Operating system structures

Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition
Chapter 2: Operating-System
Structures

2.2 Silberschatz, Galvin and Gagne ©2009
Chapter 2: Operating-System Structures
 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

Operating System Services
 Operating systems provide an environment for execution of programs and
services to programs and users
 One set of operating-system services provides functions that are helpful to the
user:
 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.
Programs need to read and write files and directories, create and delete
them, search them, list file Information, permission management.

Operating System Services (Cont.)
 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)
 Error detection – OS needs to be constantly aware of possible
errors
 May occur in the CPU and memory hardware, in I/O devices, in
user program
 For each type of error, OS should take the appropriate action to
ensure correct and consistent computing
 Debugging facilities can greatly enhance the user’s and
programmer’s abilities to efficiently use the system

Operating System Services (Cont.)
 Another set of OS functions exists for ensuring the efficient operation of the
system itself via resource sharing
 Resource allocation - When multiple users or multiple jobs running
concurrently, resources must be allocated to each of them
 Many types of resources - Some (such as CPU cycles, main memory,
and file storage) may have special allocation code, others (such as I/O
devices) may have general request and release code
 Accounting - To keep track of which users use how much and what kinds
of computer resources
 Protection and 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.

A View of Operating System Services

User Operating System Interface - CLI
 Command Line Interface (CLI) or command interpreter allows direct
command entry
 Some operating systems include the command interpreter in the kernel
 Others such as WindowsXP, treat the command interpreter as a
special program
 On systems with multiple command interpreters to choose from ,the
interpreters are known as SHELLS
 E.g.,
 Bourne shell
 C shell
 Bourne -Again shell(BASH)
 Korn shell

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)

Bourne Shell Command Interpreter

TOUCH SCREEN INTERFACES

The Mac OS X GUI

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)

Example of System Calls
 System call sequence to copy the contents of one file to another file

Example of Standard API

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)

API – System Call – OS Relationship

Standard C Library Example
 C program invoking printf() library call, which calls write() system call

System Call Parameter Passing
 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

Parameter Passing via Table

Types of System Calls
 Process control
 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
 File management
 create file, delete file
 open, close file
 read, write, reposition
 get and set file attributes

Types of System Calls (Cont.)
 Device management
 request device, release device
 read, write, reposition
 get device attributes, set device attributes
 logically attach or detach devices
 Information maintenance
 get time or date, set time or date
 get system data, set system data
 get and set process, file, or device attributes
 Communications
 create, delete communication connection
 send, receive messages
 transfer status information
 attach and detach remote devices

Examples of Windows and
Unix System Calls

Example: MS-DOS
 Single-tasking
 Shell invoked when system booted
 Simple method to run program
 No process created
 Single memory space
 Loads program into memory, overwriting all but the kernel
 Program exit -> shell reloaded

MS-DOS execution
(a) At system startup (b) running a program

Example: FreeBSD
 Unix variant
 Multitasking
 User login -> invoke user’s choice of shell
 Shell executes fork() system call to create process
 Executes exec() to load program into process
 Shell waits for process to terminate or continues with user commands
 Process exits with code of 0 – no error or > 0 – error code

FreeBSD Running Multiple Programs

System Programs
 System programs provide a convenient environment for program
development and execution. They can be divided into:
 File manipulation
 Status information
 File modification
 Programming language support
 Program loading and execution
 Communications
 Application programs
 Most users’ view of the operation system is defined by system
programs, not the actual system calls

System Programs
 Provide a convenient environment for program development and
execution
 Some of them are simply user interfaces to system calls; others
are considerably more complex
 File management - Create, delete, copy, rename, print, dump, list,
and generally manipulate files and directories
 Status information
 Some ask the system for info - date, time, amount of available
memory, disk space, number of users
 Others provide detailed performance, logging, and debugging
information
 Typically, these programs format and print the output to the
terminal or other output devices
 Some systems implement a registry - used to store and retrieve
configuration information

System Programs (Cont.)
 File modification
 Text editors to create and modify files
 Special commands to search contents of files or perform
transformations of the text
 Programming-language support - Compilers, assemblers,
debuggers and interpreters sometimes provided
 Program loading and execution- Absolute loaders, relocatable
loaders, linkage editors, and overlay-loaders, debugging systems for
higher-level and machine language
 Communications - Provide the mechanism for creating virtual
connections among processes, users, and computer systems
 Allow users to send messages to one another’s screens, browse
web pages, send electronic-mail messages, log in remotely,
transfer files from one machine to another

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

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

Simple Structure
 MS-DOS – written to provide the most functionality in the least space
 Not divided into modules
 Although MS-DOS has some structure, its interfaces and levels of
functionality are not well separated

MS-DOS Layer Structure

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

Traditional UNIX System Structure

UNIX
 UNIX – limited by hardware functionality, the original UNIX operating
system had limited structuring. The UNIX OS 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; a large
number of functions for one level

Layered Operating System

Microkernel System Structure
 Moves as much from the kernel into “user” space
 Communication takes place between user modules using message
passing
 Benefits:
 Easier to extend a microkernel
 Easier to port the operating system to new architectures
 More reliable (less code is running in kernel mode)
 More secure
 Detriments:
 Performance overhead of user space to kernel space
communication

Mac OS X Structure

Modules
 Most modern operating systems implement kernel modules
 Uses object-oriented approach
 Each core component is separate
 Each talks to the others over known interfaces
 Each is loadable as needed within the kernel
 Overall, similar to layers but with more flexible

Solaris Modular Approach

Operating-System Debugging
 Debugging is finding and fixing errors, or bugs
 OSes generate log files containing error information
 Failure of an application can generate core dump file capturing
memory of the process
 Operating system failure can generate crash dump file containing
kernel memory
 Beyond crashes, performance tuning can optimize system performance
 Kernighan’s Law: “Debugging is twice as hard as writing the code in the
first place. Therefore, if you write the code as cleverly as possible, you
are, by definition, not smart enough to debug it.”
 DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation
on production systems
 Probes fire when code is executed, capturing state data and
sending it to consumers of those probes

Operating System Generation
 Operating systems are designed to run on any of a class of machines;
the system must be configured for each specific computer site
 SYSGEN program obtains information concerning the specific
configuration of the hardware system
 Booting – starting a computer by loading the kernel
 Bootstrap program – code stored in ROM that is able to locate the
kernel, load it into memory, and start its execution

Operating system structures

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