3.10-3.11

Scheduling - The OS has to arrange the order of tasks to be done either:
            1. to make maximum use of the CPU
            2. To give one program priority

Program A making calculations = Processor hungry = Processor bound
Program B Printing Wahe slips = I/o hungry = I/O Bound

If Program B is given priority the it shares the time more efficiently and more throughput is achieved.

The objective of scheduling are to

Maximise the use of the computer
be fair to all users
Provide a reasonable response time to all users, whether they are on-line users or a batch processing user
Prevent the system failing if it is becoming overloaded
Make sure that the system is consistent by always giving similar response times to similar activities from  day to day

The following is a list of criteria which may be used to determine a schedule which will achieve the previous objectives

Priority - Give some jobs a priority to the CPU
I/O or processor bound? - Give priority to I/O bound tasks as Processor Bound would hog the CPU
Type of job - Batch processing, on-line and real time all erquire different response times
Resourse requirements - The amount of time needed to complete the job, the memory required, I/o and processor time
Resources used so far - The amount of processor time used so far, how much I/o used so far.
Waiting time - The time the job has been waiting to use the system.


Scheduling Policies
FIrst Come First Served - First to the ready queue will be the first to the running state
Shortest Job First - Jobs are sorted in the Ready queue in order of the time they will take. New jobs slot into their correct position
Round Robin - Each job gets set a 'time slice' with the processor and at the end of the 'slice' it returns to the back of the Ready Queue. If a job finishes during a slice it leaves the system
Shortest Remaining time - The ready queue is sorted by job that has the least time remaining
and 2 more...

A job joins a duifferent queue depending on how important the job is.
A job in a lower priority queue has to wait until the queues above are clear before it gets processor time.

MFQ - MFQ use the same system but also apply Tim Slices and if the job doesn't finish during the slice then it is moved to a lowere queue, the last of which is a Round Robin. 

Spooling - Preventing speed mismatch by settting up a cueing system.
The details of an output are witten to a file e.g name, location, size, fonts, images
The info file is placed in a queue

OS Organising Memory

Reasons

Programs loaded in different orders

Multi Users on one app with individual sets of data

Apps simultaneously controlling multiple sets of data for multiple users or apps = reentrant

Interrupts

I/o interrupt - From I/o device e/g Printer out of paper

Timer interrupt - From internal clock because processor has time critical activities to do.
                - OS' have to allocate certain time to certain processes. Switches to other processes because a certain activity is taking too long.

Hardware error - Power failiure = OS must attempt to close safely

Program interrupt - Program error e.g trying to use OS memory

Interrupts and interrupting interrupts - 

Register - Holds the info that is being executed and is the CPU's own form of 'RAM'. While also storing data while interrupts are occurring as interrupts happen first.
 So an activity is happening, then is interrupted, so the activity is stored in the register till the interrupt is finished.






Memory Management

A job is made up from instructions

Jobs sit in main memory when they are queueing to use the processor

If Job C finishes ,the OS needs to re-address the other jobs to Job E and 

The OS loads jobs and recalculates memory addresses to fit new jobs in.

The Loader does both these tasks

A calculation must be done for each instruction's address so a job can be moved

Relative addressing is when each instruction's new address is decided relative to where the first instruction was moved to.

Segmentation

Variable lengths blocks - Programmers divide jobs into segments

Segments = Sub-routines/ groups of subs/ data

Addressing

The segment table must include the start position and size of the segment

Virtual Memory

Dividing programs up into pages or segments means that only parts being ued need to be loaded

Disk threshing = Memory full forces constant page swapping from processor

What gets replaced in memory? Page idle the longest, First in First out

The OS flags each Page with a 0. data must not be paged out so when the data changes in is flagged with a 1.



Paging

Memory addressing becomes a big issue

A paging table is kept to record which Job page in which memory page

Virtual memory is a designated slice of the hard drive and it swaps entire pages around?
Used when RAM isn't available

Desktop PC OPerating Systems

File allocation Table (FAT)

Uses a linked list to point to where files are on the disk.

Disk is FORMATTED - Divides the disk radially and concentrically (Circle)