System Software

Operating system the main software that controls how a computer operates

control the hardware of the computer system; hard disk, graphic card manages software; loading / unloading into main memory provides security; username and password interface; allows you to interact with the computer

Kernel Device drivers User Interface System utilities

Loading / unloading applications from memory scheduling tasks to run efficiently on the CPU memory management file storage to and from secondary storage devices such as the hard disk file management data security

vary in tasks depending on the system

The OS communicates with lots of different devices and each require a device driver Drivers for the internal hardware are normally pre installed but can download from a companies website

RAM, HDD, graphics card, printer, monitor, mouse

normally come with device drivers on a CD, USB etc

Allows you to interact with the computer GUI Command Line Interface Natural Languages Menu Based

Graphical User Interface WIMP User friendly users pictures, icons to make it intuitive slower to perform tasks

Windows, Icons, Menus, Pointers

Interact with the Computer system by typing commands into the screen

reacting to voice Alexa, telephone menu system can struggle with accents good where keyboard is not practical issues in noisy environment

Provides basic facilities that run in the background print spool services cryptographic password management file management services anti-virus utility disk defragmenter utility file compression utility

Even in modern computer you can run out of RAM This is when the operating system will use ‘virtual memory’ The OS will identify RAM that isn’t in use and will copy this to secondary storage This frees up a block of RAM to be used for the program that is in use This will effect performance as it takes time to perform the swapping of data If the computer is low on RAM the operating system may spend more time swapping and less time executing the task. This is called thrashing and the operating system will try and avoid this If thrashing starts then the operating system may terminate programs that are taking up too much RAM but data might be lost

Memory is limited in size so with processes being loaded and run it needs to be managed this is to prevent processes overwriting over processes data each process is given it’s own segment of memory the memory manager assigns each process a segment of main memory these segments are further divided to each have a ‘stack’, ‘free memory’, etc A process is an executable application that’s loaded into memory

paging, segmentation and virtual memory

both allow programs to run despite insufficient memory both pages and segments are stored on disk both pages and segments are transferred into memory when needed

pages are fixed size pages are made to fit sections of memory pages are physical divisions segments are different sizes segments are complete sections of programs segments are logical divisions

If physical memory is running low, virtual memory has to be used If more time is spent moving pages/segments in and out of the memory to the disk (thrashing) than actually processing the instruction, the computer will be very slow

stack segment free memory data segment code segment

Logical division of memory. The sizes of each segments may be unequal

Code seg - program instructions Data seg - for variables the programs use stack seg - allows for subroutines - when a subroutine is called the program notes where it was called - And added to stack Free memory - for when the stack grows and shrinks

when RAM gets too full, the memory manager may mark sections of secondary storage (on the hard disk) to act as main memory. These sections are called virtual memory Very slow for the CPU to access When the CPU needs data held in virtual memory, the memory manager re-loads it into RAM, swapping out less-used data to make room for it Swapping data in and out of virtual memory is a slow process, so if virtual memory is in heavy use, the computer seems slower