Preliminaries and OS Structures

Question 1

What is the role of an OS?

Answer 1

• Provide access to resources (processors, peripherals, memory)
• Gets out of the way of applications

Question 2

What is contained in the code of an OS?

Answer 2

• Access to resources
• Protects application from itself
• Protections applications from other applications

Question 3

What are characteristics of a DOS-like OS system?

Answer 3

• No protection between kernel and user
• Single user system
• One program ran at a time
• Extensible since everything runs in the same protection domain

Question 4

What are characteristics of a monolithic OS system?

Answer 4

• Has protection between user and kernel
• Not easily extensible due to consolidation. Need to recompile to customize
• Performs well since all system services are in same address space

Question 5

What is contained in a PCB?

Answer 5

Process control block

• Volatile state: register values of the processors for this process
• Computational state for each thread

Question 6

What is the difference between threads and processes?

Answer 6

• OS doesn’t see threads, just processes
• Thread library is part of application at runtime
• Switching threads is cheap (taken care of by application)

Question 7

What can multithreads do with multicores/multiprocessors?

Answer 7

• Not a lot. Multithreads cannot run on multiple processors since processes run on processors, not threads
• If a process makes an I/O call, entire application will be blocked on the processor

Question 8

What are characteristics of user level threads?

Answer 8

• Switching among threads is voluntary
• Pre-emption is not voluntary
• All blocking calls go through the thread library

Question 9

What are characteristics of kernel level threads?

Answer 9

• It is expensive to context switch
• Thread libraries are non portable
• Threads in application are now visible to OS through libraries
• OS provides the thread library

Question 10

What services are offered by the OS?

Answer 10

• Process/thread management
• Memory management
• Inter-process communication
• Network access
• Access to mass storage and other peripherals

Question 11

How do components/structures affect the OS?

Answer 11

• Affects performance

- Border crossings are expensive

Question 12

Why are border crossings expensive?

Answer 12

• Change in locality (address space)
• Copying between kernel buffer and user buffers are expensive
• Different applications require different sources

Question 13

What are the goals of an OS?

Answer 13

• Protection
• Performance
• Flexibility
• Scalability
• Agility
• Responsiveness

Question 14

What is the process involved in a page fault?

Answer 14

• Find a free page frame (Page replacement algorithm)
• Update page table
• Resume process

Can be customizable

Question 15

What are the three classic examples of OSes?

Answer 15

• DOS-like (great performance, no protection)
• Monolithic (flexible, bad performance, good protection)
• Microkernel (Flexible)

Question 16

What are program discontinuities?

Answer 16

• Page fault
• System call
• External interrupt
• Exceptions

Question 17

What are the two highest overhead components involved in border crossings?

Answer 17

• Cache misses

- TLB misses

Question 18

What are the characteristics of a microkernel OS system?

Answer 18

• Takes a hit in performance due to number of border crossings
• Extensible
• Protection due to each service having their own address space

Question 19

What is the difference between a logical protection domain and hardware address space?

Answer 19

Protection domain: OS abstraction

Hardware address space: mechanism for implementing OS abstraction