CH 2 Operating-System Structures

Information Maintenence

get time or date, set time or date get system data, set system data get and set process, file, or device attributes

Protection and security

lThe 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

Security

4of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts

I/O Operations

A running program may require I/O, which may involve a file or an I/O device

Program loading and execution (System Programs)

Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language

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

User Interface

Almost all operating systems have a user interface (UI). Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch

iOS

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

Emulation

Can allow an OS to run on non-native hardware.

Programming-language support (System Programs)

Compilers, assemblers, debuggers and interpreters sometimes provided

Protection

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

Processor 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

File Management (System Programs)

Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories

Operating-System Debugging

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 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

Android

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

Application Programs

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

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

Mechanism

How to do it? determine how to do something

Performance Tuning

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

Protection

Involves ensuring that all access to system resources is controlled

Background services (System Programs)

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

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

Modules

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

Hybrid Systems

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)

Touchscreen Interfaces

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

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 n]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

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

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

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.

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 Used to build system-specific compiled kernel or system-tuned Can general more efficient code than one general kernel

System Calls

Process Control File Management Device Management Information Maintenance Communications Protection

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)

Communications (System Programs)

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.

Status Information (System Programs)

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

Software engineering

Specifying and designing an OS is highly creative task of ___________.

File Modification (System Program)

Text editors to create and modify files Special commands to search contents of files or perform transformations of the text

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.

Layed Approach System Structure

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

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)

Accounting

To keep track of which users use how much and what kinds of computer resources

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

System Call IOmplementation

Typically, a number associated with each system call System-call interface maintains a table indexed according to these numbers 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)

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

GUI - Operating System Interface

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).

Policy?

What will be done? decide what will be done

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.

System Boot

When power initialized on system, execution starts at a fixed memory location Firmware ROM used to hold initial boot code Operating system must be made available to hardware so hardware can start it Small piece of code - bootstrap loader, stored in ROM or EEPROM locates the kernel, loads it into memory, and starts it Sometimes two-step process where boot block at fixed location loaded by ROM code, which loads bootstrap loader from disk Common bootstrap loader, GRUB, allows selection of kernel from multiple disks, versions, kernel options Kernel loads and system is then running

File Management

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

Communications (System Call)

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

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.

Device Managment

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