Introduction to PC Operating Systems Operating System Concepts 8th Edition Written by: Abraham Silberschatz, Peter Baer Galvin and Greg Gagne John Wiley & Sons, Inc. ISBN: 978-0-470-12872-5 Chapter 2 Operating-System Structure The design of a new operating system is a major task. It is important that the goals of the system be well defined before the design begins. These goals for the basis for choices among various algorithms and strategies. What can be focused on in designing an operating system • Services that the system provides • The interface that it makes available to users and programmers • Its components and their interconnections Operating-System Services The operating system provides: • An environment for the execution of programs • Certain services to programs and to the users of those programs. Note: (services can differ from one operating system to another) A view of operating system services user and other system programs GUI batch command line user interfaces system calls program I/O file Resource communication accounting execution operations systems allocation error protection detection services and security operating system hardware Operating-System Services Almost all operating systems have a user interface. Types of user interfaces: 1. Command Line Interface (CLI) – method where user enter text commands 2. Batch Interface – commands and directives that controls those commands are within a file and the files are executed 3. Graphical User Interface – works in a windows system using a pointing device to direct I/O, using menus and icons and a keyboard to enter text. Operating-System Services Services Program Execution – the system must be able to load a program into memory and run that program. The program must be able to end its execution, either normally or abnormally (indicating error). I/O Operations – this operation may involve files and I/O devices. (i.e. recording to CD or DVD or blanking a screen) For efficiency and protection, users usually cannot control I/O devices. Therefore, the operating system must be programmed to do this. File Manipulation – this involves reading and writing files, creating and deleting files and folders. It also involves searching and listing files by name. It also involves granting and denying access to files and folders based upon ownership. Operating-System Services The command Attrib displays, sets, or removes the read-only, archive, system, and hidden attributes assigned to files or directories. Used without parameters, attrib displays attributes of all files in the current directory. attrib [{+r|-r}] [{+a|-a}] [{+s|-s}] [{+h|-h}] [[Drive:][Path] FileName] [/s[/d]] Parameters +r : Sets the read-only file attribute. -r : Clears the read-only file attribute. +a : Sets the archive file attribute. -a : Clears the archive file attribute. +s : Sets the system file attribute. -s : Clears the system file attribute. +h : Sets the hidden file attribute. -h : Clears the hidden file attribute. Operating-System Services The command chmod changes the permission of a file. Syntax • chmod [OPTION]... MODE[,MODE]... FILE... chmod [OPTION]... OCTAL-MODE FILE... Numeric Permissions: CHMOD Numeric Permissions: Permissions 400 read by owner u – Owner/User who owns the file. 040 read by group g - Group that owns the file. 004 read by anybody (other) o - Other. 200 write by owner a - All. 020 write by group r - Read the file. 002 write by anybody w - Write or edit the file. 100 execute by owner x - Execute or run the file as a program. 010 execute by group 001 execute by anybody The command chmod 755 file.txt would yield the permission - rwx r-x r-x 1 hope 123 Feb 03 15:36 file.txt File Owner group everyone else links owner size mod date file name Operating-System Services Communications – being that computer are often times communicating with other computers normally through a network, where memory is shared, or through message passing, information is moved between processes by the operating system in packets (formatted units of data). Error Detection – The operating system needs to be constantly aware of possible errors. Errors may occur in the CPU and memory hardware (such as a memory error or a power failure), in I/O devices (such as a parity error on tape, a connection failure on a network, or lack of paper in the printer), and in the user program (such as an arithmetic overflow, an attempt to access an illegal memory location , or a too-great use of CPU time. For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing. Operating-System Services Resource Allocation, Accounting and Protection and Security are operating system functions that exist for the purpose of ensuring the efficient operation of the system. Resource allocation - it becomes important when there are multiple users or multiple jobs running at the same time; resources must be allocated to each of them. (i.e. CPU cycles, main memory and file storage) Accounting – It may come a time when we may want to keep track of each users time usage of resources. This may be done for billing purposes or simply to accumulate statistics on the usage of resources in order to improve your computing service. Protection and Security – Protection involves ensuring that all access to system resources are controlled (i.e. process interference). Security through passwords authenticate each user attempting to gain access to system resources. User Operating-System Interface As mentioned earlier there are several ways for a user to interface with the operating system. 1. Command line 2. Batch processing 3. Graphical User Interfaces (Icons) Command Interpreter Some operating systems include the command interpreter in the kernel. Others, such as Windows and UNIX, treats the command interpreter as a special program that is running when a job is initiated or when a user first logs on. On system with multiple command interpreters to choose from, the interpreters are know as shells. UNIX and LINUX Shells (Korn, Bourne, C and Bourne-Again) User Operating-System Interface Command Interpreter (cont.) The main function of the command interpreter is to get and execute the next user-specified command. Many of the commands given at this level manipulate files: create, delete, list, print, copy, execute, etc. Graphical User Interfaces A second strategy for interfacing with the operating system is through a user friendly graphical user interface, or GUI. Rather than entering commands directly via a command-line interface, users employ a mouse-based window-and-menu system on a computer’s desktop. Depending on the mouse pointer’s location, clicking a mouse button will invoke a program, select a file or directory (folder), or pull down a menu that contains commands. User Operating-System Interface Graphical User Interfaces (cont.) GUI’s first appeared due in part to research taking place in the early 1970s at Xerox PARC research facility. The first GUI appeared on the Xerox Alto computer in 1973 and soon became more widespread with the advent of Apple Macintosh computers in the 1980s. Microsoft’s first version of Windows, (Windows – Version 1.0), was based on the addition of a GUI interface to the MS-DOS operating system. User Operating-System Interface System Calls System calls provide an interface to the services made available by an operating system. These calls are generally available as routines written in C and C++, although certain low-level tasks many need to be written when hardware must be accessed directly. System calls are invoked when instructions, are displayed, a file needs to read an input file, write to an output file, close both files and writing a confirmation message. User Operating-System Interface Example of how system calls are used. source file destination file Example System Call Sequence Acquire input file name Write prompt to screen Accept input Acquire Output file name Write prompt to screen Accept input Write a program using Open the input file these stages of system if file doesn’t exist, abort calls to simulate Create output file if file exits abort copying a data file. Loop Read from input file Write to output file Until read fails Close output file Write completion message to screen Terminate normally User Operating-System Interface Types of System Calls 1. Process control 2. File manipulation 3. Device manipulation 4. Information manipulation 5. Communications 6. Protection User Operating-System Interface Types of System Calls 1. Process control – the halting of a running program’s execution whether it halts normally (programs end) or abnormally (aborted). If a system call is made to terminate the currently running program abnormally, or if the program runs into a problem and causes an error trap, a dump of memory is sometimes taken and an error message generated. The dump is written to disk and may be examined by a debugger (system programmer). Under either normal or abnormal circumstances, the operating system must transfer control to the invoking command interpreter. User Operating-System Interface Types of System Calls (Process Control ) Interactive system, the command interpreter simply continues with the next command; it assumes that the user will issue an appropriate command to respond to an error. GUI system, a pop-up window might alert the user to the error and ask for guidance. Batch system, the command interpreter usually terminates the entire job and continues with the next job. User Operating-System Interface Types of System Calls (Process Control ) Summary of System calls • End , Abort • Load, Execute • Create process, Terminate process • Get process attributes, Set process attributes • Wait for time • Wait event, Signal event • Allocate and free memory Example of Windows and Unix System Calls Windows Unix CreateProcess() fork() Process ExitProcess() exit() Control WaitForSingleObject() wait() User Operating-System Interface Types of System Calls 2.