OS midterm

Question 1

4 sources of processor interrupts?

Answer 1

Device Interrupts: signal from controller that says it needs attention
I/O Events: arrival of new info
Timer
Program Errors

Question 2

Where is the state of an interrupted process saved?

Answer 2

Pushed onto Control Stack

Question 3

What is locality of reference?

Answer 3

Programs tend to remain in certain regions before moving to new ones

Question 4

What are the two examples of locality of reference discussed in class?

Answer 4

If a program executes instruction i, it is likely to execute i+1 next
Programs may iterate over a block of data in a loop, which uses the program and the data mostly

Question 5

How does locality of reference make cache memory effective

Answer 5

If the processor hardware can move the referenced blocks of memory from main
memory into cache memory when the block is first accessed, instructions and/or
data will be found in cache (hit) for all the remaining access while in a region and
the overall execution of the program / process will be that much faster.

Question 6

What are the restrictions on User Mode compared to Kernel Mode?

Answer 6

Can only access allotted memory
Can’t run privileged instructions

Question 7

What causes the OS to switch into Kernel mode from User Mode?

Answer 7

Any interrupt

Question 8

What is the name of the mechanism to request resources or a service from the OS?

Answer 8

Syscall

Question 9

What are the four steps involved in making a syscall?

Answer 9

1. Place the Syscall number and arguments in specified registers
2. Send a software interrupt. This saves the current processes’ state on the stack
3. Interrupt handler looks at the registers and executes the request
4. When finished, restore state using IRET instruction and return to user mode

Question 10

What are the four innovations of computer systems design that make modern OS possible? Name and describe

Answer 10

1. Protected memory: Kernel operates in a region of memory that can only be accessed in kernel mode
2. Privileged Instructions: instructions that can only be executed by the processor when it is in kernel mode. 1 and 2 are made to prevent corruption of the OS while running (or stealing protected data)
3. Interrupts: Way to interrupt/pause currently running program to handle asynchronous events
4. Hardware Timers: periodic interrupts

Question 11

What info is maintained in the process image?

Answer 11

The process image is the region of memory (disk or main memory) containing
the instructions / data / stack of the program being executed.

Question 12

In what 2 places is the process image stored?

Answer 12

System Drive (as a file) and Main Memory

Question 13

What are the 5 steps to create a process image?

Answer 13

1. Allocate space for the new image on the system (swap) drive. Copy the text and data segments from the executable file to the new image
2. Create PCB and other resources needed for process execution
3. Assign unique PID to process
4. Copy image from the system drive into main memory
5. Put the process in the ready state and put it into the Ready queue

Question 14

Purpose of the PCB?

Answer 14

Process Control Block: used to maintain info needed to maintain a single process. Maintained by the kernel

Question 15

What are the 3 categories of info stored in the PCB? Name and explain.

Answer 15

Process Identification: Information to identify the process. PID, PPID, Owning user ID, etc.
Process Control:
Processor state: State of the processor needed to allow the process to be context switched. Aka the state to be restored when process resumes

Question 16

Differences between the PCB in a single-threaded process vs. the PCB/Thread Control Block in a multithreaded process

Answer 16

Since a STP only has one thread, there is very little distinction between the thread control and the process control. In a MTP, though, these are separate because while the process only has one PCB, it may have many thread control blocks.

Question 17

What are the differences between how the user-mode stacks are maintained in a STP vs. an MTP

Answer 17

There is one user-mode stack per thread, so an STP only has one but a MTP has one per thread

Question 18

What are the differences between how processor state information is maintained in a STP vs. an MTP

Answer 18

This is stored in the Thread Control Block. So in a STP this is in the PCB and in a MTP this is in the Thread Control Block

Question 19

What are the differences between how OS resources are managed in a STP vs. an MTP

Answer 19

No differences, both are maintained at the process level

Question 20

3 advantages of threads over processes

Answer 20

1. Threads can be created/destroyed quicker
2. N threads require less memory than N processes
3. Efficient communication between threads through memory owned by the process

Question 21

3 Multithreading control strategies?

Answer 21

Worker Threads: Many threads take items from a FIFO and work them independently
Task Scheduling: Task(s) are periodically run. On a loop, run, then wait.
Event Handling: Events are delivered to system from many sources. Event handlers for each source can be in their own thread. Example is socket.read which is blocking

Question 22

What are the 3 advantages semaphores offer over compare and swap instructions

Answer 22

1. Semaphores cause the blocking of waiting processes / threads eliminating the
   busy-wait loop utilized by C&S instructions.
2. Semaphores offer the fair scheduling of blocked processes using FIFO queue
   of processes / treads waiting on the semaphore i.e. no starvation of waiting
   processes / threads.
3. With C&S, if the process in the critical section dies before setting the shared
   variable to zero, the waiting processes will never unblock. Because a
   Semaphore is managed by the OS, if the owning process dies while in a CS,
   the OS will release the terminated process’s ownership on any semaphores
   allowing waiting blocked processes to continue.

tldr:
1. no busy-wait loop spinning
2. fair scheduling of blocked processes
3. waiting processes will unblock if owning process dies

Question 23

How do semaphores use a FIFO queue

Answer 23

They use it to keep track of the blocked threads waiting for it. The first one to request will be the first one to get it