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Systems Software: Functions of an Operating System (Processor Scheduling …
Systems Software: Functions of an Operating System
What is an Operating System?
You need software to manage communication with your computer hardware.
The boot loader in ROM loads the Operating System (OS) into RAM when the computer is switched on.
The OS manages the hardware and provides an interface for the user and the application software.
What functions does the OS provide?
-
User interface
-
Memory management
-
Interrupt handling
-
Processor scheduling
User interface
The Operating System hides the complexity of the hardware from the user by providing a user interface
Type of interface
-
GUI
-
Command line
-
Menu
-
Form based
Memory Management
Programs and their data need to be loaded into RAM.
The Operating System must manage the allocation of RAM to the different programs.
There may not be sufficient RAM for all desired processes to be completely loaded into RAM at once.
Paging
Available memory is divided into
fixed
size chunks called
pages
.
Each page has an address.
A process loaded into RAM is allocated sufficient pages, but those pages may
not
be
contiguous
(next to each other) in physical terms
.
Process A requires two pages in RAM, process B requires three pages.
Process A ends.
Process C is started, but it needs three pages of RAM so it is allocated non-contagious pages.
Page table
A page table maps between the logical memory locations and the physical memory locations.
Segmentation
Alternatively, memory is divided into segments which can be of different lengths.
Segments can relate to parts of a program, for example a particular function or subroutine may occupy a segment.
Virtual memory
A computer has a fixed amount of RAM; the demands for memory will often exceed this amount.
An area of the hard disk can be designated as virtual memory.
Some of the pages of a current process are stored in virtual memory until they are needed, at which point they are swapped into RAM.
If many processes are running and the computer has insufficient RAM, lots of time is spent swapping pages in and out of virtual memory.
Repeatedly swapping pages can noticeably slow down the computer.
This is known as
disk thrashing
.
Interrupts
Interrupts can be sent to the CPU by software, hardware devices or the CPU's internal clock.
Interrupt examples
An I/O device sends an interrupt signal.
The printer runs out of paper.
An error occurs in a program.
A scheduled interrupt from the internal clock.
Power failure.
The CPU checks at the end of each clock cycle whether there are any interrupts to be processed.
Interrupts - using the stack
When an interrupt is detected, the processor stops fetching instructions and instead pushes the current contents of its registers onto a stack.
The CPU uses an
interrupt service routine
to process the interrupt.
-
When processing has finished, the values can be popped from the stack and reloaded into the CPU.
Interrupt priority
Interrupts have different priorities, and will be processed in order of priority.
Interrupts can themselves be interrupted if the new interrupt is of a higher priority.
-
If a higher priority interrupt whilst an interrupt is being processed, the original interrupt registers will be pushed onto the stack as well.
-
A stack is a
LIFO
data structure, so the last data to be pushed on will be the first to be retrieved.
Processor Scheduling
A single CPU can only process instructions for one application at a time.
The Operating System must schedule when each application can use the CPU.
This gives the illusion of
multitasking
- multiple applications appear to be running simultaneously.
Aims of Scheduling
-
To provide an acceptable response time to all users.
-
To maximise the time the CPU is usefully engaged.
-
To ensure fairness on a multi-user system.
Round Robin
Each job is allocated (FIFO) a
time slice
during which it can use the CPU's resources.
If the job has not completed by the end of its time slice, the next job is allocated a time slice.
First come first served
The first job to arrive is executed until it completes.
Shortest remaining time
The to completion is estimated as each new job arrives.
The job with the shortest
remaining
time to completion is executed, meaning that a shorter new job can take over from the current process.
Why does this algorithm not constantly switch between two jobs as they become closer to completion?
What do you think ‘starvation’ is, and why might it occur if this algorithm is used to schedule processor time?
Shortest job first
Also known as "shortest process next".
As with shortest remaining time,the total execution time of each job is estimated by the
user
.
The waiting job with the smallest total execution time is executed when the current job completes.
Unlike shortest remaining time, this algorithm is not pre-emptive. What do you think this means?
Preemptive scheduling:
job is allowed to move out of the running state to the ready queue.
Non preemptive:
job can only be moved from running state once complete.
Multilevel feedback queues
Multiple queues are created with different priority levels.
If a job uses too much CPU time it is moved to a lower priority queue.
Processes can also be moved to a higher priority queue if they have waited along time.
