ICS 335: Operating Systems

ICS 335: Operating Systems

Sebnem Baydere

Lecture 1:Introduction

Main Points

What operating systems do ?

Historical Perspective

1.0  Why study O/S?

·  Understand how computers work

·  Abstraction: Providing illusion of infinite resources (CPU, memory etc)

·  System Design: Tradeoffs between performance and simplicity, division of labor between HW and WS

·  O/S combines several areas in Computer Science discipline: languages, hardware, data structures, algorithms

2.0  Why build a real O/S kernel?

·  Hear and forget, see and remember, do and understand

·  Gain the confidence : difference between a computer scientist and a programmer

3.0  What is an O/S?

·  Virtual machine

·  Extension of hardware

·  Resource allocation : fair division of resources between users of the system

·  Coordinator: between applications and hardware resources (concurrency, device drivers, memory, file system, networks

·  Standard services (utility programs- libraries, window system

·  What would be without O/S?

4.0  History of O/S: change !!

Design tradeoffs change as technology changes

Typical academic computer in 1981 and 1998

What does this mean?

Techniques have to vary over time, adapt to changes in the hardware, adapt to changing tradeoffs.

4.1 Phase 1: Hardware expensive, human cheap

When computers cost millions of $s, optimize for more efficient use of the hardware!

1.  User at a console: one user at a time. O/S as a subroutine library

2.  Batch Monitor: load program, run, print (no memory protection)

3.  Interrupt Driven architecture-DMA: Overlapping I/O and CPU (computation), OS requests I/O, goes back to computing, gets an interrupt when I/O device has finished.

4.  Memory protection + relocation: Multiprogramming, several programs run at the same time; users share the system.

Benefits:

1.  Small jobs not delayed by large jobs

2.  More overlap between I/O and CPU

Multiprogramming requires memory protection to keep bugs in one

program from crashing the system or corrupting other programs.

O/s manages these and gets complicated. First: MULTICS announced in 1963 ran in 1969. (UNIX based on Multics)

OS 360 released with 1000 bugs.

4.2 Phase 2: Harware cheap, human expensive

5.  Interactive Timesharing: Use cheap terminals to let multiple users interact with the system at the same time. Sacrifice CPU time to get better response time for users.

Problem: thrashing -- performance fall off as you add users.

4.3 Phase 3: Hardware cheaper, human more expensive

6.  Graphic Interfaces: Window systems,bitmap displays, Ethernet, internet, desktop publishing

7.  Personal Computing: initially library, later with memory. Protection and multiprogramming added.

4.4  Phase 4: Distributed systems

8. Network computing : Network is the computer?? Allow different machines to share resources.

5.0 Summary

Point of change is not: How old fashioned batch processing is. It was right for the tradeoffs of the time.

Point is: have to change with changing technology.

Today: situation is more like it was in late 60's: O/Ss today are large and complex:

small O/S -- 100K lines

big O/S -- 10M lines

10-1000 people-years for development

NT is under development for 10 years, still has problems

KEY ASPECT OF THE COURSE -- understand O/Ss so that we can simplify them