Operating System Services and Structures

Outline of this lecture

In this lecture, we will cover the following:

• Operating System Services
• User Operating System Interface
• System Calls
• Types of System Calls
• System Programs
• Operating System Design and Implementation
• Operating System Structure
• Operating System Debugging
• Operating System Generation
• System Boot



To describe the services an operating system provides to users, processes, and other systems



To discuss the various ways of structuring an operating system

 To explain how operating systems are installed and customized

and how they boot

OS provides an environment for execution of programs and services to programs and users. One set of OS services provides functions that are helpful to the user:

• User Interface- - Almost all operating systems have a user interface (). Varies UI between Command-Line (CLI) , Graphics User Interface (GUI)
• Program execution – system capability to load a program into memory and to run it. OS able to end a program ’ s execution either normally or abnormally.
• 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 – The OS gives the ability to read, write, create, open, close and delete files / directories.
• Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
• 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 (packets moved by the OS, see chapter 3).

Additional functions exist not for helping the user, but rather for ensuring efficient system operations of the system itself via resource sharing.

• Resource allocation – allocating resources to multiple users or multiple jobs running concurrently, resources must be allocated to each of them.
• 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 & Security – The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other 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 If a system is to be protected and secure, precautions must be instituted  throughout it. A chain is only as strong as its weakest link.

 CLI or command interpreter allows direct

command entry

Sometimes implemented in kernel, sometimes by systems



program On systems with multiple CLI to choose from, the



interpreters are known – 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



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 in the early 1970s



Many systems now include both CLI and GUI interfaces

Microsoft Windows is GUI with CLI

“ command shell

”



Apple Mac OS X is

“ Aqua GUI interface with UNIX kernel underneath

”



and shells available Unix and Linux have CLI with optional GUI interfaces (CDE, KDE,



GNOME)

• System calls provide the interface between a running program and O/S.
• 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) (Note that the system-call names used throughout this text are generic)

 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 needs 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)



Often, more information is required than simply identity of desired system call



Exact type and amount of information vary according to OS and call

 Three general methods used to pass parameters to the OS

Simplest: pass the parameters in registers



In some cases, may be more parameters than registers





Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register

This approach taken by Linux and Solaris

 Parameters placed, or pushed, onto the stack by the program and popped



off the stack by the operating system Block and stack methods do not limit the number or length of parameters



being passed

System Calls can be grouped into 6 categories

• Process control – create/terminate process, load/execute, end/abort, etc.
• File management- create/delete file, open/close, read/write, get/set file attributes
• Device management - read/write, get/set device attributes, request/release device.
• Information maintenance- get/set time or date, get/set file attributes, get/set process/device attributes.
• Protection – Control access to resources, Get and set permissions, Allow and deny user access
• Communications - create, delete communication connection send, receive messages if message passing model to host name or  process name From client to server   Shared-memory model create and gain access to memory regions transfer status information   attach and detach remote devices

• General-purpose OS is very large program
• Various ways to structure ones
  • Simple structure – MS-DOS
  • More complex – UNIX
  • Layered – an abstraction
  • Microkernel – Mach

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

The system is divided into separate, smaller components that have specific and limited functionality.

• With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers

3. Microkernel System Structure

• Removes all nonessential components from the kernel & implementing them as system & user-level progs- smaller kernel. E.g. Mach OS
• Microkernel provides communication between client prog & various services that are running in user space using message passing.
• Benefits:
  • extension of the OS is easier when there is less kernel to modify.
  • easier to port the operating system to a different hardware platform because there's less kernel to reimplement.
  • more reliable (less code is running in kernel mode).
  • more secure (most services are running as user rather than kernel)

 Most modern operating systems actually not one pure model

Hybrid combines multiple approaches to address performance, security, usability



needs Linux and Solaris kernels in kernel address space, so monolithic, plus modular



for dynamic loading of functionality

 Windows mostly monolithic, plus microkernel providing support for separate

subsystems that run as user-mode processes.