Chapter 2: Operating-System Structures

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 Virtual Machines

Objectives Describe the services an operating system provides to users, processes, and other systems Discuss various ways of structuring an operating system Explain how operating systems are installed and customized and how they boot

Operating System Services Provide functions that are helpful to users User interface Batch, Command-Line (CLI), Graphics User Interface (GUI) Program execution Load a program into memory, Run it, End execution normally or abnormally I/O operations File-system manipulation Create, delete, read, write, and list files and directories Change permissions

Operating System Services (Cont.) Provide functions that are helpful to users Communications Exchange information on the same computer or between computers over a network Shared memory or message passing Error detection Hardware: CPU, memory, I/O devices, etc. User program For each type of error, take an appropriate action to ensure correct and consistent computing Debugging

Operating System Services (Cont.) Ensure efficient system operation via resource sharing Resource allocation 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 Essential in multi-user/networked environment Protection: Ensure that all access to system resources is controlled, e.g., mode bit, memory management Security: User authentication, access control, ...

User Operating System Interface: CLI (Command Line Interface) Direct command entry Implemented in kernel or by systems program Multiple flavors can be implemented, e.g., several command shells for UNIX Fetch a command from user and execute it Built-in (i.e., implemented in kernel) Invoke user-level programs Adding new features requires no shell modification

User Operating System Interface: GUI User-friendly desktop metaphor interface Icons represent files, programs, actions, etc Mouse buttons over objects cause various actions Invented at Xerox PARC Many systems have both CLI and GUI Microsoft Windows is GUI with CLI UNIX/LINUX is CLI with optional GUI interfaces Apple Mac OS X has Aqua GUI interface with UNIX kernel underneath

System Calls Programming interface to OS services Typically written in C or C++ Usually use high-level Application Program Interface (API) rather than direct system calls Win32 API for Windows POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X) Java API for the Java virtual machine (JVM)

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

Example of Standard API Consider the ReadFile() function in Win32 API

System Call Implementation System call interface invokes intended system call in OS and returns system status and any return values Caller needs to know nothing about how the system call is implemented Use API Hide implementation details

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 simple identity of desired system call How to pass parameters to OS Pass the parameters in registers Number & length of parameters can be limited Parameters stored in a block or table in memory, and pass the block’s address as a parameter in a register Push parameters onto stack

Parameter Passing via Table

Types of System Calls Process control File management Device management Information maintenance Communications

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

FreeBSD Running Multiple Programs

System Programs Convenient environment for program development and execution Program loading and execution File manipulation Programming language support Communications Application programs Most users’ view of the OS 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, Status information File modification Text editors to create and modify files Commands to search file contents, e.g., grep in UNIX/Linux

System Programs (cont’d) Programming language support Compilers/interpreters, assemblers, debuggers Program loading and execution Absolute loaders, relocatable loaders, linkage editors, and overlay loaders Communications Creating virtual connections among processes, users, and computer systems Send messages to somebody’s screen, send email, browse web pages, remote log in, file transfer

Operating System Design and Implementation No silver bullet but some approaches have been successful Internal structure of different OSs can vary widely Affected by choice of hardware & OS type First define goals and specifications User goals and System goals User goals: OS should be convenient, easy to learn, reliable, safe, and fast System goals: OS should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

Operating System Design and Implementation (Cont.) Important 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

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

Layered Operating System

UNIX UNIX Limited by hardware functionality, the original UNIX had limited structuring consisted of two separable parts Systems programs Kernel Consists of everything below the system-call interface and above the physical hardware Provides a large number of functions for one level: CPU scheduling, memory management, file system and other OS functions

UNIX System Structure

Microkernel System Structure Moves as much from the kernel into “user” space as possible Message passing between user modules Benefits Easier to extend a microkernel Easier to port the OS to new architectures More reliable (less code is running in kernel mode) More secure Problem Performance overhead due to communications between user space to kernel space

Mac OS X Structure Hybrid approach (microkernel + layered approach) Mach microkernel: Memory management, RPC & IPC BSD kernel: BSD command line interface, networking & file system, POSIX API’s such as Pthreads In addition, kernel environment provides an I/O kit & dynamically loadable modues

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, e.g., device drivers & system calls Similar to layers but more flexible

Solaris Modular Approach

Virtual Machines Take the layered approach Treat hardware and OS kernel as though they were all hardware Provide an interface identical to the underlying hardware An OS creates the illusion of multiple processes executing on its own processor with its own (virtual) memory

Virtual Machines (Cont.) (a) Nonvirtual machine (b) virtual machine Non-virtual Machine Virtual Machine

Virtual Machines (Cont.) Provide complete protection of system resources Each virtual machine is isolated from all other virtual machines No direct sharing of resources A perfect vehicle for operating systems research and development Develop an OS on a virtual machine Do not disrupt normal system operation. Difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

VMware Architecture

The Java Virtual Machine

End of Chapter 2 Questions?