Week 3: Processes (Model trafficing schedulling)

Question 1

Why do modern computers appear to perform multiple tasks simultaneously?

Answer 1

The processor switches between programs multiple times per second, giving the illusion of multitasking.

Question 2

What is the role of the operating system in multi-programming?

Answer 2

To keep track of running programs and manage the switching between them.

Question 3

How must processes be executed?

Answer 3

They must execute sequentially even though they may be interrupted so other processes can run.

Question 4

What is a process tree?

Answer 4

It’s a hierarchy where parent processes create child processes, which in turn can create their own child processes.

Question 5

What are the three resource-sharing options during process creation?

Answer 5

• Parent and children share all resources
• Children share a subset of the parent’s resources
• Parent and child share no resource

Question 6

What are the two resource-sharing execution options during process creation?

Answer 6

• Parent and children execute concurrently
• Parent waits until children terminate

Question 7

What are the two resource-sharing program options during process creation?

Answer 7

• The child is a duplicate of the parent
• The child has a program loaded into it

Question 8

What occurs during normal process termination?

Answer 8

The process executes its final instruction, exits, ensures output data is written, and deallocates its resources.

Question 9

Under what circumstances might a parent process terminate a child process?

Answer 9

If the child exceeds a resource limit, becomes stuck, or if the parent process itself is terminated.

Question 10

What are the five states of a process?

Answer 10

• New
• Ready
• Running
• Blocked (Waiting)
• Terminated.

Question 11

What does it mean when a process is “Blocked”?

Answer 11

The process is waiting for an event to occur.

Question 12

What typical information does the operating system hold about a process?

Answer 12

• Process management information: process state, program counter, processor registers, time process started,
  process ID, parent ID
• Memory management information: code location and size, data location and size, stack
• I/O management information: files open and modes, I/O devices in use, current directory

Question 13

Why is process information necessary?

Answer 13

To allow the operating system to resume a process in the correct state after an interruption.

Question 14

What is the purpose of process tables in an operating system?

Answer 14

To store information about each process, enabling management and state tracking.

Question 15

What happens to a process’s table entry when it is interrupted?

Answer 15

The operating system updates the process’s state and any related information.

Question 16

What are the two types of interrupts?

Answer 16

• Software
• Hardware

Question 17

What happens when an interrupt occurs?

Answer 17

The microprocessor finishes the current instruction and passes control to an interrupt handling routine.

Question 18

What is preserved during interrupt handling?

Answer 18

The state of essential registers, including the program counter, is preserved to enable the resumption of execution.

Question 19

What governs the execution of interrupt handling routines?

Answer 19

The operating system decides which code runs based on the type of interrupt.

Question 20

What is a clock interrupt?

Answer 20

A hardware interrupt triggered when an internal timer reaches zero, to interrupt user processes that are simply doing long calculations

Question 21

When is it only possible to change the value of the internal timer of the clock interrupt?

Answer 21

When the operating system is in privileged mode

Question 22

What happens during a context switch?

Answer 22

The operating system saves the state of the old process and loads the saved state of a new process to run it

Question 23

What factors influence the time taken for context switching?

Answer 23

The efficiency of the operating system and the level of hardware support.

Question 24

Why is the time spent on context switching considered overhead?

Answer 24

Because the system does no useful work while switching

Question 25

In what does the amount of time context switching take depend?

Answer 25

The operating system and the hardware

Question 26

What are the different classes of scheduler types?

Answer 26

• Long-term: selects which processes should be brought into the queue
• Medium-term: selects which process is temporarily suspended and swapped out of main memory to reduce the number of processes occupying memory
• Short-term: selects which process should be executed by the processor

Question 27

What is the race condition?

Answer 27

The condition which its result depends on the race (timing) of two processes.