OS Structures & Services: Comm, Sys Calls, UIs & VMs, Slides of Computer Science

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Operating System Structures

Chapter 2

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Operating System Services

• User interface - Almost all operating systems have a user interface (UI)
  • Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch
• Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)
• I/O operations - A running program may require I/O, which may involve a file or an I/O device.
• File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories, create and delete them, search them, list file Information, permission management.
• Communications – Processes may exchange information, on the same computer or between computers over a network - Communications may be via shared memory or through message passing (packets moved by the OS)

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User Operating System Interface - CLI

CLI allows direct command entry

• Sometimes implemented in kernel, sometimes by

systems program

• Sometimes multiple flavors implemented – shells
• Primarily fetches a command from user and

executes it

• Sometimes commands built-in, sometimes just names of programs - If the latter, adding new features doesn’t require shell modification
• Example: Unix shell programming

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MS-DOS execution

(a) At system startup (b) running a program

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User Operating System Interface - GUI

• User-friendly desktop metaphor interface
  • Usually mouse, keyboard, and monitor
  • Icons represent files, programs, actions, etc
  • Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder )
  • Invented at Xerox PARC
• Many systems now include both CLI and GUI interfaces
  • Microsoft Windows is GUI with CLI “command” shell
  • Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and shells available
  • Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)

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System Calls

• Programming interface to the services provided by the OS
• Typically written in a high-level language (C or C++)
• Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use
• Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)
• Why use APIs rather than system calls?

(Note that the system-call names used throughout this text are generic)

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System Call Implementation

• Typically, a number associated with each system call
  • System-call interface maintains a table indexed according to these numbers
• The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values
• The caller need know nothing about how the system call is implemented - Just needs to obey API and understand what OS will do as a result call - Most details of OS interface hidden from programmer by API - Managed by run-time support library (set of functions built into libraries included with compiler)

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Standard C Library Example

• C program invoking printf() library call, which

calls write() system call

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Parameter Passing via Table

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Types of System Calls

• Process control

• File management

• Device management

• Information maintenance

• Communications

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Operating System Design and

Implementation

• Design and Implementation of OS not “solvable”, but

some approaches have proven successful

• Internal structure of different Operating Systems can

vary widely

• Start by defining goals and specifications
• Affected by choice of hardware, type of system
• User goals and System goals
  • User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
  • System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

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Operating System Design and

Implementation (Cont.)

• Important principle to separate

Policy: What will be done?

Mechanism: How to do it?

• Mechanisms determine how to do something,

policies decide what will be done

• The separation of policy from mechanism is a very

important principle, it allows maximum flexibility

if policy decisions are to be changed later

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UNIX: Also “Simple” Structure

• UNIX – limited by hardware functionality

• Original UNIX operating system consists of two

separable parts:

• Systems programs
• The kernel
  • Consists of everything below the system-call interface and above the physical hardware
  • Provides the file system, CPU scheduling, memory management, and other operating-system functions;
  • Many interacting functions for one level

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UNIX System Structure

User Mode

Kernel Mode

Hardware

Applications

Standard Libs