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قديم 07-08-2011, 12:36 AM   #29
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افتراضي رد: M350 - Chapter 1: Introduction


Single-Processor systems

Most systems use a single processor which has one main CPU talent --> executing a general-purpose instruction set, including instructions from user processes

If there is only one general-purpose CPU, then the system is a single-processor system

------
Almost all systems have other special-purpose processors : like

come in the form of device-specific processors, such as disk, keyboard

come in the form of more general-purpose processors

-------
All of these special-purpose processors run a limited instruction set and do not run user processes

The use of special-purpose microprocessors is common and does not turn a single-processor system into a multiprocessor

=============================================

التعديل الأخير تم بواسطة nil ; 07-08-2011 الساعة 12:38 AM
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قديم 07-08-2011, 01:07 AM   #30
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افتراضي رد: M350 - Chapter 1: Introduction


Multiprocessor Systems = Parallel Processing Systems = Tightly coupled systems

Multiprocessor Systems

Multiprocessor systems have three main advantages: are

Increased throughput: By increasing the number of processors, we expect to get more work done in less time

Economy of scale: Multiprocessor systems can cost less than equivalent multiple single-processor systems, because they can share peripherals, mass storage, and power supplies

Increased reliability: If functions can be distributed properly among several processors, then the failure of one processor will not halt the system, only slow it down (processors share the work of the failed processor)

--------------
gracfull degradation: the ability to continue providing service relative to the level of surviving hardware.

Some systems go beond gracfull degradation : called
fault tolerant : they can suffer a failure of any single component and still continue operations but they need special mechanism to allow failure detect.


-------------

The multiple-processor systems in use today are of two types: are

asymmetric multiprocessing:each processor is assigned a specific task. A master processor controls the system; the other processors either look to the master for instruction or have predefined tasks. This scheme defines a master-slave relationship



symmetric multiprocessing (SMP): each processor performs all tasks within the OS. SMP means that all processors are peers; no master-slave relationship exists between processors

---------------------------

التعديل الأخير تم بواسطة nil ; 07-08-2011 الساعة 01:20 AM سبب آخر: add pictures
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قديم 07-08-2011, 01:20 AM   #31
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افتراضي رد: M350 - Chapter 1: Introduction


multiprocessing can cause a system to change it's memory access model from (UMA) uniform memory access to (NUMA) non-uniform memory access

UMA: the situatuion in which access to any RAM from CPU takes the same ammount time

multiple compute cores on a single chip is

efficient than multipla chips , faster , less power and well suited for servers system
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قديم 07-08-2011, 01:27 AM   #32
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افتراضي رد: M350 - Chapter 1: Introduction


blade servers: are a recent development in wich multiprocessor boards , I/O boards and networking boards are placed in the same frame.

these servers consist of multible independent multiprocessors systems - each one run it's own operating system
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قديم 07-08-2011, 02:06 AM   #33
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افتراضي رد: M350 - Chapter 1: Introduction


Clustered Systems

Like multiprocessor systems, but multiple systems working together ,Usually sharing storage via a storage-area network (SAN) or (LAN)

clustor Provides a high-availability service which survives failures

Asymmetric clustering has one machine in hot-stand by mode

hot-stand by mode monitor the active server if that server fails , the hot-standby host becomes the active srever

Symmetric clustering has multiple nodes running applications, monitoring each other - it more eficient because it use all available hardware

Some clusters are for high-performance computing (HPC)wich give more power to use

Paralleization : dividing a program into seperate components that run in parallel on individual computers in the cluster. each node solve its portion of the problem.

Other advanced form of clusters, cluster includes paralleled-clusters over WAN, these allow multiple host to access the same data on the shared storage, via Distributed Lock Manager (DLM), to ensure no conflict.



SANs : storage area networks
SANs allow many system to attach to group of storage shareing with each other.
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قديم 07-08-2011, 06:09 AM   #34
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افتراضي رد: M350 - Chapter 1: Introduction


Operating-System Structure

A time-sharing ( multi-user multi-tasking ) OS : requires

Memory management , Process management , Job scheduling , Resource allocation strategies , Swap space / virtual memory in physical memory , Interrupt handling , File system management , Protection and security , Inter-process communications

=========================================

Multiprogramming

Single user cannot keep CPU and I/O devices busy at all times even if he has multible programs running

Operating System has ability to multiprograming: because

it increase CPU utilization by organizes jobs (code and data) so CPU always has one to execute. the operation done by subset of total jobs in system is kept in main memory (main memory too small to accommodate all) in job pool



Job Pool: consists all of all processes residing on disk awaiting allocation of main memory

after that operating picks and begins to excute One job selected and run via job scheduling. When it has to wait (for I/O for example), OS switches to another job . when first job finish waiting and get the CPU back so it will keep busy

=============================================

Time-sharing (multitasking) is logical extension in which CPU executes multiple jobs by switching among them so frequently, that users can interact with each program while it is running

sharing require interactive (hands on computer system) direct communication between user and system

Response time : time to do operation should be less than 1 second

Time shared operating system : As the system switches rapidly from one user to the next, each user is given the impression that the entire computer system is dedicated to his use, even though it is being shared among many users

Processes : program are swapped in and out of main memory to the disk and excuted . In effect, we are now "memory sharing" between competing users (programs). This idea leads to a mechanism called virtual memory

Virtual memory : is a technique that allows the execution of a process that is not completely in memory. The main advantage of the virtual-memory scheme is that it enables users to run programs that are larger than actual physical memory

------------

If several jobs are ready to be brought into memory, and if there is not enough room for all of them, then the system must choose among them. Making this decision is job scheduling

When the OS selects a job from the job pool, it loads that job into memory for execution. Having several programs in memory at the same time requires some form of memory management

In addition, if several jobs are ready to run at the same time, the system must choose among them. Making this decision is CPU scheduling not job scheduling be noticed

CPU scheduling will : make these things
abstracts main memory into a large
uniform array of storage
separating logical memory as viewed by the user from physical memory

This arrangement , make programmers free and they will not worry about over memory-storage limitations

==========================================
Time-sharing systems provide a file system exist in a collection of disks

time-sharing systems provide a mechanism for protecting resources from inappropriate use

To ensure orderly execution, the system must provide mechanisms for job synchronization and communication, and it may ensure that jobs do not get stuck in a deadlock, forever waiting for one another

================================================== =====

التعديل الأخير تم بواسطة nil ; 07-08-2011 الساعة 06:13 AM سبب آخر: arrnging
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قديم 07-08-2011, 06:27 AM   #35
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افتراضي رد: M350 - Chapter 1: Introduction


Operating-System Operations

Modern OSs are interrupt driven. If there are no processes to execute, no I/O devices to service, and no users to whom to respond, an OS will sit quietly, waiting for something to happen

Events are signaled by the appearing of an interrupt or a trap

trap (or an exception): is a software-generated interrupt caused either by an error (for example, division by zero or invalid memory access) or by a specific request from a user program that an operating-system service be performed

With sharing, many processes could be badly affected by a bug in one program. More errors can occur in a multiprogramming system also

So , without protection against these sort of errors either the computer must excute only one process or all output must be suspect

==================================================

we can ensure computer is protected and controlled by : Dual mode operation & Timmer

================================================
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قديم 07-08-2011, 06:50 AM   #36
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افتراضي رد: M350 - Chapter 1: Introduction


Dual-Mode Operation : Transition from user to kernel mode


we need two separate modes of operation: user mode and (kernel mode=supervisor mode=system mode=privileged mode)

kernek mode: referred to as system mode, is one of the two distinct modes of operation of the CPU (central processing unit)
user mode: a non-privileged mode for user programs, that is, for everything other than the kernel

Mode bit : to indicate the current mode we use a bit (0,1): like
kernel (0) or user (1)

When a user application requests a service from the OS (via a system call), it must transition from user to kernel mode to fulfil the request



====================================
privileged instructions : machine instructions that may cause harm
====================================

I/O protection: all I/O operations are privileged; so user programs can only access I/O by sending a request to the (controlling) OS


Memory protection: make limitation to registers , memory management unit; so user programs can only access the memory that the OS has allocated


CPU control: control timer (alarm clock), context switch; so user programs can only read the time of day, and can only have as much CPU time as the OS allocates

==================================

The dual mode of operation provides us with the means for protecting the OS from errant users: how???

If an attempt to execute a privileged instruction in user mode, the hardware does not execute the instruction but rather treats it as illegal and traps it to the OS ,control is switched to kernel mode

================================================== ===
Some hardware protection technique

System calls provide the means for a user program to ask the OS to perform tasks reserved for the OS

A system call usually takes the form of a trap to a specific location in the interrupt vector and from interrupt vector to a service routine in the OS, and the mode bit is set to kernel mode.

================================================== ======

التعديل الأخير تم بواسطة nil ; 07-08-2011 الساعة 06:57 AM
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قديم 07-08-2011, 07:03 AM   #37
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افتراضي رد: M350 - Chapter 1: Introduction


Timer

We must ensure that the OS control the CPU ( pervent getting stuck in an infinite loop or not calling system services ) we can use a timer

timer : can be set to interrupt the computer after a specified period

==============================================

timer technique :

Set up before scheduling process to regain control or terminate program that exceeds a specified time

Set interrupt after specific period

Every time the clocks ticks, Operating system decrements counter

When counter zero, generate an interrupt
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قديم 07-08-2011, 07:29 AM   #38
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افتراضي رد: M350 - Chapter 1: Introduction


===============================================
Process Management

A process needs certain resources-including CPU time, memory, files, and I/O devices-to accomplish its task

===============================================
program by itself is not a process; a program is a passive entity

process = unit of work in a system (an active entity)

system = collection of processes

-------------------

operating-system processes = execute system code
user processes = those that execute user code

------------------
single-threaded :process has one program counter specifying the next instruction to execute .The CPU executes one instruction of the process after another(sequential), until the process completes.

multithreaded process : has multiple program counters, each pointing to the next instruction to execute
-----------------

All these processes can potentially execute concurrently by multiplexing the CPU among them on a single CPU.
===============================================

process management activities: are

Creating and deleting both user and system processes
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling

========================================
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قديم 07-08-2011, 07:40 AM   #39
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افتراضي رد: M350 - Chapter 1: Introduction


================================================== =====

Memory Management

In a very simple OS, only one program at a time is in the memory. To run second program, the first one has to be removed and the second one placed in memory (Single Tasking System)

More sophisticated OSs allow multiple programs to be in memory at the same time(Multi Tasking System)

Main memory is a large array of words or bytes with open range each word or byte has its own address.

There are different memory-management Schemas reflect various approaches and algorithms depends on many factors – especially the H/W design

================================================== =====
memory management activities: are

Keeping track of which parts of memory are currently being used and by whom

Deciding which processes (or parts thereof) and data to move into and out of memory

Allocating and deallocating memory space as needed

================================================
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قديم 07-08-2011, 07:44 AM   #40
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افتراضي رد: M350 - Chapter 1: Introduction


======================================

Storage Management

To make the computer system convenient for users: the OS

provides logical view of information storage
abstracts from the physical properties of its storage devices to define a logical storage unit, the file
maps files onto physical media and accesses these files via the storage devices

================================================

Subsections : are

File-System Management
Mass-Storage Management
Caching
I/O Systems

================================================
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قديم 07-08-2011, 08:29 AM   #41
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افتراضي رد: M350 - Chapter 1: Introduction


File-System Management


File management is the most visible components of an OS. Computers can store information on several different types of physical media

Each mode is controlled by a device and has it own unique properties, such as a disk drive

These properties include access speed, capacity, data-transfer rate, and access method (sequential or random)

file : is a collection of related information defined by its creator. Commonly, files represent programs and data

Block : special files are used to model devices that consist of a collection of randomly addressable blocks, such as disks

Character : special files are used to model printers, modems, and other devices that accept or output character stream

---------------------------------------------------

activities of file management: are

Creating and deleting files

Creating and deleting directories to organize files

Supporting primitives for manipulating files and directories

Mapping files onto secondary storage

Backing up files on stable (nonvolatile) storage media

================================================== =====

Mass-Storage Management

computer system must provide secondary storage to back up main memory : because

main memory is too small to accommodate all data and programs, and the data are lost when power is lost

The correct management of disk storage is : central importance to a computer system
--------------------------------------------

activities of disk management:

Free-space management
Storage allocation
Disk scheduling

----------------------------------
Because secondary storage is used frequently, it must be used efficiently

The entire speed of operation of a computer may turning on the speeds : of the disk subsystem and algorithms that manipulate that subsystem

================================================== ======

Caching

Caching is an important principle of computer systems , performed at many levels in a computer (in hardware, operating system, software)

When we need a particular piece of information, we first check whether it is in the cache. If it is, we use the information directly from the cache; if it is not, we use the information from the source, putting a copy in the cache under the assumption that we will need it again soon


There are caches implemented in hardware , Without this cache, the CPU would have to wait several cycles while an instruction was fetched from main memory

Because caches have limited size, cache management is an important design problem

storage performance comparison in large workstations and small servers that shows the need for caching.



The movement of information between levels of a storage hierarchy may be either explicit or implicit

Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the
most recent value in their cache

Distributed environment situation even more complex



================================================== ===============

I/O Systems
One of the purposes of an OS is to hide the peculiarities of specific hardware devices from the user

------------------------------------------------

The I/O subsystem consists of several components: like

A memory-management component that includes buffering, caching, and spooling

A general device-driver interface, that is, applies to many or all I/O devices equally well

Drivers for specific hardware devices

Only the device driver knows the peculiarities of the specific device to which it is assigned

================================================== =============
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افتراضي رد: M350 - Chapter 1: Introduction


========================================
Protection and Security

Protection : any mechanism for controlling access of processes or users to resources defined by the computer system (authorized/unauthorized usage)

Security : defense the system against internal and external attacks like:
Huge range, including denial-of-service, worms, viruses, identity theft, theft of service

-------------------------

Systems generally first distinguish among all it's users : by

User identities (user IDs, security SID) include name and associated number, one per user

User ID then associated with all files, processes of that user to determine access control

Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file

Privilege escalation to gain extra permission for an activity, allows user to change to effective UID with more rights

=====================================
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