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Operating-System Structures

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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 (Cont.):

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)

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 - CPU cycles, main memory, file storage, I/O devices.

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

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A View of Operating System Services

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User Operating System Interface - CLI

CLI or command interpreter allows direct command entry

Sometimes implemented in kernel, sometimes by systems program
Sometimes multiple flavors implemented – shells (Bourne shell, C shell, Bourne-Again shell, Korn shell)
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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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 is “Aqua” GUI interface with UNIX kernel underneath and shells available
Unix and Linux have CLI with optional GUI interfaces (CDE, KDE, GNOME)

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Touchscreen Interfaces

Touchscreen devices require new interfaces

Mouse not possible or not desired
Actions and selection based on gestures
Virtual keyboard for text entry

Voice commands.

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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 Programming 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

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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 the 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

create process, terminate process
end, abort
load, execute
get process attributes, set process attributes
wait for time
wait event, signal event
allocate and free memory
Dump memory if error
Debugger for determining bugs, single step execution
Locks for managing access to shared data between processes

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Types of System Calls

File management

create file, delete file
open, close file
read, write, reposition
get and set file attributes

Device management

request device, release device
read, write, reposition
get device attributes, set device attributes
logically attach or detach devices

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Types of System Calls (Cont.)

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 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

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Types of System Calls (Cont.)

Protection

Control access to resources
Get and set permissions
Allow and deny user access

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Examples of Windows and Unix System Calls

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Standard C Library Example

C program invoking printf() library call, which calls write() system call

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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 sometimes stored in a File modification
Programming language support
Program loading and execution
Communications
Background services
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.)

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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System Programs (Cont.)

Background Services

Launch at boot time

Some for system startup, then terminate
Some from system boot to shutdown

Provide facilities like disk checking, process scheduling, error logging, printing
Run in user context not kernel context
Known as services, subsystems, daemons

Application programs

Don’t pertain to system
Run by users
Not typically considered part of OS
Launched by command line, mouse click, finger poke

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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 the design 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 (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

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Operating System Structure

General-purpose OS is very large program
Various ways to structure ones

Simple structure – MS-DOS
More complex -- UNIX
Layered – an abstrcation
Microkernel -Mach

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Simple Structure -- MS-DOS

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

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Traditional UNIX System Structure

Beyond simple but not fully layered

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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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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

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End of Chapter 2

Operating System Concepts – 9^th Edition