Download What is an Operating System?

CS 571
Operating Systems
Prof. Sanjeev Setia
Fall 2001
Overview
? Prerequisites
? Computer Architecture (CS 365)
? C++/C/Java progamming
? Textbook
? Silbershatz,Galvin,Gagne – Operating Systems Concepts
? Grading
? One midterm exam (25%), Final Exam (25%)
? Assignments (50%)
? Three Programming Assignments
? Concurrent Programming
? RPC/RMI
? Distributed File System
? First two to be done individually, third may be done in groups of
two
? Late submission penalty – 10% per day
? Can “redo”first two assignments to improve grade to some
extent (can make up half the difference between old grade and
max grade)
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Logistics
? Class Web Page
? http://www.cs.gmu.edu/~setia/cs571/
? Slides, Handouts, Old Exams, Useful Links
? Slides
? More than 90% of slides taken from slides made available by
authors of textbook
? http://www.bell-labs.com/topic/books/os-book/
? Office Hrs
? Monday, 2 – 4 pm, Room 347, S&T II
? [email protected]
? TA
? Vamshi Kalakuntla
? Office Hrs
? Office
? Computer Accounts (IT&E or your own machine at home/work)
Operating Systems
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Chapter 1: Introduction
? What is an Operating System?
? Mainframe Systems
? Desktop Systems
? Multiprocessor Systems
? Distributed Systems
? Clustered System
? Real -Time Systems
? Handheld Systems
? Computing Environments
Operating Systems
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What is an Operating System?
? A program that acts as an intermediary between a user of
a computer and the computer hardware.
? Operating system goals:
? Execute user programs and make solving user problems
easier.
? Make the computer system convenient to use.
? Use the computer hardware in an efficient manner.
Operating Systems
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Computer System Components
1. Hardware – provides basic computing resources (CPU,
memory, I/O devices).
2. Operating system – controls and coordinates the use of
the hardware among the various application programs for
the various users.
3. Applications programs – define the ways in which the
system resources are used to solve the computing
problems of the users (compilers, database systems,
video games, business programs).
4. Users (people, machines, other computers).
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Abstract View of System Components
Operating Systems
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Operating System Definitions
? Resource allocator – manages and allocates resources.
? Control program – controls the execution of user
programs and operations of I/O devices .
? Kernel – the one program running at all times (all else
being application programs).
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Mainframe Systems
? Reduce setup time by batching similar jobs
? Automatic job sequencing – automatically transfers
control from one job to another. First rudimentary
operating system.
? Resident monitor
? initial control in monitor
? control transfers to job
? when job completes control transfers pack to monitor
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Memory Layout for a Simple Batch System
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Multiprogrammed Batch Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.
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OS Features Needed for Multiprogramming
? I/O routine supplied by the system.
? Memory management – the system must allocate the
memory to several jobs.
? CPU scheduling – the system must choose among
several jobs ready to run.
? Allocation of devices.
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Time-Sharing Systems–Interactive Computing
? The CPU is multiplexed among several jobs that are kept
in memory and on disk (the CPU is allocated to a job only
if the job is in memory).
? A job swapped in and out of memory to the disk.
? On-line communication between the user and the system
is provided; when the operating system finishes the
execution of one command, it seeks the next “control
statement”from the user’s keyboard.
? On-line system must be available for users to access data
and code.
Operating Systems
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Desktop Systems
? Personal computers – computer system dedicated to a
single user.
? I/O devices – keyboards, mice, display screens, small
printers.
? User convenience and responsiveness.
? Can adopt technology developed for larger operating
system’often individuals have sole use of computer and
do not need advanced CPU utilization of protection
features.
? May run several different types of operating systems
(Windows, MacOS, UNIX, Linux)
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Parallel Systems
? Multiprocessor systems with more than on CPU in close
communication.
? Tightly coupled system – processors share memory and a
clock; communication usually takes place through the
shared memory.
? Advantages of parallel system:
? Increased throughput
? Economical
? Increased reliability
? graceful degradation
? fail-soft systems
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Parallel Systems (Cont.)
? Symmetric multiprocessing (SMP)
? Each processor runs and identical copy of the operating
system.
? Many processes can run at once without performance
deterioration.
? Most modern operating systems support SMP
? Asymmetric multiprocessing
? Each processor is assigned a specific task; master
processor schedules and allocated work to slave
processors.
? More common in extremely large systems
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Symmetric Multiprocessing Architecture
Operating Systems
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Distributed Systems
? Distribute the computation among several physical
processors.
? Loosely coupled system – each processor has its own
local memory; processors communicate with one another
through various communications lines, such as highspeed buses or telephone lines.
? Advantages of distributed systems.
? Resources Sharing
? Computation speed up – load sharing
? Reliability
? Communications
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Distributed Systems (cont)
? Requires networking infrastructure.
? Local area networks (LAN) or Wide area networks (WAN)
? May be either client-server or peer-to-peer systems.
Operating Systems
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General Structure of Client-Server
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Clustered Systems
? Clustering allows two or more systems to share storage.
? Provides high reliability.
? Asymmetric clustering: one server runs the application
while other servers standby.
? Symmetric clustering: all N hosts are running the
application.
Operating Systems
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Real-Time Systems
? Often used as a control device in a dedicated application
such as controlling scientific experiments, medical
imaging systems, industrial control systems, and some
display systems.
? Well-defined fixed-time constraints.
? Real-Time systems may be either hard or soft real-time.
Operating Systems
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Real-Time Systems (Cont.)
? Hard real-time:
? Secondary storage limited or absent, data stored in short
term memory, or read-only memory (ROM)
? Conflicts with time-sharing systems, not supported by
general-purpose operating systems.
? Soft real-time
? Limited utility in industrial control of robotics
? Useful in applications (multimedia, virtual reality) requiring
advanced operating-system features.
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Handheld Systems
? Personal Digital Assistants (PDAs)
? Cellular telephones
? Issues:
? Limited memory
? Slow processors
? Small display screens.
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Migration of Operating-System Concepts and Features
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Chapter 2: Computer-System Structures
? Computer System Operation
? I/O Structure
? Storage Structure
? Storage Hierarchy
? Hardware Protection
? General System Architecture
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Computer-System Architecture
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Computer-System Operation
? I/O devices and the CPU can execute concurrently.
? Each device controller is in charge of a particular device
?
?
?
?
Operating Systems
type.
Each device controller has a local buffer.
CPU moves data from/to main memory to/from local
buffers
I/O is from the device to local buffer of controller.
Device controller informs CPU that it has finished its
operation by causing an interrupt.
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Common Functions of Interrupts
? Interrupt transfers control to the interrupt service routine
?
?
?
?
Operating Systems
generally, through the interrupt vector, which contains the
addresses of all the service routines.
Interrupt architecture must save the address of the
interrupted instruction.
Incoming interrupts are disabled while another interrupt is
being processed to prevent a lost interrupt.
A trap is a software-generated interrupt caused either by
an error or a user request.
An operating system is interrupt driven.
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Interrupt Handling
? The operating system preserves the state of the CPU by
storing registers and the program counter.
? Determines which type of interrupt has occurred:
? polling
? vectored interrupt system
? Separate segments of code determine what action should
be taken for each type of interrupt
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Interrupt Time Line For a Single Process Doing Output
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I/O Structure
? After I/O starts, control returns to user program only upon
I/O completion.
? Wait instruction idles the CPU until the next interrupt
? Wait loop (contention for memory access).
? At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
? After I/O starts, control returns to user program without
waiting for I/O completion.
? System call – request to the operating system to allow user
to wait for I/O completion.
? Device-status table contains entry for each I/O device
indicating its type, address, and state.
? Operating system indexes into I/O device table to determine
device status and to modify table entry to include interrupt.
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Two I/O Methods
Synchronous
Operating Systems
Asynchronous
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Device-Status Table
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Direct Memory Access Structure
? Used for high-speed I/O devices able to transmit
information at close to memory speeds.
? Device controller transfers blocks of data from buffer
storage directly to main memory without CPU
intervention.
? Only on interrupt is generated per block, rather than the
one interrupt per byte.
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Storage Structure
? Main memory – only large storage media that the CPU
can access directly.
? Secondary storage – extension of main memory that
provides large nonvolatile storage capacity.
? Magnetic disks – rigid metal or glass platters covered with
magnetic recording material
? Disk surface is logically divided into tracks, which are
subdivided into sectors.
? The disk controller determines the logical interaction
between the device and the computer.
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Moving-Head Disk Mechanism
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Storage Hierarchy
? Storage systems organized in hierarchy.
? Speed
? Cost
? Volatility
? Caching – copying information into faster storage system;
main memory can be viewed as a last cache for
secondary storage.
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Storage-Device Hierarchy
Operating Systems
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Caching
? Use of high-speed memory to hold recently-accessed
data.
? Requires a cache management policy.
? Caching introduces another level in storage hierarchy.
This requires data that is simultaneously stored in more
than one level to be consistent.
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Migration of A From Disk to Register
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Hardware Protection
? Dual-Mode Operation
? I/O Protection
? Memory Protection
? CPU Protection
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Dual-Mode Operation
? Sharing system resources requires operating system to
ensure that an incorrect program cannot cause other
programs to execute incorrectly.
? Provide hardware support to differentiate between at least
two modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also kernel mode or system mode) –
execution done on behalf of operating system.
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Dual-Mode Operation (Cont.)
? Mode bit added to computer hardware to indicate the
current mode: monitor (0) or user (1).
? When an interrupt or fault occurs hardware switches to
monitor mode.
Interrupt/fault
monitor
user
set user mode
Privileged instructions can be issued only in monitor mode.
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I/O Protection
? All I/O instructions are privileged instructions.
? Must ensure that a user program could never gain control
of the computer in monitor mode (I.e., a user program
that, as part of its execution, stores a new address in the
interrupt vector).
Operating Systems
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Use of A System Call to Perform I/O
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Memory Protection
? Must provide memory protection at least for the interrupt
vector and the interrupt service routines.
? In order to have memory protection, add two registers
that determine the range of legal addresses a program
may access:
? Base register – holds the smallest legal physical memory
address.
? Limit register – contains the size of the range
? Memory outside the defined range is protected.
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Use of A Base and Limit Register
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Hardware Address Protection
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Hardware Protection
? When executing in monitor mode, the operating system
has unrestricted access to both monitor and user’s
memory.
? The load instructions for the base and limit registers are
privileged instructions.
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CPU Protection
? Timer – interrupts computer after specified period to
ensure operating system maintains control.
? Timer is decremented every clock tick.
? When timer reaches the value 0, an interrupt occurs.
? Timer commonly used to implement time sharing.
? Time also used to compute the current time.
? Load-timer is a privileged instruction.
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Network Structure
? Local Area Networks (LAN)
? Wide Area Networks (WAN)
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Local Area Network Structure
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Wide Area Network Structure
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Chapter 3: Operating-System Structures
? System Components
? Operating System Services
? System Calls
? System Programs
? System Structure
? Virtual Machines
? System Design and Implementation
? System Generation
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Common System Components
? Process Management
? Main Memory Management
? File Management
? I/O System Management
? Secondary Management
? Networking
? Protection System
? Command-Interpreter System
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Process Management
? A process is a program in execution. A process needs
certain resources, including CPU time, memory, files, and
I/O devices, to accomplish its task.
? The operating system is responsible for the following
activities in connection with process management.
? Process creation and deletion.
? process suspension and resumption.
? Provision of mechanisms for:
? process synchronization
? process communication
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Main-Memory Management
? Memory is a large array of words or bytes, each with its
own address. It is a repository of quickly accessible data
shared by the CPU and I/O devices.
? Main memory is a volatile storage device. It loses its
contents in the case of system failure.
? The operating system is responsible for the following
activities in connections with memory management:
? Keep track of which parts of memory are currently being
used and by whom.
? Decide which processes to load when memory space
becomes available.
? Allocate and deallocate memory space as needed.
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File Management
? A file is a collection of related information defined by its
creator. Commonly, files represent programs (both
source and object forms) and data.
? The operating system is responsible for the following
activities in connections with file management:
? File creation and deletion.
? Directory creation and deletion.
? Support of primitives for manipulating files and directories.
? Mapping files onto secondary storage.
? File backup on stable (nonvolatile) storage media.
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I/O System Management
? The I/O system consists of:
? A buffer-caching system
? A general device-driver interface
? Drivers for specific hardware devices
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Secondary-Storage Management
? Since main memory (primary storage) is volatile and too
small to accommodate all data and programs
permanently, the computer system must provide
secondary storage to back up main memory.
? Most modern computer systems use disks as the
principle on-line storage medium, for both programs and
data.
? The operating system is responsible for the following
activities in connection with disk management:
? Free space management
? Storage allocation
? Disk scheduling
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Networking (Distributed Systems)
? A distributed system is a collection processors that do not
?
?
?
?
share memory or a clock. Each processor has its own
local memory.
The processors in the system are connected through a
communication network.
Communication takes place using a protocol.
A distributed system provides user access to various
system resources.
Access to a shared resource allows:
? Computation speed-up
? Increased data availability
? Enhanced reliability
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Protection System
? Protection refers to a mechanism for controlling access
by programs, processes, or users to both system and
user resources.
? The protection mechanism must:
? distinguish between authorized and unauthorized usage.
? specify the controls to be imposed.
? provide a means of enforcement.
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Command-Interpreter System
? Many commands are given to the operating system by
control statements which deal with:
? process creation and management
? I/O handling
? secondary-storage management
? main-memory management
? file-system access
? protection
? networking
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Command-Interpreter System (Cont.)
? The program that reads and interprets control statements
is called variously:
? command-line interpreter
? shell (in UNIX)
Its function is to get and execute the next command
statement.
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Operating System Services
? Program execution – system capability to load a program into
memory and to run it.
? I/O operations – since user programs cannot execute I/O
operations directly, the operating system must provide some
means to perform I/O.
? File-system manipulation – program capability to read, write,
create, and delete files.
? Communications – exchange of information between processes
executing either on the same computer or on different systems
tied together by a network. Implemented via shared memory or
message passing.
? Error detection – ensure correct computing by detecting errors
in the CPU and memory hardware, in I/O devices, or in user
programs.
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Additional Operating System Functions
Additional functions exist not for helping the user, but rather
for ensuring efficient system operations.
• Resource allocation – allocating resources to multiple users
•
•
Operating Systems
or multiple jobs running at the same time.
Accounting – keep track of and record which users use how
much and what kinds of computer resources for account
billing or for accumulating usage statistics.
Protection – ensuring that all access to system resources is
controlled.
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System Calls
? System calls provide the interface between a running
program and the operating system.
? Generally available as assembly-language instructions.
? Languages defined to replace assembly language for
systems programming allow system calls to be made
directly (e.g., C, C++)
? Three general methods are used to pass parameters
between a running program and the operating system.
? Pass parameters in registers.
? Store the parameters in a table in memory, and the table
address is passed as a parameter in a register.
? Push (store) the parameters onto the stack by the program,
and pop off the stack by operating system.
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Passing of Parameters As A Table
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Types of System Calls
? Process control
? File management
? Device management
? Information maintenance
? Communications
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MS-DOS Execution
At System Start-up
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Running a Program
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UNIX Running Multiple Programs
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Communication Models
? Communication may take place using either message
passing or shared memory.
Msg Passing
Shared Memory
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System Programs
? System programs provide a convenient environment for
program development and execution. The 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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MS-DOS System 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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MS-DOS Layer Structure
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UNIX System Structure
? 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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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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An Operating System Layer
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OS/2 Layer Structure
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Microkernel System Structure
? Moves as much from the kernel into “user” space.
? 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
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Windows NT Client-Server Structure
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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 creates the illusion of multiple
processes, each executing on its own processor with its
own (virtual) memory.
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Virtual Machines (Cont.)
? The resources of the physical computer are shared to
create the virtual machines.
? CPU scheduling can create the appearance that users have
their own processor.
? Spooling and a file system can provide virtual card readers
and virtual line printers.
? A normal user time-sharing terminal serves as the virtual
machine operator’s console.
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System Models
Non-virtual Machine
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Virtual Machine
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Advantages/Disadvantages of Virtual Machines
? The virtual-machine concept provides complete
protection of system resources since each virtual
machine is isolated from all other virtual machines. This
isolation, however, permits no direct sharing of resources.
? A virtual-machine system is a perfect vehicle for
operating-systems research and development. System
development is done on the virtual machine, instead of on
a physical machine and so does not disrupt normal
system operation.
? The virtual machine concept is difficult to implement due
to the effort required to provide an exact duplicate to the
underlying machine.
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Java Virtual Machine
? Compiled Java programs are platform-neutral bytecodes
executed by a Java Virtual Machine (JVM).
? JVM consists of
- class loader
- class verifier
- runtime interpreter
? Just-In-Time (JIT) compilers increase performance
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Java Virtual Machine
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System Implementation
? Traditionally written in assembly language, operating
systems can now be written in higher-level languages.
? Code written in a high-level language:
? can be written faster.
? is more compact.
? is easier to understand and debug.
? An operating system is far easier to port (move to some
other hardware) if it is written in a high-level language.
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