C191 (Operating Systems for Programmers) Unit 1

Operating System (OS)

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

Operating System Goals

-Execute user programs and makes solving user problems easier
-Makes the computer system convenient to use
-Use the computer hardware in an efficient manner

Computer System

Hardware <--> OS <--> Applications <--> Users

What exactly is an Operating System (OS)

-Resource Allocator: decides between conflicting requests for efficient and fair resource use
-Control Program: Controls execution of programs to prevent errors and improper use of a computer

Resource Allocator

decides between conflicting requests for efficient and fair resource use

Control Program

Controls execution of programs to prevent errors and improper use of a computer

Kernel

The one program running at all times on the computer

Bootstrap Program

loaded at power-up or reboot

What is true about I/O and CPU?

They can execute concurrently.

How can an Interrupt occur?

By the device controller informing the CPU that it's finished with an operation

An interrupt transfers control to?

It transfers control to the interrupt service generally though the interrupt vector, which contains the addresses of all service routines

How interrupts are handled?

Incoming interrupts are disabled while another interrupt is being processed.

Trap

software-generated interrupt caused either by an error or a user request

System Call

Request to the operating system 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

How device status table and OS work together?

OS indexes the I/O device table to determine device status and to modify the table entry to include interrupt

Storage Structure

-main memory
-secondary storage
- disk

Main Memory

RAM (Random Access Memory), volatile

Secondary Memory

SRAM (Secondary Random Access Memory), extension of main memory. provides large non volatile storage

Disk

Divided into tracks which are subdivided into sectors. Disk controller determines logical interaction between the device and computer

Caching

Copying information into faster storage systems

Multiprocessor Systems

-Increased throughput, economy of scale, increased reliability
-Can be asymmetric or symmetric
-Clustered Systems

Clustered Systems

Linked multiprocessor systems

Multiprogramming

Provides efficiency via job scheduling

Timesharing

CPU switches jobs so frequently that each user can interact with each job while it is running (interactive computing)

Dual Mode

Operation allows OS to protect itself and other system components
-user mode
-kernel mode

Privileged

Some instructions only being executable in kernel mode

Single threaded vs multi threaded processes

-Single Threaded : has one program counter
-Multi Threaded : has one Program counter per thread

Protection

Mechanism for controlling access of processes or users to resources defined by the OS

Security

Defense of a system against attacks

User IDs

One per user, and Group IDs, determine which users and groups of users have which privileges