2.1 System Sofware

OS manages hardware and provides an interface for the users and the application software

• User interface • Memory management • Interrupt handling • Processor scheduling

Allows the operating system to hide the complexity of hardware

• Programs and data need to be loaded into RAM • Operating systems must manage allocation of RAM to different programs • There may not be sufficient RAM for all desired processes to be completely loaded into RAM at once

• Available memory is divided into fixed size chunks called pages • Each page has an address

• A process loaded into RAM is allocated sufficient pages, but those pages may not be contiguous in physical terms

A page table maps between the logical locations and the physical memory locations

Spaces of memory that can relate to parts of a program

Dividing memory into segments

Some of the pages of a current process are stored in virtual memory until they are swapped into RAM

Where the computer has to constantly swap pages between virtual and primary memory and therefore slows down the computer

CPU instructions can be interrupted when necessary

CPU checks at the end of each clock cycle whether there are any interrupts to be processed

The processor stops fetching instructions and instead pushes the current contents of its registers onto a stack

Interrupts can themselves be interrupted if the new interrupt is of a higher priority

OS schedules when each application can use the CPU

• Each job is allocated by FIFO a time during which it can use the CPU’s resources • If the hob has not been completed by the end of its slice, the next job is allocated a time slice

The first job to arrive is executed until it completes

• The time to completion is estimated as new job arrives • The job with the shortest remaining time to completion is executed, meaning that a shorter new job can take over from the current process

• Multiple queues are created with different priority levels • If a job uses too much CPU time it is moved to a lower priority queue • Processes can also be moved to a higher priority queue if they have waited a long time

When high priority processes keep executing and low priority processes get blocked for indefinite time