OS

Question 1

What is a process?

Answer 1

It is a program/application in execution

Question 2

What is a thread?

Answer 2

It is the unit of execution within a process. A process has at least one thread.

Question 3

What is a PCB?

Answer 3

PCB stands for a Process Control Block. It is a block of memory which represents a process/task within OS.
Some items in the PCB:
- PID
- Process state
- CPU registers
- Program counter
- Memory limits
- List of opened files
- Kernel stack
- Trap frame

Question 4

What is a context switch in terms of OS?

Answer 4

It is a process of saving a state of currently executing process and restoring state of another process which is chosen to be executed on CPU.

Question 5

What are the some benefits of using threads?

Answer 5

• Increasing responsiveness
• Resource sharing
• No overhead of creating separate processes in memory
• Utilization of a multi-processor architecture

Question 6

What data an OS kernel keeps for each process in the system?

Answer 6

1. Process Control Block of a process
2. Kernel stack for the process
3. Page tables for the process

Question 7

What is CPU burst and I/O burst?

Answer 7

CPU burst is the time of execution of the process on CPU.

I/O burst is the time when a process is waiting for I/O for further execution

Question 8

What is a short-term CPU scheduler?

Answer 8

It is a kernel service which selects a process from the processes in the READY queue and allocates the CPU for that process.

Question 9

What is a dispatcher in the context of CPU scheduling?

Answer 9

Dispatcher is a module that gives a control over CPU to the process selected by the short-term scheduler.

The time it takes for the dispatcher to stop one process and start another process is known as dispatch latency.

Question 10

What are criteria of a CPU scheduling algorithm?

Answer 10

• CPU utilization, percentage of time CPU is busy working
• Throughput, number of processes that are completed per unit of time
• Turnaround time, sum of periods spent waiting to get in the memory, waiting in the ready queue, running on the CPU and doing I/O
• Waiting time, sum of periods spent waiting in the ready queue
• Response time, time from the submission of a process until end of the first execution

Question 11

What are states in a process lifecycle?

Answer 11

• New
• Ready
• Running
• Blocked/Waiting
• Zombie/Terminated

Question 12

What is an interrupt and what its purpose?

Answer 12

An interrupt is a type of event in a computer system produced by a hardware to notify about change in its state or about an error.

Question 13

Classify general events in a computer system?

Answer 13

Event types:

1. Interrupts (also known as hardware interrupts)
2. Traps (software interrupts like printing on the screen)
3. Exceptions, faults (recoverable like page fault), aborts (non-recoverable like division by zero)

Question 14

What happens when there is an interrupt?

Answer 14

1. Hardware device emits interrupt
2. After each instruction of a process CPU checks whether there is an interrupt occurred
3. If CPU detected interrupt then it switches to kernel stack if necessary and switches to privileged mode
4. CPU saves some CPU registers in the kernel stack and suspends execution of a current process
5. CPU finds and executes Interrupt Handler Routine
6. IRH saves rest of the current process states and responds to the interrupt
7. CPU restores user’s task

Question 15

What is a state of CPU?

Answer 15

The state of CPU is a current content of all its registers

Question 16

What is a process kernel stack and why it is needed?

Answer 16

A process kernel stack is a special block of memory in the process memory space which is used during system calls (kernel routines).
It is necessary to have a separate kernel stack for each process in the system so the system call can be resumed if context switch happens during its execution.

Question 17

What is preemptive scheduling of processes?

Answer 17

Preemptive scheduling is a type of scheduling when the system allocates CPU to a process for a limited time and then takes it away to another process.

Examples: Round Robin, Shortest Remaining Time First (SRTF), Priority (preemptive version)

Question 18

What is non-preemptive scheduling of processes?

Answer 18

In non-preemptive (cooperative) systems a process gets executed on CPU until its termination or transitioning to WAIT state due to I/O. In other words, a running process completely owns the CPU and the system doesn’t interrupt its execution.

Examples: Shortest Job First (SJF), Priority (non-preemptive version)

Question 19

When context switch occurs in OS?

Answer 19

• timer interrupt occurs to preempt the currently running process
• a process enters the kernel with a system call and has to wait on some resources
• running process voluntarily yields the CPU

Question 20

What are two types of multi-tasking?

Answer 20

1. Preemptive

2. Cooperative

Question 21

What is virtual memory?

Answer 21

VM is one of the two major abstractions in the modern computer systems which helps to solve several problems of the memory management.

Question 22

What are two major abstractions used in the computer systems?

Answer 22

1. Logical Control Flow. Process execution is interleaved, so each process seems to have an exclusive use of the CPU
2. Virtual Address Space. Address spaces are managed by the virtual memory system and each process gets its own private virtual memory space => each process seems to have an exclusive use of the main memory

Question 23

What problems Virtual Memory solves?

Answer 23

• external fragmentation
• fitting large virtual memory space into physical memory space which is order of magnitude smaller
• managing a memory space of multiple processes
• protecting processes’s memory space
• allowing processes to share common parts of memory

Question 24

What is a Page Table?

Answer 24

PT is an array of page table entries (PTEs) that maps virtual pages to physical frames.