Software applications

Provides an interface between the user/ software and the hardware

Without an OS, the computer would have less flexibility to run a variety of applications as each piece of software would have to be designed for the specific hardware it was run on

The means by which a user of the computer interacts with the computer and receive the information that is processed

Ensures that resources are protected from unauthorised access through the use of permissions and passwords

During paging, logical address space is physically divided into units called pages, which are fixed size and can be stored non-contigously

Memory is split using Logical divisions which can be different sizes and can be stored non-contigously

Fixed size vs Different sizes Pages made to fit sections of memory, segments are complete sections of programs Physical divisions vs logical divisions

Where the OS swaps parts of the process that are not in use from main memory into allocated space on the hard disk and bring it back when needed

Pro: Increases the potential memory storage capacity available Con: Accessing data from virtual memory is slow and negatively effects performance

Where an OS allocates processing time to different tasks and switches between them giving the illusion that multiple things are occurring at once

A signal that is sent to the processor to request immediate attention

Any 2: Hardware device signaled it has data to process Hardware device has completed a task Software process needs OS to perform a function An allocated amount of time has expired and an action needs to be performed Hardware failure needs addressing

• Processor checks the interrupt queue to see of there is a high priority interrupt • If there is, the processor moves the current contents of the registers into a stack • The processor then loads the appropriate ISR into the RAM and sets a flag to show the ISR has begun • The processor services the interrupt then checks if there is another. • If not the flag is removed and the stack loaded back

The interrupt must wait in a queue for the process before the ISR occurs

Processor time is split into equal time slices. If a process isn’t completed within a time slice is suspended and will only continue at its next time slice

The first process has as much processing time it needs to finish. Other processes must wait their turn. Like a queue at the shop

Pro: ensures that every task is allocated processor time without waiting to long Con: Does not scale well, as the more processes run the less time slices each has

Pro: Once a job starts it is guaranteed to be completed, no starvation Con: Long queues may cause long waits for some processes

When a process cannot complete its execution because it has been constantly denied processor time

The process than needs the shortest time to complete goes first

Pro: shortest processes removed quickly which saves time for longer jobs Cons: Longer jobs may face starvation is shorter jobs keep getting added

The process with the shortest time to completion goes first until a different task is added with a shorter time to completion, which takes priority

Pro: shortest time processes completed quickly so more time for longer processes Cons: Longer processes face starvation if shorter time processes keep getting added

Pro: shortest time processes completed quickly so more time for longer processes Cons: Longer processes face starvation if shorter time processes keep getting added