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Operating Systems

Lecture 03:

Operating System Overview

Today

• Announcements:

• Proposal: change in TAs schedule
• Piazza
• Slides posted before class on course page:

• Operating Systems Overview:

• Definitions (as many variants as textbooks…)
• Historical perspective

• Solution today’s “jeopardy”

• Grade yourself and hand it to me

Operating System

• An interface between user applications and

hardware

• A service provider
• A program that manages computer resources

The Operating System as a

User/Computer Interface

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The Operating System as a

Service Provider

• Program development (not really part of OS)
• Program execution
• Access I/O devices
• Controlled access to files
• System access
• Error detection and response
• Accounting

Operating System

as Software

• Functions in the same way as ordinary

computer software

• Program, or suite of programs, executed by

the processor

• Frequently relinquishes control and must

depend on the processor to allow it to regain control

Evolution of Operating Systems

 A major OS will evolve over time for a

number of reasons:

Hardware upgrades

New types of hardware

New services

Fixes

In the Beginning…

 In the beginning: Expensive Hardware, Cheap People.

 Goal: maximize hardware utilization.

 (Now: Cheap Hardware, Expensive People.)

 Goal: make it easy for people to use computer.

1940s to mid 1950s:

 computers were huge machines, expensive to buy, run and

maintain

 used in single user, interactive mode

 programmers interact with the machine at a very low

 no OS.

The Growth of the OS (2)

• Problem: Scheduling:
  • Setup time: a considerable amount of time was spent just on setting up the program to run
  • Hardcopy sign-up sheet to reserve computer time - Solution: - Hire specialized human - Faster, but still human - Build a batch monitor - Store jobs on disk - computer loads them into memory and executes them - (one at a time) - Huge improvement! - Debugging offline from printouts and memory dumps

Resident Batch Monitor

• Monitor controls the sequence of events
  • user no longer has direct access to processor
  • job is submitted to computer operator who batches them together and places them on an input device
  • program branches back to the monitor when finished
• Resident Monitor is software always in memory
• Monitor reads in job and gives control
• Job returns control to monitor

Job Control Language (JCL)

Special type of programming language used to provide instructions to the monitor

what compiler to use

what data to use

Desirable Hardware

Features

• while the user program is executing, it must not alter the memory area containing the monitor

Memory protection for monitor

• prevents a job from monopolizing the system

Timer

• can only be executed by the monitor

Privileged instructions

• gives OS more flexibility in controlling user programs

Interrupts

Simple Batch System Overhead

• Processor time alternates between execution of user programs and execution of the monitor
• Sacrifices:
  • some main memory is now given over to the monitor
  • some processor time is consumed by the monitor
• Despite overhead, the simple batch system improves utilization of the computer

The Growth of the OS (3)

• Problem: at any given

time, the job is either using the OS or I/O (resource underutilization)

• Problem: one job

cannot keep both CPU and I/O busy

• Solution: allow the job to overlap I/O and computation - Buffering and interrupt handling added to the subroutine library
• Solution: multiprogramming