CPU-intro

Question 1

By running one process, then stopping it running another, the OS promote the illusion that many virtual CPUs exist when in fact there is only one physical CPU (or a few). This technique is also called.

Answer 1

Time sharing of the CPU

Question 2

What does time sharing allow users to do?

Answer 2

Run many concurrent processes

Question 3

Negatives with time sharing of CPU

Answer 3

Performance cost, as each will run more slowly if the CPU(s) must be shared.

Question 4

Low-level machinery of the OS is called

Answer 4

Mechanisms

Question 5

Explain mechanisms

Answer 5

Are low-level methods or protocols that implement a needed piece of functionality. Mechanisms are the implementations that enforce policies, and often depend to some extent on the hardware on which the operating system runs.

Question 6

Name an example of low-level machinery

Answer 6

Context switch, it is a time-sharing mechanism employed by the OS

Question 7

What ability does the context switch give the OS?

Answer 7

Ability to stop running one program and starting running another on a given CPU

Question 8

Explain what policies are

Answer 8

Policies are algorithms for making some kind of decision within the OS.

Question 9

Give an example of a policy within the OS

Answer 9

Scheduling policy (deciding which processes should run)

Question 10

The abstraction provided by the OS of a running program.

Answer 10

Process

Question 11

What a program can read or update when it is running

Answer 11

Its machine state

Question 12

Name components of machine state that comprises a process

Answer 12

Memory, registers, storage device

Question 13

The memory that the process can address

Answer 13

Address space

Question 14

What are the process registers?

Answer 14

Many instructions explicitly read or update registers, important to the execution of the process.

Question 15

Give an example of the processes registers

Answer 15

Program counter (PC)

Question 16

What is the program counter

Answer 16

It tells us which instruction of the program that is currently being executed.

Question 17

Explain stack pointer and fram pointer

Answer 17

Are used to manage the stack for function parameters, local variables, and return addresses.

Question 18

Name some of the API that must be included of an operating system.

Answer 18

Create, destroy, wait, miscellaneous control, status

Question 19

Explan the method create

Answer 19

An operating system must include some method to create new processes.

Question 20

Explain the method destroy

Answer 20

A way to destroy proesses forcefully.

Question 21

Explain the method wait

Answer 21

Wait for a process to stop running.

Question 22

Explain miscellaneous control

Answer 22

For example can be a method to suspend a process, and then resume it.

Question 23

Explain status

Answer 23

Interfaces to get some status information about a process as well, how long it has run for or its state.

Question 24

How does the OS get a program up and running?

Answer 24

OS load its code and any static data (initilized variable) into memory, into the address space of the process. Programs reside on disk in some kind of executable format. The OS reads those bytes from disk and place them in memory.

Question 25

What does it mean when an operating system loads the process eagery

Answer 25

All at once before running the program.

Question 26

Loads the process lazily

Answer 26

Loads pieces of code or data only as they are needed during program execution.

Question 27

Explain what the stack is

Answer 27

Contains the programs local variables, function parameters and return addresses. OS will ikely initilized the stack with arguments (fill the parameters to main function, ie arg and argv).

Question 28

Explain heap

Answer 28

Heap is used for explicitly requested dynamically-allocated data.

Question 29

Which methods does programs have to request and release space on the heap?

Answer 29

malloc() and free().

Question 30

Which file descriptors does each process by efault have?

Answer 30

Standard input, output and error.

Question 31

How does the OS transfers control to the CPU to the newly-created process?

Answer 31

Jumping to the main() routine. And the program begins its execution.

Question 32

Explain the three states a process can be in

Answer 32

Running: A process is running on a process, it is executing instructions.
Ready: Process is ready to run but is OS has chosen not to run it at this given moment.
Blocked: A process has performed some kind of operation that makes it not ready to run until some other event takes place (process init an I/O request).

Question 33

Process being moved from ready to running

Answer 33

Process has been scheduled.

Question 34

Process being moved from running to ready

Answer 34

Descheduled

Question 35

When does a blocked process (through I/O init) becomes ready?

Answer 35

I/O completion.

Question 36

Explain process list

Answer 36

A list for all processes that are ready, as well as some additional information to track which process is currently running. It is a datastructure of the OS.

Question 37

What is a register context?

Answer 37

It holds for a stopped process, the contents of its registers.

Question 38

What is a zombie state?

Answer 38

A process has exited but has not been cleaned up.

Question 39

What happens when a parent waits for its child?

Answer 39

Wait for the completion of child, also indicate to the OS that is can clean up any relevant data structures that referred to the now-extinct process.

Question 40

Why do we seperate the OS mechanisms from the policies?

Answer 40

Allows to change policies without having to rethink the mechanism and form a modularity (general software design principle).

Question 41

What is another name of the individual structure that stores information about a process?

Answer 41

Process Control Block (PCB).