° The processors in the system are connected through a communication network. ° Communication takes place using a protocol. ° A distributed system provides user access to various system resources. ° Access to a shared resource allows: ° Computation speed-up ° Increased data availability ° Enhanced reliability
Protection System ° Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources. The protection mechanism must: ° ° distinguish between authorized and unauthorized usage. ° specify the controls to be imposed. ° provide a means of enforcement.
Command-Interpreter System ° Many commands are given to the operating system by control statements which deal with: ° process creation and management ° I/O handling ° secondary-storage management ° main-memory management ° file-system access ° protection ° networking
Command-Interpreter System (Cont.) ° The program that reads and interprets control statements is called variously: ° command-line interpreter ° shell (in UNIX) Its function is to get and execute the next command statement.
Operating System Services ° Program execution – system capability to load a program into memory and to run it. ° I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O. ° File-system manipulation – program capability to read, write, create, and delete files. ° Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing . ° Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
Additional Operating System Functions Additional functions exist not for helping the user, but rather for ensuring efficient system operations. • Resource allocation – allocating resources to multiple users or multiple jobs running at the same time. • Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics. • Protection – ensuring that all access to system resources is controlled.
System Calls ° System calls provide the interface between a running program and the operating system. ° Generally available as assembly-language instructions. ° Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, C++) ° Three general methods are used to pass parameters between a running program and the operating system. ° Pass parameters in registers . ° Store the parameters in a table in memory, and the table address is passed as a parameter in a register.
- One '20