Ch. 2 - Processes

Question 1

fork()

Answer 1

Creates an identical copy of the calling process with the same data, values, and registers. The new process is the child process and it has a different pid than its parent.

Question 2

wait()

Answer 2

Allows a parent process to check the running status of the child and wait until it is complete.

Question 3

Uniprogram Systems

Answer 3

Current program must finish before the next program can be executed.

Question 4

Multi-programming Systems

Answer 4

CPU switches automatically from process to process. The CPU only runs one process at any given time.

Question 5

How can several processes share one CPU?

Answer 5

-Fair Scheduling (every process gets to run eventually)
-Protection (different processes cannot modify each other’s state)
-Resource sharing

Question 6

What is the dispatcher responsible for?

Answer 6

The dispatcher schedules processes. It runs a process for a short period of time then stops the process and saves its state. Finally, the dispatcher loads the state of another process and runs the process.

Question 7

List the different states of a process

Answer 7

New (give program resources)
Ready (ready for execution)
Active (process currently executing)
Blocked (voluntary stop)
Stopped (involuntary stop)
Exiting (process leaving CPU)

Question 8

How does the dispatcher regain control of the CPU?

Answer 8

“Sleeping Beauty Approach” - Trust the process to wake up the dispatcher when it is done
“Alarm Clock” - Wake up the dispatcher after a certain amount of time has passed (superior approach)

Question 9

What is an issue of using the “sleeping beauty approach” to give the dispatcher control of the CPU?

Answer 9

A process may loop indefinitely. Thus, it never finishes so it never wakes up the dispatcher.

Question 10

Context Switching

Answer 10

OS saves the state of the active process and restores the state of the interrupt service routine (ISR)

Question 11

Why must the OS disable all interrupts while saving the state of an active process?

Answer 11

If the OS is interrupted, it cannot save the state as it must service the interrupt which could also be interrupted.

Question 12

What are the 2 ways of creating a new process? Which method is more efficient?

Answer 12

Building a process from scratch
Cloning an existing process

Cloning an existing process is faster as no disk read is necessary.

Question 13

fork() return values

Answer 13

Returns the PID of the process that was created
Value greater than 0 - parent process
Value = 0 - child process
Value less than 0 - fork() failed

Question 14

Zombie Process

Answer 14

A process that has completed execution but still has an entry in the PCB

Question 15

What are the reasons why a process can enter the exiting state?

Answer 15

Normal Completion - exit() or abort called explicitly or implicitly.
Abnormal Termination - programming error, runtime error, I/O error, process killed.

Question 16

What happens when the stack and heap in memory intersect?

Answer 16

You have no more memory to allocate

Question 17

What are the similarities between threads and processes?

Answer 17

Every thread/process has a state.
Every thread/process has its own stack
Execute sequentially
Can issue system calls

Question 18

What are the differences between threads and processes

Answer 18

Creating a thread is less expensive (no process spawned)
Threads within a process share resources while processes have different address spaces
Threads within a process are not independent and not protected against each other

Question 19

Thread Safe Code

Answer 19

Code that functions correctly during simultaneous execution by multiple threads.