Operating Systems (Midterm)

Question 1

What are a computer’s two modes of operation?

Answer 1

1. User mode (application)
2. Kernel mode (OS kernel)

Question 2

What are the transitions between user/kernel mode?

Answer 2

1. Hardware interrupt: HW device requests OS services (asynchronous, event-triggered)
2. System call (aka trap): User program requests OS services (synchronous, program-triggered)
3. Exception: Error handling (Invalid memory access, invalid permission, etc.)

Question 3

Describe a monolithic kernel

Answer 3

Monolithic kernel: A large process running entirely in a single address space (single binary file)
- All kernel services execute in kernel address space

Question 4

What are the pros and cons of a monolithic kernel?

Answer 4

Pros:
- Fast
Cons:
- Huge kernel, harder to maintain
- No protection between kernel components
- Complex dependencies, not easily extensible

Question 5

Describe a microkernel

Answer 5

Microkernel: Kernel broken down to separate processes (aka servers)
- Servers kept separate and run in different address spaces
- Communication is done via message passing (servers communicate through IPC)

Question 6

What are the pros and cons of a microkernel?

Answer 6

Pros:
- Modular design, easily extensible
- Easy to maintain
- More reliable + secure
Cons:
- Performance loss
- Complicated process management

Question 7

On a magnetic disk, read/write data is a 3-stage process. What are the three steps and which is the longest?

Answer 7

1. Seek time: Position the arm over the proper track (longest time)
2. Rotational latency: Wait for the desired sector to rotate under the read/write head
3. Transfer time: Transfer a block of bits (sector) under the read/write head

Question 8

How do you make information generated by HW device events available to running applications?

Answer 8

1. Polling
2. (HW) Interrupt
3. DMA

Question 9

Explain polling

Answer 9

Polling (busy waiting):
- I/O device puts information in a status register.
- OS periodically checks the status register
(SIMPLE, WASTES CPU CYCLES, LATENCY)

Question 10

Explain HW interrupt

Answer 10

Interrupt:
- Whenever an I/O device needs attention form the processor, it interrupts the processor from what its currently doing
- Types on interrupts: I/O, Timer
(MORE RESPONSIVE, COMPLEX, BETTER PROCESSING EFFICIENCY)

Question 11

Briefly explain DMA

Answer 11

Direct memory access, delegate I/O responsibility from CPU

Question 12

What is a process?

Answer 12

An instance of a program running on a computer.
An abstraction that supports running programs.

Question 13

What is the difference between a program and a process?

Answer 13

• Program is static code + data
• Process is a dynamic instantiation of code + data + files
• No 1:1 mapping
  
  • A program can run invoke many processes (Running a program twice, or a program contains fork())

Question 14

What does the fork() function do?

Answer 14

The Fork system call creates an exact copy of the calling process.
- After fork, caller is parent, newly creates process is child
- Returns new PID to parent, and 0 to child
- Same memory image, environment settings, and opened files
- Child program calls execve to change its memory image + run a new program

Question 15

What makes output deterministic?

Answer 15

Deterministic: Output stays the same no matter no matter how many times it’s run

Question 16

What are 5 process management system calls?

Answer 16

1. fork: Creates a new process
2. exec: Execute a new process image
3. exit: voluntary process termination
4. wait: parent wait for child to finish
5. kill: send a signal to process (or group), can cause involuntary process termination

Question 17

What is the 5-state process model?

Answer 17

1. New: New process is first accepted into the queue
2. Ready: Process is waiting to be selected to run
3. Blocked: Process is waiting for some event
4. Running: Process current being executed by CPU
5. Terminated: Process released due to completion or by some issue

Question 18

What is a thread?

Answer 18

A program in execution w/out dedicated address space: threads of the same process share address space

Question 19

Processes vs threads, describe some characteristics of threads.

Answer 19

Threads:
- No data segment or heap
- Multiple can coexist in a process
- Share code, data, heap, I/O
- Have own stack and registers
- Inexpensive to create
- Inexpensive context switching
- Efficient communication

Question 20

Processes vs threads, describe some characteristics of processes.

Answer 20

Processes:
- Have data/code/heap
- Include at least one thread
- Have own address space, isolated from other processes
- Expensive to create
- Expensive context switching
- IPC can be expensive

Question 21

What are the pros and cons of user-level threads?

Answer 21

Pros:
- No OS support needed
- Lightweight (thread switching)
- Local procedure
- Customized scheduling algorithm
Cons:
- Blocking system
- Page fault
- Do not have access to the system clock interrupt system

Question 22

What are the pros and cons of kernel-level threads?

Answer 22

Pros:
- Threads known to OS, can be scheduled by OS, which improves performance for multiple threads
- Less likely for a single thread to affect others
Cons:
- Slow (context switch overhead)
- Expensive to create and switch

Question 23

What are the different types of CPU scheduling?

Answer 23

1. First-come-first-serve (FCFS)
2. Shortest job first (SJF)
3. Round Robin (RR)

Question 24

What is mutual exclusion and critical regions?

Answer 24

Mutual exclusion: If one process is using a shared variable or file, other processes will be excluded from doing the same thing
Critical region(s): part(s) of a program where the shared resource is accessed.

Question 25

What is busy waiting? What are the pros and cons of busy waiting?

Answer 25

Busy waiting: Continuously testing a variable until some value appears (spin lock)
Pros:
- Avoids expensive context switching when CR is very short
Cons:
- Wastes CPU cycles
- Priority inversion problem

Question 26

What is sleep-and-wakeup? What are the pros and cons of sleep-and-wakeup?

Answer 26

Sleep-and-wakeup: Uses IPC primates (sleep and wakeup) that manage access to a shared variable.
Pros:
- Less CPU-costly
- Avoids priority inversion problem
Cons:
- Requires context switching

Question 27

When should you use busy waiting? When should you use sleep-and-wakeup?

Answer 27

Busy waiting is used for a few processes or when critical region (CR) is very short. Sleep-and-wakeup is used for multi-threaded applications with many processes.
Sleep-and-wakeup used on single-core systems, busy waiting on multi-core systems.