Process

Question 1

What is an OS’s task?

Answer 1

1) Abstraction: hide hardware details from applications/programmers/users
2) Resource Management: resources must be multiplexed in a fair way between applications/users
3) Protection: Different applications shouldn’t be able to disturb/manipulate each other
4) PROVIDING AN EXECUTION ENVIRONMENT FOR APPLICATIONS

Question 2

Why is abstraction a central task of an OS

Answer 2

• to hide implementation details of the hardware from the applications
• make it easier to develop applications
• increase application compatibility
• improve reliability by taking away complexity

Question 3

Why is protection still as important although most devices run an OS used by a single user only?

Answer 3

• protective measures such as the separation into user and kernel modes increases the reliability of the system
• a buggy application might crash the whole system, leading to data loss
• to protect user’s data from untrusted sources in today’s interconnectivity of the computers

Question 4

What is the advantage of fixed-size integer types like uint32_t?

Answer 4

The normal int types have different sizes depending on the OS. The fixed-size types help writing portable code; code that works on multiple different systems without modification

Question 5

Explain all functions of * and & operators

Answer 5

• *: in front of a variable name declares that variable to be a pointer: *pointer1
• *: in front of a pointer variable dereferences the pointer: return *pointer1
  -&: in front of a variable returns the address of that variable, which is a pointer: pointer2 = &notapointer

Question 6

User mode

Answer 6

-only non-privileged processor instructions can be executed

Question 7

kernel mode

Answer 7

• allow the execution of all instructions
• some activities that control & manage the system as a whole may only be performed in kernel mode
• it is needed to avoid other applications to use the system according to their own needs while negatively affecting other applications; by occupying the system resources as long as they need

Question 8

What are privileged instructions and give examples

Answer 8

Instructions that
- manipulate control registers for memory translation
- disable or enable interrupts
- access platform devices
+Load and store control registers
+Halt processor
+Write model-specific registers
+Read time-stamp counter

Question 9

Basic data types in C

Answer 9

-char: 1B
-short: 2B
-int: 4B
-long long: 8B
-float: 4B
-double: 8B

Question 10

Global variables

Answer 10

• live while the program runs
• placed in the data segment

Question 11

Local variables

Answer 11

• accessible and valid only in the scope of the block/function in which they are declared
• placed on the stack or in CPU registers

Question 12

What is a process?

Answer 12

A process is a program in execution - an instance of a program
Each process is associated with a OS data structure called process control block (PCB) that holds all state information

Question 13

Concurrency vs parallelism

Answer 13

Concurrency: Multiple processes on the same CPU
Parallelism: Processes truly running at the same time with multiple CPUs

Question 14

What is direct execution and its problems?

Answer 14

Direct execution: Allow user process to run directly on hardware, OS creates process and transfers control to starting point
Problems:
- Process can harm each other
- Could read/write other process data
- Process could run forever
- Process could do something small

Question 15

80:20 rule

Answer 15

Programs can see more memory than available:
80% of process memory idle, 20% active working set

Question 16

What are three kinds of data

Answer 16

1. Fixed size data items
2. Data that is naturally free’d in reverse order of allocation
3. Data that is allocated and free’d dynamically at random

Question 17

Fixed-size data and code segments

Answer 17

1. BSS: variables that haven’t been initialized, the executable file typically contains the starting address and size of BSS, the entire segment is initially zero
2. Data segment: initialized data elements
3. Read-only data segment: constant numbers and segments

Question 18

Typical process address space layout

Answer 18

OS: Addresses where the kernel is mapped
Stack: Local Variables, function call parameters, return addresses
->

<-
Heap: Dynamically allocated data (malloc)
BSS: uninitialized data
Data: initialized data
RO-Data: Read-only data, strings
Text: Program, machine code

Question 19

Processes vs threads

Answer 19

thread -> lightweight process
an execution stream that shares an address space
multiple threads within a single process
(multiple stacks within the same address space)

Question 20

What are three occasions that invoke the kernel and switch to kernel mode?

Answer 20

-Syscalls: user mode process requires higher privileges
- interrupts: external device sends a signal to the CPU
- exceptions: CPU realizes a notable condition

Question 21

types of syscalls

Answer 21

• process control
• memory management
• file management
• device management
• communication
• information maintenance
• system management

Question 22

Trap instruction

Answer 22

The trap instruction switches the CPU to kernel mode and enters the kernel in the same predefined way for every syscall.
It is a procedure call that synchronously transfers the control

Question 23

System Call Handler

Answer 23

1. Saves registers
2. Reads parameters that were passed by the caller
3. Checks parameters
4. Checks if the process has permission to perform the requested action
5. Performs the requested service on behalf of the process
6. Returns to the caller with a success or error code

Question 24

What are possible events that cause processes to be created?

Answer 24

1. System initialization
2. Process creation syscall issued by a running process
3. User request to create a new process
4. Initiation of a batch job

Question 25

What test does the kernel perform when receiving the address of a buffer as a syscall parameter?

Answer 25

The kernel ensures that the address lies within the user address range. That way, the user can't fool the kernel into overwriting its own data structures or leaking critical system private information to the user process.

Question 26

Program vs process

Answer 26

Program: Passive entity, just a file on disk Process: Active entity, has an execution context

Question 27

What is a zombie and how to prevent it

Answer 27

When a child process terminates, the parent may want to know the child's exit status. To make this possible a stub of the child will stay in the process table after termination as a zombie. The parent needs to collect the exit status of the terminated processes with the wait() or waitpid() syscalls to free the zombies.

Question 28

What is the context of a process? Name its properties.

Answer 28

The context is made up of properties that identify the current runtime state of a process. - General purpose registers, Instruction Pointer, Stack pointer - Process state - Process priority - Unique process ID - Open files - Network connections - Security credentials

Question 29

Single- vs multiprogramming

Answer 29

Single: allows only one process to run at a time, no overlapping on the CPU so the CPU would be idle up to 50% of the time Multi: enables n > 1 processes to execute concurrently, sharing the physical resources

Question 30

CPU-bound and I/O-bound processes

Answer 30

CPU: rarely invokes I/O-operations, not very likely to block I/O: perform only short computations between I/O jobs, expected to block often

Question 31

Kernel entries can be classified as voluntary/involuntary and synchronous/asynchronous. Associate the kernel entries.

Answer 31

1. Interrupts: involuntary & asynchronous: the interrupted thread didn't call for the interrupt or didn't specify when the interrupt should occur; the activity that raised the interrupt executes independently of the current thread 2. Exceptions: involuntary & synchronous: the application caused the event while executing, a specific instruction triggered the kernel entry synchronously with the program's execution; the kernel entry is a side-effect rather than a desired action 3. Syscalls: voluntary & synchronous: the kernel entry is caused by the execution of a specific instruction; the whole purpose of these instructions is to enter the kernel

Question 32

How to pass parameters to the kernel when performing a syscall?

Answer 32

-registers -stack -main memory + location in register

Question 33

Process states

Answer 33

-new: it has been created but never run -ready: it is waiting to be assigned to a processor -running: instructions are currently being executed -waiting: it is waiting for some event to occur -terminated: it has finished execution (zombie state)