CSE 2431 Midterm Review

What Operating Systems Do

Controls and coordinates use of hardware among various applications and users.

User view

In this view the OS is made for ease of use

System view

 In this view the OS is a resource allocator

Operating Systems views

user and system

OS definition

no real adequate definition, no universally accepted definition 

\n  Loosely – “everything a vendor ships when you order the ‘operating system’” 

kernel

OS is the one programming running all the time

Computers are

organized into one or more CPUs and device controllers connected through a common bus that provides access between components and shared memory.

Interrupt

hardware mechanism that enables a device to notify the CPU that it needs attention 

Interrupt timeline

I/O changes after I/O request and at transfer done

\
CPU changes at interrupt signaled and interrupt handled

Storage

RAM (Random Access Memory \[volatile typically\]), HDD (Hard Disk Drive), NVM (Non-volatile Memory \[faster than HDD\]), tertiary storage (optical storage, magnetic tape)

Von Neumann architecture

structure of computer where process instructions and data are stored in the same main memory 

 I/O Structure

CPU/memory/device (including DMA) 

System call

request to the OS to allow user to wait for I/O completion

Device-status table

contains entry for each I/O device indicating its type, address, and state

Single processor

There is one main CPU capable of executing a general purpose instruction set.

Multiprogramming and multitasking

o Multiprogram - increases CPU utilization by organizing jobs so that the CPU always has one to execute.

\
o  Multitasking - CPU executes multiple jobs by switching between them but the users can interact with each program while it is running. 

Dual mode and multi-mode 

o Dual - Allows OS to protect itself and other system components

\
o Multi - A system that can support systems/service that operate within the physical data center and virtual/cloud/software-defined environment. 

 Process vs program (active vs passive)

o Process - Active entity (A program in execution)

o Program - Passive entity (does nothing unless instructions are executed by CPU)

 Protection

any mechanism for controlling the access of processes or users to the resources defined by a computer system. 

 Security

 defend a system from external and internal attacks.

Virtualization, emulation, guest operating systems

Makes it possible to run many OS on top of one core system.

Peer-to-peer and cloud computing

o Peer to Peer - Client and server is not distinguished from one another (all nodes are considered peers and can act as client or server)

\
o Cloud - Delivers computing, storage, even apps as a service across a network. 

Operating System Services

o Layered approach - A modular approach in which an operating system is broken into a number of layers (levels). The bottom layer (layer 0) is the hardware; the highest (layer N) is the user interface.

 System calls and APIs

System call provide an interface to the services made available by an operating system.

System calls and API examples

Process control, device management, file management, info maintenance, communication, protection

Process Control

The operating systems mechanism for directing the control flow of different processes. 

Device management

The operating systems mechanism for managing user’s access to different devices within a computer, whether they’re hardware or abstract (files).

File management

The operating systems mechanism for creating, deleting, opening, closing, reading from, and writing to files

Information maintenance

The operating systems mechanism for transferring information (i.e. date, debug info) between the user program and the operating system.

Communication

The way in which different processes communicate

Two ways to communicate

Message passing model and shared memory model

Message-Passing Model

* communicating processes exchange messages with one another to transfer information

Shared-Memory Model

processes use shared memory create and shared memory attach system calls to create and gain access to regions of memory owned by other processes

Protection

Provides a mechanism for controlling access to the resources provided by a computer system

System Services/System Utilities

Provide a convenient environment for program development and execution.

System services/system utility examples

file management, status info, file modification, program-language support, program load and execution, and communications

File management

Programs that manage files (create, destroy, etc.)

Status information

 Programs that ask the system for different kinds of info

File modification

Programs, such as text editors, that allow you to edit file contents

Programming-language support

Compilers, assemblers, debuggers, interpreters

Communications

Programs that provide interprocess and system communications

Relocatable object file

*  source code compiled into object files designed to be loaded into any physical memory location

Linker

Combines relocatable object files into a single binary executable file.

Loader

* Brings executable files into memory to be executed.

Relocation

* Assigns final addresses to program parts and adjusts code and data in program to match those addresses.

OS design goals

User vs System Goals AND Policy vs mechanisms

User goals

Operating system should be convenient to use, easy to learn, reliable, safe, and fast

Systems goals

Operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

Policy

*what* will be done

mechanisms

determines *how* to do it.

OS structures

Monolithic, Layered, Microkernels, modules, hybrid, mobile

Monolithic

The kernel takes a massive amount of functionality as its responsibility

Layered

Operating system is divided into layers, each built on top of the lower layers. Lower layers service the upper layers.

Microkernels

Removed all non-essential components from the kernel and implements them as system and user-level programs.

Modules

The best modern implementation for operating systems. The kernel has a set of core components and links in additional services via loadable modules.

Hybrid

Most modern operating systems are not usually 1 pure model. Combines multiple of the aforementioned approaches to address performance, security, and usability.

Mobile

Android and iOS

Android

*  Structured on modified Linux kernel.

iOS

* structured on Mac OS X with added functionality.

Booting an OS

* When power initialized on system, execution starts at fixed memory location
* OS must be made available to hardware so hardware can start it
* Bootstrap loader, BIOS, stored in ROM locates kernel, loads it into memory, and starts it
* Modern systems replace BIOS with Unified Extensible Firmware Interface (UEFI)
* Common bootstrap loader, GRUB, allows selection of kernel from multiple disks and versions.
* When kernel loads, system is running

Debugging

Finding and fixing errors or bugs

Failures and performance issues 

* OS generates log files containing error info
* Application failure can generate core dump file capturing memory of failed process
* OS failure can generate crash dump file containing kernel memory
* Performance tuning optimizes system performance
* Tracing - Collects data for a specific event, such as steps involved in a system call invocation

 Concept of what a Process is

Programs being executed

 Components within a process

* Text section, program counter, stack, data section, and heap

Process states 

* New - Process being created
* Running - Instructions are being executed
* Waiting - The process is waiting for an event to occur
* Ready - Process is waiting to be assigned to processor
* Terminated - Process has finished execution

Process control block 

Components are Process State, Program Counter, CPU register, CPU scheduling info, memory management info, accounting info, and I/O status info.

Threads

Multiple threads of control → threads

Process Scheduling

selects among available processes for next execution on CPU core

\n

Queues

Ready, Wait, Job

Ready Queue

* set of all processes residing in main memory, ready and waiting to execute

Wait Queue

 set of processes waiting for an event

Job Queue

consists of all processes in a system

Context switching

When the CPU switches from one process to another, the system must save the state of the old process and load the saved state for the new process via a context switch.

Context switch

 when CPU switches from one process to another

Operations on processes

create (fork, exec) and termination

Create (fork, exec) 

* fork() creates new process
* exec() used after fork() to replace the process’ memory space with a new program. 
* Parent process calls wait() for the child to terminate

termination

* Process executes the last statement and then asks the operating system to delete it using exit(). 
* Parents may terminate the execution of children processes using abort().

 Independent processes 

* It can’t affect or be affected by other processes executing in the system.

cooperating processes

* It can affect or be affected by other processes executing in the system. 

Shared memory

* region of memory that is shared by cooperating processes.

Non-Shared memory

communication takes place by means of messages exchanged between the cooperating processes.

IPC synchronization

Communication between processes takes place through calls to send() and receive() primitives.

Shared memory (i.e., POSIX shm_open, mmap, ftruncate)

an area of memory shared among the processes that wish to communicate. Communication controlled by user processes.

Message passing

mechanism for processes to communicate and synchronize their actions. Can be blocking or non-blocking. 

\n

Blocking

considered synchronous

Non Blocking

considered asynchronous

Blocking send

*  sender is blocked until message is received

Blocking receive

receiver is blocked until message is available

Nonblocking send

* sender sends message and continue

Non Blocking receive

* receiver receives null or valid message

Pipes

Acts a conduit allowing two processes to communicate

Named Pipes

 can be accessed without a parent-child relationship

Ordinary Pipes

* can’t be accessed from outside the process that created it. 

Sockets

An endpoint for communication (concatenation of IP address \[host\] and port)

\n Ex: 161.25.19.8:1625

 Remote Procedure Calls (RPC)

abstracts procedure calls between processes on networked systems. 

\n

What is a thread and why we have it

A thread is a basic unit of CPU utilization. Allows for a process to create multiple threads to handle different tasks, rather than making more processes and incurring overhead.

\n

Benefits of Thread

Responsiveness, resource sharing, economy, scalability

Responsiveness

* May allow continued execution if part of process is blocked, especially important for user interfaces

resource sharing

Threads share resources of processes they belong to, easier than shared memory or message passing between processes.

economy

Cheaper than process creation, thread switching lower overhead than context switching