1.2 Software and Software development

• A collection of programs that work together to provide an interface between the user and computer

• Memory management • Resource management • I/O management • Interrupt management • Security • User interface • Utility software

Both used when RAM space is insufficient, can cause disk thrashing

• Paging uses equal sized sections called pages - Segmentation uses variable-sized, logical sections called segments

• A section of the hard drive is used to act as RAM • Sections of programs not currently being used are temporarily moved into virtual memory through paging. • This frees up memory for other programs in RAM

• A signal generated by software or hardware to indicate to the processor that a process needs attention.

• Interrupt register checked at the end of each FDE cycle • If an interrupt with a higher priority exists, the current contents in the CPU are transferred to a stack. • The relevant ISR is loaded into RAM • A flag is set to signal the ISR had started • Flag is reset when finished • Interrupt register checks if there are any other high priority interrupts, if so it is repeated. • If there are none, contents of the stack are popped off and the FDE cycle carries on.

• Jobs are actively made to start and stop by the operating system.

• Once a job is started, it is left alone until it is completed.

• Round Robin • Shortest remaining time • Multi-level feedback queues

• First come first served - Shortest job first

• Takes into consideration different job priority - Difficult to implement

• Real-time operating systems perform a task within a guaranteed time frame. They are used in systems where a response within a given time frame is critical e.g. life support systems, self-driving cars and power systems.

• Running tests upon the computer’s start-up: • POST (Power-on self test) which makes sure that all hardware is correctly connected and functional • Checking CPU clock, memory and processor • Testing for external memory devices

Enable the operating system to interact with hardware devices

• Testing programs • Protection from malware • Running software compatible with different versions and types of OS’s

• Advantage: Platform independent, portable - Disadvantage: Slower execution

• Applications software is utilised directly by the end-user whereas systems software ensures high performance of the computer

• Compression • Disk de-fragmentation • Antivirus • Automatic backup • Automatic updating

• Thorough, regular and well-tested updates • Expert support and user manuals from company • High levels of security as it is developed in a professional, controlled environment

• Doesn’t require a license to be used • Distributed with the source code • Can be modified and sold on

• Faster execution - Doesn’t require a compiler to run

• Platform independent • Runs instantly without time waiting for compilation • Useful for debugging

• A low level language that has almost a one-to-one relationship with machine code. It is platform specific