2.01
Operating-System Structures
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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
Virtual Machines
Operating System Debugging
Operating System Generation
System Boot
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Objectives
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
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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
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Operating System Services (Cont)
One set of operating-system services provides functions that are helpful to the user:
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.
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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Operating System Services (Cont)
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
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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.
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A View of Operating System Services
User Operating System Interface – CLI
Command Line Interface (CLI) or command interpreter 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
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Bourne Shell Command Interpreter
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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Touchscreen devices require new interfaces
Mouse not possible or not desired
Actions and selection based on gestures
Virtual keyboard for text entry
Voice commands.
Touchscreen 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)
Why use APIs rather than system calls?
(Note that the system-call names used throughout this text are generic)
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Example of System Calls
System call sequence to copy the contents of one file to another file
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Example of Standard API
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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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API – System Call – OS Relationship
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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
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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
Protection
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Examples of Windows and Unix System Calls
C program invoking printf() library call, which calls write() system call
Standard C Library Example
MS-DOS execution
(a) At system startup (b) running a program
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FreeBSD Running Multiple Programs
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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
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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
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System Programs (cont’d)
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
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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
The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later (example – timer)
Specifying and designing an OS is highly creative task of software engineering
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Implementation
Much variation
Early OSes in assembly language
Then system programming languages like Algol, PL/1
Now C, C++
Actually usually a mix of languages
Lowest levels in assembly
Main body in C
Systems programs in C, C++, scripting languages like PERL, Python, shell scripts
More high-level language easier to port to other hardware
But slower
Emulation can allow an OS to run on non-native hardware
General-purpose OS is very large program
Various ways to structure ones
Simple structure – MS-DOS
More complex — UNIX
Layered – an abstrcation
Microkernel -Mach
Operating System Structure
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
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Non Simple 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
Traditional UNIX System Structure
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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
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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
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Microkernel System Structure
Moves as much from the kernel into user space
Mach example of microkernel
Mac OS X kernel (Darwin) partly based on Mach
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
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Microkernel System Structure
Many modern operating systems implement loadable 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
Linux, Solaris, etc
Modules
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
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Solaris Modular Approach
Most modern operating systems are 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 for different subsystem personalities
Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment
Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions)
Hybrid Systems
Mac OS X Structure
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Apple mobile OS for iPhone, iPad
Structured on Mac OS X, added functionality
Does not run OS X applications natively
Also runs on different CPU architecture (ARM vs. Intel)
Cocoa Touch Objective-C API for developing apps
Media services layer for graphics, audio, video
Core services provides cloud computing, databases
Core operating system, based on Mac OS X kernel
iOS
Developed by Open Handset Alliance (mostly Google)
Open Source
Similar stack to IOS
Based on Linux kernel but modified
Provides process, memory, device-driver management
Adds power management
Runtime environment includes core set of libraries and Dalvik virtual machine
Apps developed in Java plus Android API
Java class files compiled to Java bytecode then translated to executable than runs in Dalvik VM
Libraries include frameworks for web browser (webkit), database (SQLite), multimedia, smaller libc
Android
Android Architecture
Virtual Machines
A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware
A virtual machine provides an interface identical to the underlying bare hardware
The operating system host creates the illusion that a process has its own processor and (virtual memory)
Each guest provided with a (virtual) copy of underlying computer
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Virtual Machines History and Benefits
First appeared commercially in IBM mainframes in 1972
Fundamentally, multiple execution environments (different operating systems) can share the same hardware
Protect from each other
Some sharing of file can be permitted, controlled
Communicate with each other, other physical systems via networking
Useful for development, testing
Consolidation of many low-resource use systems onto fewer busier systems
“Open Virtual Machine Format”, standard format of virtual machines, allows a VM to run within many different virtual machine (host) platforms
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Virtual Machines (Cont)
(a) Nonvirtual machine (b) virtual machine
Non-virtual Machine
Virtual Machine
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Debugging is finding and fixing errors, or bugs
OS 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
Sometimes using trace listings of activities, recorded for analysis
Profiling is periodic sampling of instruction pointer to look for statistical trends
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.”
Operating-System Debugging
VMware Architecture
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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
Improve performance by removing bottlenecks
OS must provide means of computing and displaying measures of system behavior
For example, “top” program or Windows Task Manager
Performance Tuning
DTrace
DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation on production systems
Probes fire when code is executed within a provider, capturing state data and sending it to consumers of those probes
Example of following XEventsQueued system call move from libc library to kernel and back
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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
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System Boot
Operating system must be made available to hardware so hardware can start it
Small piece of code – bootstrap loader, locates the kernel, loads it into memory, and starts it
Sometimes two-step process where boot block at fixed location loads bootstrap loader
When power initialized on system, execution starts at a fixed memory location
Firmware used to hold initial boot code
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Application
Program
File
System
Device
Driver
Interprocess
Communication
memory
managment
CPU
scheduling
messagesmessages
microkernel
hardware
user
mode
kernel
mode
Application
Program
File
System
Device
Driver
Interprocess
Communication
memory
managment
CPU
scheduling
messagesmessages
microkernel
hardware
user
mode
kernel
mode
graphical user interface
Aqua
application environments and services
kernel environment
Java Cocoa Quicktime BSD
Mach
I/O kit kernel extensions
BSD
graphical user interface
Aqua
application environments and services
kernel environment
Java Cocoa Quicktime
BSD
Mach
I/O kit kernel extensions
BSD
fg02_17.eps
Cocoa Touch
Media Services
Core Services
Core OS
Applications
Application Framework
Android runtime
Core Libraries
Dalvik
virtual machine
Libraries
Linux kernel
SQLite openGL
surface
manager
webkit libc
media
framework
Applications
Application Framework
Android runtime
Core Libraries
Dalvik
virtual machine
Libraries
Linux kernel
SQLite openGL
surface
manager
webkit libc
media
framework