Last Week Logic gates are built out of transistors There are many - PDF document

10/26/2009 Last Week  Logic gates are built out of transistors  There are many different logic gates  NAND is functionally complete  Digital circuits process data using gates  Half and full adder 1

10/26/2009 This week  Software  Operating systems  Kernels  Shells 2

10/26/2009 Programs  Programs tell the computer what to do  Very earliest computers had fixed programs  Program was part of computer  like e.g. a digital calculator  Programming involved physical redesign Stored programs  Since Manchester ―Baby‖ (1948)  Program is data  Program can now be processed by computer  Programmer can easily change it  Computer can change its own programs 3

10/26/2009 Hardware & Software  Hardware: physical parts of the computer  e.g. processor, RAM, mass storage  to improve you replace  Software: programs stored on computer  e.g. operating system, browser,  to improve you can update Firmware  Computer program run on non-CPU hardware  Can be updated by software  Examples  BIOS (Basic Input/Output System) controls bootup of PC  Optical drive (e.g. DVD) controller, can be updated as technology evolves  Halfway between software and hardware 4

10/26/2009 Software  System software  Basic operations of computer  Operating system: manages all activity  Utility software: supports routine tasks ○ e.g. defrag, virus scan, compression, ...  Application software: supports users’ needs  e.g. web browser, word processor 5

10/26/2009 Operating Systems  Software that manages activities and resources of computer  Windows, Linux, OS X, ...  Allows user to start programs  Environment for programs to run  Kernel surrounded by shell Operating systems  What is an operating system?  It is a program that manages all other programs. These other programs or applications make requests from the operating system (ex. Open program).  PC Operating systems ○ DOS ○ Windows 3.1, 95, 98, NT, 2000,ME, XP, Vista ○ Linux  Mac Operating systems ○ OS 8.6, 9.0, 9.1, X.2 , X.4… 6

10/26/2009 Why Study Operating Systems?  Most likely you won't write an OS, so why study them?  Primary intersection point: ○ It draws on many CS areas - software engineering, computer architecture, data structures, networking, algorithms.  Learn programming techniques: ○ One can apply data structures, conflict resolution, concurrency issues, resource management, etc., used in OS to other areas.  "Grungy" things sometimes do require modifications to OS. ○ You can't do this if you don't understand them!  Curiosity – ―look under the hood.‖ What is an Operating System?  An Operating System (OS) is a program that controls the execution of computer programs and act as the interface between the user and the hardware.  An OS functions as:  A resource manager ○ It manages and allocates resources such that the computer hardware can be used in an efficient manner. ○ Examples of resources: processes, CPU(s), memory, file system, networking, etc.  A virtual machine: ○ OS hides the details of the underlying hardware ○ OS provides a common API to applications and services.  An OS makes the machine convenient and efficient to use. 7

10/26/2009 History of Operating Systems.  Once upon a time …  Human Computer History of Operating Systems (cont)  Single program execution  First generation  Why? 1945 - 1955  Hardwire “programming”  vacuum  Programming slow, not “offline”! tubes, plug Plug board and punch cards.  boards No programming language.   Programmer spent quite a lot  ENIAC time to find the real “Bug”. (mid- 1940’s)  Just part of it!! 8

10/26/2009 History of Operating Systems (cont)  Second generation 1955 - 1965  transistors, batch systems.  Early batch system (overlapped CPU & I/O operations)  Buffer slow I/O onto fast tape drives connected to CPU, replicate I/O devices.  Spool data to disk.  Programming languages: Fortran or assembler (on punch cards!).   Two main applications: Scientific.  Data processing.  History of Operating Systems (cont)  Multi-programming  Third generation systems: 1965 – 1980  Run several programs  IBM 360 series (mid- at the same time. 1960s)  Spooling as jobs finished.  New problems:  Response time.  Thrashing.  File-systems. 9

10/26/2009 History of Operating Systems (cont)  Interactive timesharing systems:  Lots of cheap terminals and one computer ○ All users interact with the system at once ○ Debugging is much easier  Disks are cheap so put programs and data online ○ 1 punch card = 100 bytes  New problems: ○ Need pre-emptive scheduling to maintain adequate response time ○ Need to avoid thrashing (swapping programs in and out of memory too often) ○ Need to provide adequate security measures  UNIX developed at Bell Labs (Thompson, Ritchie) History of Operating Systems (cont)  Fourth generation 1980 – present  Personal computing  CPUs are cheap enough for everyone, yet powerful enough to be useful. There can be only one…IBM PC The only real competitor left… vs. IBM 8086 SUPERBOWL 10

10/26/2009 OS’s: The Bad News...  Modern operating system are:  Enormous: ○ Linux v2.6 – approx. 5.9 million lines of code (128MB RAM) ○ Win2000 - approx. 35 million lines of code (64MB RAM) ○ WinXP – approx. 40 million lines of code (128MB RAM) ○ Win2003 – approx 50 million lines of code (256MB RAM)  Complex: ○ Poorly understood – too large for one person to comprehend.  (Always) full of bugs and (often) unreliable.  Dependent on hardware in strange ways (makes porting difficult). Commercial-Released O/S A commercial-release operating systems is any operating system which has all of the following attributes:  It costs money, typically more than $50  Source code for the system is not available  There are strict limits as to how the system may be copied.  Consequently, Windows, MacOS, DEC Ultrix, Solaris, OS/2, and other similar platforms are commercial operating systems. 11

10/26/2009 The problems  The needs of the computer user have to be balanced with the companies need for profit.  Here is some of the issues:  Slow release pattern  High cost  Tech Support?  Lack of source code availability  Planned obsolescence of hardware  Crash-prone 12

10/26/2009 Windows  Hybrid kernel (NT)  System libraries: C:\windows\system32\  Shells: cmd.exe (CL) and explorer.exe (GUI)  System tools: cmd.exe 13

10/26/2009 The Unix Operating System  The UNIX operating system was designed to let a number of programmers access the computer at the same time and share its resources.  Bell Laboratories created the UNIX OS in 1969.  There goals were to design an operating system to satisfy the following objectives:  Simple and elegant  Written in a high level language rather than assembly language (It is written in C)  Allow re-use of code The Unix Structure  Structure: Kernel and shell  kernel (in assembly language, small)  shell (in C).  The benefit of this structure is that UNIX is highly customizable and new features are relatively simple to add . 14

10/26/2009 What is UNIX? 29 The Unix Kernel  This is the heart of the UNIX Operating System.  The kernel is at the core of each UNIX system and is loaded in whenever the system is started up  The kernel creates the same virtual machine 30 15

10/26/2009 The Unix Kernel  It performs the tasks that create and maintain the UNIX environment managing the entire resources of the system. It: ◦ Manages the machine's memory ◦ Schedules the work done by the CPU ◦ Organizes the transfer of data from one part of the machine to another ◦ Accepts instructions from the shell and carries them out ◦ Enforces access permissions ◦ Keeps track of the disks, tapes, printers, terminals, communication lines etc attached to the computer.  You do not need to know anything about the kernel in order to use a UNIX system 31 The Kernel  Core of the operating system  Coordinates activity of CPU, memory & I/O  Provides basic services to other software  Open a file  Create a directory  Connect to network host  ... 16

10/26/2009 Parts of the Kernel  File manager  Where files are stored, and free space  Which users allowed access  Device drivers  Communicate with attached I/O devices  Memory manager  Assign RAM space to individual tasks  Paging: when RAM is full it can overflow onto hard disk Kernels In Action  Optimised for different applications  Space: embedded devices (PalmOS)  Reliability: no-access environments (VxWorks)  Real-time response: guidance systems (QNX)  Scalability: mainframes (z/OS) 17

10/26/2009 Kernel Design  Monolithic: a single kernel does all the work  Microkernel: a simple kernel delegates services to many servers  Hybrid: core services in kernel + some servers The Shell  Interface between user and kernel  Command-line shell  GUI (Graphical User Interface) shell 18