10/26/2009. Last Week

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1 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

2 This week Software Operating systems Kernels Shells 2

3 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

4 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

5 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

6 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

7 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

8 History of Operating Systems. Once upon a time Human Computer History of Operating Systems (cont) First generation vacuum tubes, plug boards ENIAC (mid s) Just part of it!! Single program execution Why? Hardwire programming Programming slow, not offline! Plug board and punch cards. No programming language. Programmer spent quite a lot time to find the real Bug. 8

9 History of Operating Systems (cont) Second generation 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) Third generation IBM 360 series (mid- 1960s) Multi-programming systems: Run several programs at the same time. Spooling as jobs finished. New problems: Response time. Thrashing. File-systems. 9

10 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

11 OS s: The Bad News... Modern operating system are: Enormous: Linux v2.6 approx. 5.9 million lines of code (128MB RAM) Win 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

12 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

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

14 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 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

15 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

16 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

17 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

18 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

19 The Shell The Shell is a program that provides an interpreter and interface between the user and the UNIX Operating System. Upon login the user is placed in their own shell which prompts for and reads commands. The commands are translated and passed on to the kernel for execution and the results are then displayed at the terminal. Input can be via the keyboard or read from file. Files containing commands may be created, allowing users to build their own commands. In this manner, users may tailor UNIX to their individual requirements and style. 37 The Shell Standard Shell features: Create an environment that meets your needs Write shell scripts Define command aliases Manipulate the command history Edit the command line Some shells provide more facilities than others 38 19

20 The Shell There are several shells available for Unix; You can use any one of these shells if they are available on your system. You can switch between the different shells once you have found out if they are available. Bourne shell (sh). C shell (csh). TC shell (tcsh). Korn shell (ksh). Bourne Again SHell (bash). 39 Operating Environment What is common to (most) installations of OS OS kernel System libraries Common functionality for applications One or more shells for user interaction Utility software, for easy access to OS services 20

21 The Main Features of UNIX The features: Multitasking capability, Multi-user capability, Portability Security Most computers hosting WWW sites use UNIX (or Linux) as their operating system WHY? 21

22 Reasons for its Popularity Only a very small amount of code in UNIX is written in assembly language, making it easier for a computer vendor to get UNIX running on their system. The user benefit is that UNIX runs on a wide variety of computer systems. The application program interface allows many different types of applications to be easily implemented under UNIX without writing assembly language. These applications are relatively portable across multiple vendor hardware platforms. Reasons for its Popularity Third party software vendors can save costs by supporting a single UNIX version of their software rather than four completely different vendor specific versions requiring four times the maintenance. Vendor-independent networking allows users to easily network multiple systems from many different vendors. 22

23 Free Operating Systems Linux (including Red Hat Official Linux, though that is a borderline case) and FreeBSD are free operating systems. They are often available for the cost of the media ($2 to $5), The source code is available, often included with the main distribution; There are few limits to redistributing the system. 23

24 Linux Monolithic kernel, Unix-like System libraries, e.g. /lib/ Shells: POSIX shells (e.g. bash) and desktop environments (e.g. GNOME) System tools: GNU The Linux Operating System As UNIX spread, many people wanted to adapt it for their own needs. In 1987 Professor Andrew S. Tanenbaum invented Minix, an open-source OS that cloned UNIX. A young computer student in Finland named Linus Torvalds became intrigued by the possibilities of MINIX In 1991, he posted a note in the MINIX newsgroup that he had started work on a free operating system and asked his fellow programmers to help Over the next few years, developers for this new OS, dubbed Linux (a mix of Linus and UNIX) swelled from the hundreds to the thousands. 24

25 Linux Operating System Characteristics Unix like Open source (software that can be freely shared with no one person owning the code) requires less system resources to operate smoothly in its native form, Linux has a command line interface xwindows interface The fact that Linux is being constantly updated and refined with the latest technologies ensures that it is reliable and secure Reasons for its Popularity Linux is Network-friendly more networks set up in homes as costs for basic computer hardware and networking equipment continue to fall. Linux is capable of acting as client and/or server to any of the popular operating systems in use today Linux is Multi-user Built on the Unix design philosophy Linux is Open Gives the ability to write kernel extensions and drivers as needed Linux is Reliable and Backwards-Compatible Linux is "Free" 25

26 OS X Hybrid kernel (XNU), Unix varient Core OS is Darwin, based on BSD System libraries: /usr/lib/ Shells: POSIX shells, e.g. Bash Desktop GUI (Aqua, Finder) built on top of OS 26

27 OS X for Mac The kernel for the OS X, was created in conjunction with Apple's own engineers and computer scientists from Carnegie-Mellon and the University of California at Berkley. Apple has decided to embrace aspects of open source code creation (While Microsoft is well known for maintaining an airtight lid on its Windows source code, ). Benefits: encourage developers to submit modifications and enhancements. programmers will be able to customise Mac OS X to meet their specific needs. whole host of Macintosh products from Apple and third parties that incorporate its exciting new technologies, new networking standards lets you automatically create a network of applications, printers and other peripheral devices without having to manually configure drivers or settings. 27

28 IBM OS/2 and OS/2 WARP OS/2 was developed by Microsoft and IBM, but was later abandoned by Microsoft when it developed its own Windows operating system The "revolutionary" version was OS/2 warp 3, produced in 1994 Although it often has been declared extinct, OS/2 is still used among financial institutions and also has a small but enthusiastic group of desktop users. More Recent Development in OS Parallel Operating Systems. Distributed Operating Systems. Clustered Operating Systems. Real time Operating Systems. Embedded Operating Systems. 28

29 OS Components Operating Systems Multiuser: several users on same OS Multitasking: each running several programs But a processor can only do one thing at a time! Multiprogramming: allocates time slices of a single processor to task Load balancing: allocates tasks among multiple processors 29

30 Computer Platforms War PC (most of the market share) competitive open mass market, drives price down varying quality lower cost speedy software developments and high availability Apple Macintosh (small part of the market share) strict licensing/approval of third party products higher cost can be higher quality specialist applications - example: audio, graphics CPU I.E. Central Processing Unit The brain of the computer. All incoming events and commands are interpreted here. Instructions or calculations are carried out and the results of these are output. This is what happens when you type something and it subsequently appears on the screen. Each processor in the CPU is measured by how fast it oscillates (spins). If a processor has a speed of 500 million instructions per second (500 million oscillations), its speed is 500MHz Examples of processors: Pentium and PowerPC 30

31 CPU Protection Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly. Provide hardware support to differentiate between at least two modes of operations. User mode execution done on behalf of a user. Monitor mode (also kernel mode or system mode) execution done on behalf of operating system. CPU Protection (cont) Mode bit added to computer hardware to indicate the current mode: kernel (0) or user (1). When an interrupt or fault occurs hardware switches to kernel mode. Interrupt/fault kernel user set user mode Privileged instructions can be issued only in monitor mode. 31

32 Memory Structure Typical memory hierarchy numbers shown are rough approximations Memory Protection One base-limit pair and two base-limit pairs 32

33 I/O Devices I/O devices and the CPU can execute either synchronously or asynchronously. Each device controller is in charge of a particular device type. Each device controller has a local buffer. CPU moves data from/to main memory to/from local buffers. I/O is from the device to local buffer of controller. Device controller informs CPU that it has finished its operation by causing an interrupt. I/O Devices Protection All I/O instructions are privileged instructions. Must ensure that a user program could never gain control of the computer in monitor mode (I.e., a user program that, as part of its execution, stores a new address in the interrupt vector). 33

34 OS Interfaces (Shell, API) Command interpreter: Interface between OS and your fingers (or mouth or toes!) Commands to manipulate files, run and kill programs, access remote machines, change protections on files, etc. Some OS s have built-in command interfaces, some allow an interface to be plugged in Unix: shell (csh, tcsh, bash, ash, zsh, ) System calls - Programmer Interface System calls are the programmer s interface to the operating system In modern systems, generally available from a high-level language such as C Examples: fopen(), ioctl() (see MOS Fig 1.18 for more systems calls) Previously: Most of OS was written in assembly and system calls were made from assembly language 34

35 OS Structure Or Not? Simplest system structure: little or none Example: MS-DOS Written to squeeze the most out of limited hardware Small, some structure, but programmer is free to go around whatever structure is present anyway Monolithic kernel vs. Micro-kernel Monolithic approach: OS provides: File system, Scheduling, Device management, and etc. Components are separated by discipline and hacking requires care. For example, the traditional Unix has two layers. - Systems programs, user programs. Micro-kernel approach: Make kernel very small (provides only a limited amount of the total OS functionality, like interprocess communication & basic H/W control). Plug in virtual memory system, file system, etc. Gives encapsulation w/o strict layering. 35

36 Virtual Machines Virtual machine: fancy software which exports an interface that looks like a particular type of machine Can create illusions of multiple computer systems, which can be like the physical hardware or completely different! Sometimes a VM environment actually produces a virtual machine that's faster than the real thing on the same hardware! Example: VDM's under OS/2 (virtual dos machines) How can this be? The Java Virtual Machine Java achieves (relative) platformindependence through the JVM (Java Virtual Machine) JVM (java) is implemented for each target platform Java compiler (javac) emits bytecode (as *.class files) rather than architecturespecific machine code JVM interprets the bytecode 36

37 The Java Virtual Machine, Cont. Problem: Speed! Java code typically executes more slowly than equivalent C or C++ code Solutions (more like workaround!) JIT (Just In Time) compiler, which compiles the bytecode into native machine code immediately before execution AOT (Ahead of Time) compiler, which compiles the bytecode into native machine code. Interesting issue: caching Summary Stored program concept: program is data Software & hardware (& firmware) OS = Kernel + Shell Kernels designed for different applications Monolithic, microkernel and hybrid kernels GUI & command-line shells Reading: Brookshear 3 37