FUNCTIONS OF OPERATING SYSTEMS

Interfaces

Memory Management

Security

Utility

Interrupt Handling

Security

• Login• back up/system restore• utility software

JOB SCHEDULING

– FCFS – fist come, first served– RR or round robin – every user gets a small amount of time

with the processor before it goes onto the next user (multi access OS)

– SJF – shortest job first. OS estimates length of time to carry out the task and places accordingly in the queue

– SRT – shortest remaining time, similar to SJF except jobs will obviously get shorter and hence nearer the top of the queue as they are processed. Jobs are returned to the queue when they have to stop for some reason like waiting for input from a keyboard. Problem is that longer jobs may never get started!!

– MFQ – multi-level feedback queues. Very complete algorithm involving a number of queues set up according to original rules and acting like a set of league table.• As jobs are given a long amount of processor time without

finishing, or when they require long periods of peripheral time, they will drop down the league tables, sometimes getting relegated!

• When a job is relegated or is finished and leaves the system, another job can be promoted.

• Jobs are not allowed to spend too much time monopolising the processor

SPOOLING

• If a job requires printout then there could be a sizable delay in carrying on with the processing as the processor has to send data to the printer for printing

• Printers are relatively slow at carrying out its tasks– Causes a speed mismatch between processor and printer– Buffers/interrupts is a solution to this

• If the job is going to take a long time, requiring multiple downloads to the printer, or printer is already busy – the result will be a slowing down of the processor

DISK THRASHING• Involves the disk continually being searched for pages.• Splitting the memory & jobs into pages of equal sizes makes things fairly

straight forward to control as indexing system keep track of everything.• It is not a very sensible way to split up pieces of code.• More sensible to have a procedure in a page and then another page for

the next procedure and so on.• If the procedures are smaller than a page then more than one page is

required and we are back to having arbitrary dividing lines.

STARTIN’ UP THE OS

• POST = POWER OF SELF TEST

• Checks that all the parts that it needs to ‘come to life’ are available• It clears the registers in the CPU & loads the address of the first instruction in the

boot program into the program counter

•Boot programGets the system ready to accept an operating system.unalterable

•When a computer is switched on, it can only use data that is stored in the ROMGenerally a small instruction set

•Boot fileContains some of the parameters by which the system will operate.Partial user-definable

oProgram finds the file and reads

•Computer is now ready to load the OS. – Usually found on hard drive

Index on a hard drive is known as FAT:•File allocation tableWhen the surface of the disk is formatted it is divided into sectors and tracks and groups of sectors are combined into clusters on the disk surface.The FAT has all the clusters on the disk surface listed in order & the file that is stored there.Files that require multiple clusters because of their file size will have the clusters linked together in the table, and clusters that are not in use will be tagged as available.