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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
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