1.2.1 systems software

• memory is split up into equal sized sections known as pages
• pages are swapped between main memory and the hard disk as needed
• memory is split up into logical sized divisions, called segments
• segments vary in size and represent the structure and logical flow of the program
• a section of the hard drive acts as RAM when the space in main memory is insufficient to store programs being used
• sections of programs not currently being used are temporarily moved into virtual memory through paging
• signals generated by software or hardware to indicate to the processor that a process needs attention
• interrupts have different priorities and this is considered when allocating processor time
• processor checks the interrupt register at the end of each fetch-decode-execute cycle
• operating system ensures all sections of programs being run receive a fair amount of processing time using scheduling
• pre-emptive jobs are actively made t start and stop by the operating system
• non pre-emptive jobs are left alone until it is completed
• distributed OS - runs across multiple devices

  means the load of a task is spread across multiple computer processors
• embedded OS - built to perform a small range of specific tasks,

  catered towards a specific device,

  limited functionality and hard to update,

  consume less power than other types of OS
• multi tasking OS - enables a user to carry out tasks simultaneously,

  time slicing is used to switch quickly between programs and applications in memory
• multi user OS - multiple users make use of one computer,

  a scheduling algorithm is used to allocate processor time fairly between jobs and prevent processor starvation
• real time OS - use in time critical computer systems and are designed to perform a task within a guaranteed time frame
• Basic Input Output System is the first program that runs when a computer system is switched on
• the BIOS runs a series of tests before the operating system is loaded into memory
• POST (power on self test) ensures that all hardware is correctly connected and functional
• the BIOS checks the CPU clock, memory and processor, it also tests for external memory devices
• device drivers allow the operating system to interact with hardware
• device drivers are specific to the computer's architecture, and specific to the OS
• a virtual machine is a theoretical computer and a software implementation of a computer system
• virtual machines provide an environment with a translator for the intermediate code to run