Functions of an Operating System (it's a big one) (for now)

it is what it is really

what is an operating system?

what is an operating system?

• a program or set of programs that manages the operations of the computer for the user

• acts as a bridge between user and hardware

• held in permanent storage

• when a computer is switched on, the loader in ROM sends instructions to load the OS by copying it from storage into RAM

what are the basic functions of an operating system?

• use the mindblowingly shit acronym BUMPII to remember them

• BACKING store management

• USER interface management (you shouldn’t really use this in an exam)

• MEMORY management

• PROCESSOR scheduling

• INTERRUPT service routines

• INPUT and output management

what is memory management?

• each program/file that the user is using must be allocated a specific area of memory while the computer is running

• if a user wants to switch applications, each one must be stored in memory simultaneously

• the allocation and management of space in RAM is controlled by the operating system

what is paging?

• memory is divided into fixed size pages of 4Kb each

• a process currently in memory may be held in several non-contiguous pages

• in logical memory, the pages appear to be next to each other as the user sees them

• in physical memory, the pages can be stored in memory locations anywhere within the physical memory space

what is segmentation?

• the logical division of address space into varying length segments which depend on the program structure

• it is possible to load only part of a program into memory initially

what is virtual memory? (again)

• when there is not enough space in RAM to store all programs and data, some pages of inactive programs are swapped out to an area on the hard disk, which acts as a slower form of RAM

• this is so that the OS can make room for more active programs in RAM

• if a large number of programs are loaded, pages may need to be constantly swapped in and out of RAM, which is called disk thrashing

• as the OS is spending so much time swapping pages in and out, the performance deteriorates and the CPU slows down

what are interrupts?

• a signal from the software, hardware or the clock to the CPU

• eg. when an application terminates or requests services from the OS, when an I/O operation is complete, paper jam

• interrupts are triggered regularly by a timer to indicate that it’s the next process’s turn to have processor time

• at the end of each FDE cycle, a test for the presence of interrupts is carried out

• interrupts help multi-tasking to take place

what happens when the CPU receives an interrupt?

• the running program or process is suspended

• any lower priority interrupts are disabled (i/o interrupts < clock interrupts < power fail interrupts)

• the values stored in the registers are saved onto the system stack

• an interrupt service routine is called which is different depending on the type of interrupt

• after the interrupt has been serviced, the original values of the registers are retrieved from the stack

• the process resumes from the point it left off

what is processor scheduling?

• the OS allocates processor time to each application as it competes for the CPU, so the CPU can run applications simultaneously

• when one application is using the CPU, the OS can queue up the next process required by another application to make efficient use of the processor

• by carrying out small parts of multiple larger tasks in turn, the processor can give the appearance of carrying out tasks simultaneously (multitasking)

what is the scheduler and what does it do?

• an OS module which ensures processor time is used efficiently

• can be very complex on larger networks

• the scheduler aims to maximise throughput, be fair to all users, provide good response times to all users, ensure hardware resources are kept as busy as possible

what is round robin scheduling?

• the OS allocates a time slice / quantum to each job in which it can use the CPU

• if the job is not completed by the end of the time slice, it returns to the back of the queue

• the next process then takes its turn

• an interval timer generates interrupts at specific times

• this method helps guarantee reasonable response times for all users

what is first come first served scheduling?

• first job is executed until it completes

• the time that each process takes to execute is irrelevant

• inefficient if small tasks have to wait for large ones

what is shortest remaining time scheduling?

• the process with the shortest remaining time to completion is run next

• processes can be suspended if a higher priority process joins the queue, so a shorter new job can take over from the current process

• reduces the number of small jobs waiting behind big jobs and requires knowledge of how long a job will take

• possible for batch jobs or scientific / commercial jobs which are run regularly

what is shortest job first scheduling?

• the OS picks the process with the shortest amount of time and runs it until it’s finished

• scheduler must know duration of each process

what is multi-level feedback queues scheduling?

• multiple queues are created with different priority levels

• jobs can be moved to different queues depending on how much processor time they use

• the algorithm gives preference to short jobs, gives preference to I/O bound processes, separate processes into categories based on their need for the processor

• it maximises processor use by keeping the I/O devices as busy as possible because they’re much slower than processor speed

what is backing store management?

• when files and applications are loaded, they are transferred from backing storage into memory

• the OS keeps a directory of where files are stored so they can be quickly accessed

• it needs to know which areas of storage are free

• the file management system enables a user to move files, delete files and protect files

what is peripheral management?

• different applications require different I/O devices

• the operating system needs to communicate with the printer to check it is online, check it’s a printer and begin communication

• the data to be printed is transferred to an area of memory called a buffer so that the CPU can continue with another task

• the purpose of the buffer is to compensate for the difference in speed between the printer and the CPU