks

Distributed Multi-User Real-time Multi-Tasking Embedded

• Shares processing between the processors on a network • Allow multiple computers to be treated as one • Multiple computers work on the same problem, completing it quicker Example: Video Rendering

• Allows multiple users to use the same computer at the same time • Each user is given a time slice of the processor Example: Mainframe

• Offers a guaranteed response time • Will make decisions and act immediately Examples: Self-Driving Cars & Airplane Auto Pilot

• Has limited function and is read-only – stored in ROM. • System software used to manage the device • Built into the device itself • Specific to the hardware/purpose Examples: Washing machines & Microwaves

• Allows the user to run more than one program at the same time. • Example uses include running word processing software whilst checking emails. Examples: Typical home/personal computer OS

• A program that allows the operating-system to communicate with external hardware • Translates operating-system instructions into a form that devices can use • Allows peripherals from different manufacturers to work with different OS • Abstracts hardware complexity Examples: Printer drivers| Graphics Card drivers| Sound Card drivers

Basic Input Output System (BIOS) • Contains the computer start up instructions • Loads settings • Performs the Power On Self-Test (Tests hardware is working) and reports errors • Determines the drive on which the OS is stored • Stores and loads the Bootstrap loader (used to boot up the OS) • Can change settings such as which storage device the computer boots from