1.2.1 Systems Software
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What is an operating system?
A collection of programs that work together to provide an interface between the user and computer
Give the 7 functions of an operating system.
Memory management Resource management Input/Output management Interrupt management Security User interface Utility software
Why are paging & segmentation used?
To manage memory to avoid disk thrashing
What is disk thrashing?
Slowed system performance Occurs when operating system uses disk for memory due to excessive & frequent reading or writing operations
What is paging?
A memory management technique that divides memory into fixed-sized sections called pages
What is segmentation?
A memory management technique that divides memory into variable-sized, logical sections called segments
Describe how virtual memory is used.
A section of hard drive is used to act as RAM Sections of programs not currently being used are temporarily moved into virtual memory throuugh paging Frees up memory for other programs in RAM
What is an interrupt?
A signal generated by software or hardware to indicate to the processor a need for immediate attention
Outline the stages of the Interrupt Service Routine.
Interrupt register checked at end of each FDE cycle If an interrupt with a higher priority than the current process exists, the contents of the registers are transferred onto stack Relevant ISR is loaded into RAM A flag is set to signal ISR has begun & reset once ISR is finished Interrupt register checked again, process is repeated if any of higher priority found When there are no interrupts with higher priority, the contents of the stack are popped off and loaded back into registers The FDE cycle renews as before
Name the 5 scheduling algorithms.
Round Robin (RR) Shortest Remaining Time (SRT) Shortest Job First (SJF) Multi-Level Feedback Queue (MLFQ) First Come First Served (FCFS)
Which scheduling algorithms are pre-emptive?
Round Robin Shortest Remaining Time Multi-Level Feedback Queue
Which scheduling algorithms are non-preemptive?
First Come First Served Shortest Job First
Describe the First Come First Served scheduling algorithm.
The first job enqueued is the first to enter the running state
Describe the Shortest Job First scheduling algorithm.
Sorts the jobs in order of expected time to complete
Describe the Shortest Job First scheduling algorithm.
Sorts the jobs in order of expected time to complete
Describe the Round Robin scheduling algorithm.
Gives each job a maximum length of processor time Once the time is complete, the job is put to the back of the queue and the job at the front is passed to the processor If a job completes before its time slice has ended it leaves the system
Describe the Multi-Level Feedback Queue scheduling algorithm.
Several queues of different priorities with jobs migrating downwards
Describe the Shortest Remaining Time scheduling algorithm.
The job with the shortest expected remaining time is put to the front of the queue.
Whats an advantage of using Multi Level Feedback Queues for scheduling?
Services most urgent interrupts first
Whats a disadvantage of using Multi Level Feedback Queues for scheduling?
Hard to implement
Define a real-time operating system.
Perform a task within a guaranteed time frame Used in systems were a response within a given time frame is critical
State the function of the BIOS.
Running tests upon the computer’s startup
Name the 3 tests carried out by the BIOS.
Power-On Self Test (POST) Checking the CPU clock, memory & processor Testing for external memory devices
Describe the function of the POST.
Ensures all hardware is correctly connected & functional