Lecture 3: Processes and threads

Question 1

What is the execution sequence?

Answer 1

1. Fetch instruction at PC and save it in IR (instruction register)
2. Decode instruction in IR
3. Execute (possibly using registers)
4. Write results to registers or memory
5. Update PC
6. Repeat

Question 2

What does this mean:

lw r1, 100

Answer 2

Load the word from 100 into register r1.

Question 3

What does this mean:

add r3, r1, r2

Answer 3

add r1 + r2 and save it in r3

Question 4

What does this mean:

sw r3, 100

Answer 4

store the word that’s in r3 into address 100

Question 5

What is a context (of execution)?

Answer 5

1. The value of the registers (PC, stack pointer, heap pointer, etc.) - from the CPU
2. What’s written in the memory (code, data, stack, heap)
3. Internal data in the OS relevant to the program (user, priority, etc.)

Question 6

When does the context change?

Answer 6

Every time there’s a change in the value of a register or the memory.

Question 7

What is a process?

Answer 7

A process is a dynamic instance of an application execution
(an abstraction of “an individual computer”).

Moving between contexts is the process.

Question 8

Resident process

Answer 8

A process that is currently executing on a CPU

Question 9

Non-resident processes

Answer 9

Process that is not currently running in the CPU. The context is stored in a special location in memory.

Question 10

When does a program become a process?

Answer 10

when the executable file is loaded into the memory (by the loader which is a component of the OS)

Question 11

Every time you run a program it creates a new ____.

Answer 11

Every time you run a program it creates a new process.

Question 12

A program can create ____ processes.

Answer 12

A program can create multiple processes.

Question 13

Address space

Answer 13

The set of accessible addresses in the memory by the process (Given by two parameters: base and bound)

Question 14

Who verifies that a process is accessing memory only in [base, base+bound]?

Answer 14

The CPU

Question 15

When a process specifics address x, what does it actually mean?

Answer 15

base + x

This means that the same pointer address in different processes point to different memory.

Question 16

Kernel memory

Answer 16

The OS’s address space

Question 17

User memory

Answer 17

All the memory that isn’t the kernel’s memory (The OS’s address space)

Question 18

When switching between processes, what needs to be saved?

Answer 18

The processes context (including everything in the CPU)

Everything in the memory does NOT need to be copied. (because no other process will be able to access it, because it is outside of the process’s address space)

Question 19

PCB

Answer 19

Process control block:

A data structure that saves all the data the OS needs about a process.

Question 20

What information is saved in a PCB?

Answer 20

1. Process state (resident, reday, etc. and info for calculating the process’s priority)
2. Process number
3. Registers (including PC)
4. Memory limits - base and bound
5. List of open files

Question 21

What are the states of the process? (Life-cycle)

Answer 21

1. New - being created
2. Running - being executed
3. Waiting (=blocked) for some event (disk, terminal, timer)
4. Preemption - going from running directly back to ready (without first waiting)
5. Terminated

Question 22

What is preemption and when does it occur?

Answer 22

Preemption is when a process is running and then gets kicked off but goes directly to the ready queue (no waiting).
Can only happen in time sharing and not multiprogramming.

Question 23

What causes a process from being in a running state to a ready state?

Answer 23

An interrupt (for example sys call)

Question 24

What causes a process from being in a ready state to a running state?

Answer 24

Scheduler dispatch

Question 25

What causes a process from being in a running state to a waiting state?

Answer 25

I/O or event wait

Question 26

What causes a process from being in a waiting state to a ready state?

Answer 26

I/O or event completion

Question 27

CPU Scheduler

Answer 27

CPU Scheduler: Decide which process should run on the CPU (and sometimes for how long) (“Short-term scheduling”; tens/hundreds times a second)

Question 28

Dispatcher

Answer 28

Dispatcher: The module responsible for executing the CPU scheduler decisions (context switch, switching back to user mode)

Question 29

Schedule vs dispatcher

Answer 29

Scheduler - המוח Dispatcher - הכוח Scheduler decides who should run and for how long. The dispatch is the one who copies the registers to the PCB, and the PCB to the CPU, and changes the mode from user to kernel, (or kernel to user) etc.

Question 30

How does the OS save the PCBs?

Answer 30

As linked lists. There is a linked list for the processes that are in the ready state, terminated state, waiting, etc.

Question 31

When can the PCB be updated?

Answer 31

Only during the context switch when the OS is the resident program in the CPU. During the running of the program, the OS isn't running , so the PCB can't be updated. Only in kernel mode can the data structure of processes be accessed, so we need to wait for the resident program to be the OS.

Question 32

What are the steps in a context switch between processes?

Answer 32

1. Save processor context, including program counter and other registers 2. Update the PCB with the new state and any accounting information 3. Move PCB to appropriate queue - ready, blocked/waiting 4. Select another process for execution 5. Update the PCB of the process selected 6. Update memory-management data structures 7. Restore context of selected process

Question 33

When are processes switched?

Answer 33

1. Interrupts (from the clock or from I/O) 2. Memory fault 3. System call (program purposely gives up the CPU) 4. Exception (error occurred)

Question 34

What is the purpose of the clock's interrupt?

Answer 34

To pass the control back to the OS.

Question 35

When are processes created?

Answer 35

1. Initializing system 2. fork() - a process creates another process 3. User request (e.g., double click on icon, typing a command) 4. initiating batch job.

Question 36

What type of function is "fork()"?

Answer 36

A system call

Question 37

What does the function "fork()" do?

Answer 37

Creates a new process with an identical PCB (except for process ID), allocate memory for the new process and update bounds. The child process returns a 0 and the parent returns the child's ID (nonzero)

Question 38

pid

Answer 38

Process ID

Question 39

When a process is running and there's a sys call, what state does the process transfer to?

Answer 39

Ready

Question 40

Why does the CPU do the calculation of variable+base?

Answer 40

This way if a process is forked, there will be two areas of memory, so the stack pointer will be in relation to the base of the current process.

Question 41

When the fork() functions finishes running, which process will run (the parent or son)?

Answer 41

Depends! The scheduler will decide.

Question 42

Why is it important for the parent process to have the son's ID?

Answer 42

Often time the parent process overseas\controls the son processes, and therefore it needs to know their IDs.