Chapter 2 Operating System Structures Modified Considerably by your Instructor Chapter 2 Operating System Structures Chapter 2 1 First Part Operating System Services User Operating System Interface System Calls Types of System Calls Chapter 2 2 Second Part System Programs Operating System Design and Implementation Operating System Structure Virtual Machines Operating System Generation System Boot Operating System Concepts 2 2 Silberschatz Galvin and Gagne 2005 0 Introductory Remarks Objectives At a rather superficial level yet we will overview some of the main operating system structures Operating Systems can be viewed from a number of perspectives Perspectives center around the Services operating systems provide and how they provide these services These services are many and varied Interface provided to those using the computing system programmers and users Components and interconnections provided by operating systems We will look at all three in this chapter but reserving a much more detailed look for later chapters Operating System Concepts 2 3 Silberschatz Galvin and Gagne 2005 1 Operating System Services User services include User interface All operating systems have a user interface UI Will either be a Command Line CLI Graphics User Interface GUI Batch Many kusers are more used to one form than another CS people Information Systems users IT engineers Each have advantages and disadvantages adherents and opponents Program execution The system must be able to load a program into memory run that program terminate execution either normally or abnormally I O operations Running programs may require I O involve file or I O device Because I O operations involved shared devices these devices and the programs users who access them must be protected File system manipulation The file system is of particular interest User services include the ability to read and write files and directories create and delete them search them list file Information and manage permissions access in a wide variety of ways Operating System Concepts 2 4 Silberschatz Galvin and Gagne 2005 Operating System Services Cont Other operating system services helpful to the user include Communications Processes often need to exchange information on the same computer or between computers over a network Further Communications may be via shared memory or through message passing packets moved by the OS Discussed ahead Error detection OS needs to be constantly aware of possible errors Hardware May occur in CPU I O devices in user program and memory hardware or User Errors attempting to access protected areas of the OS common programming errors such as division by zero subscripts out of range timer runout and more Silberschatz Galvin and Gagne 2005 Operating System Concepts 2 5 Operating System Services Cont Other OS services center around ensuring efficient utilization of resources for smooth running of the entire computing system Resource allocation When multiple users or multiple jobs running concurrently resources must be allocated to each of them Typical resources include CPU cycles main memory and file storage OS cannot permit resource monopolization by one process OS must schedule prioritize and otherwise control access via various queues and other data structures Accounting Keeping track of which users use how much and what kinds of computer resources CPU time disk storage and more are often billed to customers Prioritized e g Billed customers students production Operating System Concepts 2 6 Silberschatz Galvin and Gagne 2005 Operating System Services Cont Protection and security two critical services Protection involves ensuring that all access to system resources is controlled Security of the system from outsiders requires user authentication extends to defending external I O devices from invalid access attempts All of these services quite a number that we typically take for granted are critically important and will be discussed in topic specific areas such as process control memory management and I O control and management much later in this course Let s now address the second feature we mentioned Interfaces Operating System Concepts 2 7 Silberschatz Galvin and Gagne 2005 2 2 Operating System Interfaces Introduction At a very low level operating systems provide capabilities for programs running to request a long list of services from the operating system via a mechanism we refer to as system calls At a higher level a shell or command interpreter provides an interface to the user to request these services without writing a program Some of the handling of system calls are built into the command interpreter others services are programs in files that may be invoked directly from a terminal when in interactive mode The command interpreter often realizes sequences of system calls to handled involved requests for services Operating System Concepts 2 8 Silberschatz Galvin and Gagne 2005 Operating System Interfaces Three primary mechanisms for interfacing with the OS 1 Command line interface CLI allows user to interface directly via specific commands 2 Graphical User Interface GUI provides the user an interface with icons etc that represent programs and 3 Batch interface very important interface deserving discussion too Let s look at these Operating System Concepts 2 9 Silberschatz Galvin and Gagne 2005 2 2 1 CLI Interface Sometimes the CLI is implemented in the kernel Other times the CLI is implemented as a special program as for Unix Windows XP shells Here CLI acts like an interface to the kernel In UNIX environment are several command interpreters Here the program a shell is running when a job is initiated or when user first logs on for interactive systems These various command interpreters are called shells Present roughly the same capabilities prompts Normally Korn Operating System Concepts a matter of taste shell Bourne shell C shell Bash and others 2 10 Silberschatz Galvin and Gagne 2005 CLI Interface more Main purpose of the command interpreter is to execute the desired command entered by the user Primarily accepts a command from user and executes it Two general ways issued commands can be implemented 1 Sometimes the executable code exists as part of command interpreter A command thus requires a branch to a specific area in the command interpreter where the desired code e g create delete list is found Of course this makes the command interpreter larger 2 Commands such as cp file1 file2 implemented by a systems