Operating Systems Exam 1

Operating system

A program that acts as an intermediary between a user of a computer and the computer hardware

Roles of the operating system

• Resource allocator

• Control program: controls executions of programs to prevent errors and improper use

• Protection and security

Non-virtual machine vs. virtual machine

• In a non-virtual machine, the operating system communicates normally between the hardware and the processes

• In a virtual machine, the operating system communicates with the hardware through a virtual machine manager

Process

A program in execution

Address space

Contains all of the memory state of the running program

Text section

Program code, located at the bottom of the stack (address space)

Data section

Contains all the global variables, located at the second to last block of the stack

Heap

Contains memory dynamically allocated during runtime, located above the data in the address space

Stack

Contains temporary data, such as function parameters, return addresses, and local variables

New state

The process is being created

Running state

Instructions are being executed

Waiting state

The process is waiting for some event to occur, such as input/output

Ready state

The process is waiting to be assigned to a processor

Terminated state

The process has finished execution

Process control block

Information associated with each process:

• Process state

• Process number

• Program counter

• Registers

• Memory limits

• List of open files

Context switch

When a process switches to another process on the CPU mid-execution. The CPU will attempt to save as much data from the process as possible before ending one process and load the data of the next one about to start

Thread

Short for thread of control/execution

Data parallelism

Distributes subsets of the same data across multiple cores, same operation on each

Task parallelism

Distributing threads across cores, each performing a unique operation

User threads

Management done by user-level threads, created by the user

Kernel threads

Threads supported/created by the kernel

Thread control block

A data structure in an operating system kernel that contains thread-specific information needed to manage the thread

Semaphore

An integer value that represents the number of resources available

Semaphore with Queue

When a semaphore has a queue, process waiting to acquire the resource but cannot because it is in use by another process will enter the semaphore queue. When the thread obtains the resource, it enters the ready queue after waking up.

Atomic operation

An operation that can only occur completely from start to finish. If the process cannot be completed fully, it will not be ran at all

wait / P()

Used to obtain a semaphore resource

Signal / V()

Releases a semaphore after use

Deadlock

A situation where no further process can occur. Happens when all 4 of its conditions are true.

Mutual exclusion

Only one process at a time can use a resource

Hold and wait

A process holding at least one resource is waiting to acquire an additional resource held by other processes

No preemption

A resource can be released only voluntarily by the process holding it, after that process has completed its task

Circular wait

A process is waiting for a resource that is being used by the process in front of it and so on

Domain

Set of access rights

Access rights

A subset of all valid operations that can be performed on the object, such as read, write, and execute

Domain switching

Can be performed by changing file system, using passwords, or through commands

Global table

Used to store ordered triples that contain permissions in a table, but could be too large to fit into main memory

Access control lists

Each column is implemented as an access list for one object

Capability list

Each row of the table can be kept as a list of the capabilities of that domain

Lock key

• Each object has a list of unique bit patterns, called locks

• Each domain has a list of unique bit patterns, called keys

• Process in a domain can only access object if domain has key that matches one of the locks