CALectureWeek8.ppt

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CARC103 – Computer Architecture
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Prescribed Text
Bird, S. D. (2017), Systems Architecture, 7th ed, Cengage Learning
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Systems Architecture,

Seventh Edition
Chapter 11
Operating Systems
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Systems Architecture, Seventh Edition

Chapter Objectives
In this chapter, you will learn to:
Describe the functions and layers of an operating system
List the resources allocated by the operating system and describe the allocation process
Explain how an operating system manages processes and threads
Compare CPU scheduling methods
Explain how an operating system manages memory

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Systems Architecture, Seventh Edition

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FIGURE 11.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Operating Systems – Why Do They Exist?
Early computers didn’t need operating systems because:
They were only able to execute one program at a time in batch mode
The set of hardware resources was relatively simple
Users were highly trained (they had to be!)
Increasing hardware complexity created a chain of events:
Hardware resources became more numerous and varied (e.g., multiple storage and I/O devices
More complex and diverse hardware enabled more powerful programs but they were more difficult to write because of all the hardware control and interface issues
Hardware became powerful enough to run multiple programs at the same time but there was no easy way to keep them from interfering with one another
Programmers found themselves writing code to do the same sorts of things over and over again, such as:
Read or write data from files stored in disk
Interact with printers and other I/O devices
Manage memory contents (e.g., implementing buffers)

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Systems Architecture, Seventh Edition

Operating System Functions
Operating systems address the issues on the previous slide by:
Serving as a central point of control over hardware and software resources (e.g., install, execute, and delete application programs)
Coordinating access to shared resources, for example:
Load two or more programs into memory at the same time and prevent them from accessing each others’ memory
Coordinate access to shared I/O devices such as printers
Coordinate access to shared storage (i.e., a common directory structure)
Implementing access controls and authentication (usernames, passwords, and permission to run programs and to access files and I/O devices)
Providing a set of utility programs to perform functions need by multiple application programs, for example:
Managing I/O buffers
Reading from and writing to files on disk
Sending packets to/from the network
Implementing common “look and feel” features of a graphical user interface

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Systems Architecture, Seventh Edition

OS Management Function Summary
FIGURE 11.2 OS management functions
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Operating System Layers
Kernel
Directly interacts with hardware via device drivers and “logical” commands
Allocates hardware resources to users and programs and controls access
Service Layer – A large set of utility functions, subroutines, or methods that application programs “call” to perform common tasks such as:
File I/O
Network interaction
Window management
Command layer – The common user interface elements and set of tools to manage hardware and software, for example:
Usually a graphical user interface (GUI) though command languages are also used
Programs and tools to manage files (Windows Explorer)
Tools to add or update hardware and software (Windows update, Add/remove hardware devices, …)
Tools to execute application programs (e.g., double-click on an icon on the desktop or in Explorer)

FIGURE 11.3 OS layers
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Service Layer
As computer hardware and related capabilities such as graphical user interfaces have grown more complex so have the number and complexity of service layer functions
Modern OSs provide hundreds or thousands of different service layer functions
The service layer is a form of code reuse
Application programs perform I/O and other functions by calling a subroutine or method that already exists in the OS – the application programmer reuses program code embedded in the operating system
Application programs are simpler – thus, cheaper to develop and update
Application programmers need fewer skills to write programs
The service layer also places interaction with important resources under exclusive control of the OS – required if the OS is to correctly manage use of those resources and their underlying hardware
For example, application programs are prevented from interfering with each others files because all file functions are routed through the service layer which in turn relies on kernel resource allocation functions

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Systems Architecture, Seventh Edition

Kernel and Device Drivers
The hardware interface functions of the kernel are performed by a set of device drivers – one or more per device:
Device drivers enable the service layer to “talk” to multiple devices of the same type in the same way
For example, all disk drives “look” the same to file I/O functions – a linear address space with basic read/write commands
Device drivers can be updated without changing any other OS or application components
This enables OS functions to be extended to new or “improved” hardware

FIGURE 11.4 Components of the kernel
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Device Drivers – Windows Properties
FIGURE 11.5 Windows device driver properties
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Exercise
On a Windows computer do the following:
Right-click My Computer and select Properties
Select Device Manager
Expand the category “Sound, video, and game controllers (click the + sign)
Right-click on the audio device and select Properties
Examine the contents of the three tabs, especially the one titled Driver

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Systems Architecture, Seventh Edition

Resource Allocation
Resource allocation is a needed function whenever multiple “things” compete for access to a limited set of resources, for example:
Departments and employees competing for budget
Packages competing for space within trucks, planes, and warehouses
Vehicles competing for access to roadways and intersections
Resource allocation is the process of deciding:
Who gets to use what resource
How much of resource they get
When they get it
When they have to give it up
Complexity increases as the number and diversity of resources and competitors rises
For example, consider the complexity of budgeting and room scheduling in a 4-room elementary school vs. a university

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Systems Architecture, Seventh Edition

Single-Tasking Resource Allocation
If a computer supports only one running application at a time it is said to be single-tasking
Single-tasking resource allocation is simple since there’s little competition for hardware resources
Two possible ways to “implement” resource allocation:

Let the application programs do it themselves (the approach taken in the earliest computers)
Provide a simple OS to manage resources shared by multiple programs over time – e.g., files on disk (the approach taken in computers of the 1950s and early 1960s)
In the latter case there are two real-time “competitors” for resources:
The OS itself
Whatever application program is currently running

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Systems Architecture, Seventh Edition

Multitasking and Resource Allocation
A multitasking environment enables multiple application programs to “run” concurrently (at the same time)
Early OSs didn’t support multitasking because hardware resources were too limited to support multiple executing programs (e.g., how many running programs can you fit in 4 KBytes of RAM?)
As hardware improved, multitasking became possible and desirable
Multitasking “ups the ante” for OS resource allocation
Decisions must ensure that programs don’t “hurt” one another
Elaborate “accounting methods” are required to keep track of which resources are free, which are allocated, and to which programs
Decisions about allocating scare resources such as CPU time and memory must be made and changed quickly (dynamically in real time)
The application environment changes rapidly so decisions and “accounting” updates must also happen rapidly
Decisions must be made in such a way that they don’t impact overall performance – for example, what is the “best” use of the CPU at this precise moment

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Systems Architecture, Seventh Edition

Resource Allocation as Management
One of the most important tasks performed by managers in a modern organization is resource allocation:
Managers either make the allocation decisions or set the policies that drive manual or automated allocation decisions
Managers are responsible for “accounting” for resources though that task is usually delegated to supporting personnel or systems
Key management-related questions in organizational design include:
How many managers are needed and how should work be distributed among them?
What resources are used by management (a.k.a, management or system overhead) as opposed to the parts of the organization that do the “real work”
Some key management goals include:
Minimize resources consumed by management functions to maximize those available for real work
Have “just enough” management to ensure that organizational components doing the real work don’t interfere with one another
Make decision that maximize productivity of the organization as a whole

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Systems Architecture, Seventh Edition

Exercise
Consider a two-person catering company with one truck and one small kitchen
How often do “orders” arrive?
What resources do the people use to “fill” the orders?
How do they coordinate their use of resources?
Consider a large kitchen in a busy restaurant with hundreds of customers at a time and dozens of employees
How often do orders arrive?
What resources are used to do “fill” the orders?
How do employees coordinate their use of resources?

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Systems Architecture, Seventh Edition

Real and Virtual Resources
A real resource physically exists (e.g., my laptop has one dual core CPU and 4 GB of RAM)
A virtual resource “looks” real to the user (program) but may or may not physically exist:
My laptop is concurrently executing ten programs – each program “thinks” it has its own CPU
The sum of memory each program “thinks” it has is 10 GB
How is that possible?
A single program can be actively running, idle, or waiting – it needs few/no real resources in a non-running state
Real resources can be rapidly shifted among programs as they enter/leave a running state
To the extent possible, cheaper resources can be substituted for more expensive ones
For example, part of the instructions or data that program #6 “thinks” are stored in RAM are actually on disk

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Systems Architecture, Seventh Edition

Real and Virtual Resources – Continued
Advantages of virtual resource management
More tasks are completed because:
More programs (workers) are present
Resources are more efficiently used (e.g., when one program is suspended another uses the CPU)
Programs are simpler:
For example, Microsoft Word doesn’t need any internal code to handle:
“How to hand off the CPU to another program when I’m idle?”, or
“What do I do when I want to write to disk but another program is using it?”
Programs (and their programmers) simply assume that the resources they need will be available when needed
Scarce and/or expensive resources can be applied to their “highest purpose” and shifted rapidly in response to changing demands – for example,
The OS decides which programs have disk I/O cached in RAM for rapid access and which don’t
When total memory demand exceeds supply the OS decides which parts of what programs stay in RAM and which are swapped to disk
When multiple programs want to print output the OS decides who’s first

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Systems Architecture, Seventh Edition

Taking Virtualization to the Next Level
An OS virtualizes CPU, memory, secondary storage, and I/O resources for programs
Each resource is virtualized individually
For example, a program may have exclusive control of one CPU but some of its allocated memory content may actually be stored on disk
A hypervisor is an OS that virtualizes a complete set of hardware resources collectively
Each collection of virtualized hardware resources is called a virtual machine (VM)
A traditional OS such as Linux or Windows is installed on each VM

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Systems Architecture, Seventh Edition

Hypervisors
With a hypervisor, there are two layers of virtualization

The hypervisor virtualizes an entire computer system or cluster and divides it into VMs – the hypervisor and entire computer system are the “host”
“Client” OSs control each VM and virtualize individual resources for programs running within the VM
FIGURE 11.4
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Hypervisor Types and Applications
A bare-metal hypervisor installs directly on the computer system or cluster
VMware ESX, Microsoft Hyper-V, and Citrix Xen are examples
The hypervisor is a complete OS, though it is specialized to creating and managing VMs and is “bare bones” in most other respects
Used for applications such as:
Server consolidation – one hardware platform supports multiple virtual servers
Desktop virtualization – one hardware platform supports multiple “desktop” VMs
A virtualization environment is an application that installs under a traditional OS and enables VMs to be created and run within it
VMware Workstation and Microsoft Virtual PC are examples
It runs as an application, though typically with elevated privileges/priority
Used for applications such as:
Multi-platform software development and testing
Running two OSs on the same desktop or laptop machine – for example, VMware Fusion

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Systems Architecture, Seventh Edition

Process Management
From the OS’s point of view, programs (including the OS itself) are divided into processes to which virtual resources are allocated
Processes are further subdivided into threads – more on this in a few slides
As programs are started the OS adds information about their processes to a process list or process queue, which stores information such as:
Identification – a process number
Who owns the process
What state the process is in (e.g., running or idle)
What priority the process has for resource allocation

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Systems Architecture, Seventh Edition

Exercise
On a Windows machine do the following:
Start -> Run, enter taskmgr, press enter (or, Ctrl-Alt-Delete and select Start Task Manager)
Click the Processes tab and click the check box to show process from all users
Right-click on the wininit.exe process, note the contents of the pop-up menu, and select Open File Location. Note the location within C: of the file, then close then Explorer window.
Click View on the Task Manager menu bar, select Columns, and check a few of the unselected columns – e.g.,
CPU Time
I/O Read Bytes
I/O Write Bytes
I/O Other Bytes
Which processes have consumed the most CPU resources?
Which processes have consumed the most I/O resources?

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Systems Architecture, Seventh Edition

Process Groups and Architecture
A process is unit of software that can be in a running or idle state and to which any virtual resource can be allocated
A process can create, or spawn, other processes by calling the appropriate OS service layer function
A parent process spawns one or more child processes – the entire group of related processes is a process family
Spawning enables programs to subdivide themselves into independent units and minimize their resource footprint:
Functions that aren’t needed all the time are omitted from the parent process
When the function needs to be performed the parent process spawns a child process to do it
Resources for the function aren’t allocated until the child process is spawned
Resources are freed when the child process exits
This is a modular architecture for building and executing large programs
The alternative, a.k.a. monolithic architecture:
Requires more resources (the ones allocated to idle modules)
Makes program design much more complex
Makes program update more complex (can’t swap out smaller modules)

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Systems Architecture, Seventh Edition

Threads
Subdividing programs into larger and larger numbers of smaller and smaller process yields diminishing returns because:
The process list grows very large – inefficient due to the need for frequent updates
Resource allocation decision complexity increases as the number of “competitors” for the resources increases
System overhead rises, reducing resources available to do the “real work”
To allow further subdivision of programs/processes without the above problems, most OSs use process components called threads
A thread is:
A separate unit of executable code
Owned and controlled by a process
A “lightweight” process:
Allocated CPU time by the OS
Shares other resources (e.g., memory, open files, network ports) with its parent process and other related threads
Managing threads is less complex than managing processes because less information is tracked for them and fewer resource types are allocated to them

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Systems Architecture, Seventh Edition

Threads and Parallelism
A process that can spawn and concurrently execute multiple threads is aid to be multithreaded
Multithreaded processes (and the OSs that support them) can improve performance through parallelism, for example:
A web server executes a process that listens for connection requests from users
When a connection request is received it spawns a new thread to handle that connection/user
The OS allocates CPU and memory resources to a thread/connection when it’s busy and to other threads when it’s idle
When the connection is terminated so is the thread
Another example:
A global-scale weather forecast is subdivided into regional forecasts which are later combined into the global forecast
The regional forecasts are spawned as threads
Each thread executes concurrently on its own CPU(s) of a single computer system or node(s) of a cluster

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Systems Architecture, Seventh Edition

CPU Allocation
Processes and threads can only progress toward completion when they are actively executing instructions on a CPU

A multitasking OS has dozens to thousands of threads competing for a small number of CPUs
The maximum number of threads actually executing at one time is limited to the number of CPUs
A larger number may be “active”
The OS rapidly shifts the CPU among active threads so that all “appear” to be advancing toward completion – this is called concurrent execution or interleaved execution

FIGURE 11.7 Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Thread States
Threads move among three states:
Ready – Not currently executing on a CPU but can be allocated to one
Running – Actively executing on a CPU
Blocked – Waiting for a resource or for an error to be resolved, thus not eligible for the running state
The OS dispatches a thread from the ready state to the running state

FIGURE 11.8 Thread movement between states
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Interrupts and Thread States
A thread automatically moves out of the running state when an interrupt is detected
Which state it moves to depends on the type of interrupt:
Timer interrupt – generated automatically by hardware every N clock cycles – provides an opportunity for the OS to dispatch another thread or return the same thread back to the running state
Service call – implies a resource request by the thread – thread moves to the blocked state unless the resource can be immediately provided
Error condition – If caused by the thread then it’s blocked until the error can be resolved – If not caused by thread (e.g., UPS) then thread moves to ready state
Peripheral device – Usually indicates completion of a previous processing request – currently running thread moves to ready state and thread(s) that were waiting for the peripheral device move from blocked to ready state
Regardless of the interrupt source/type, every interrupt provides an opportunity to the OS to dispatch a different thread than was running when the interrupt occurred – this is known generically as preemptive scheduling

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Systems Architecture, Seventh Edition

Preemptive Scheduling
Processing steps on left occur after Thread 1 makes an I/O service call
Processing steps on right occur after I/O device completes I/O operation

FIGURE 11.9 Interrupt processing
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Scheduling
How does the OS decide what thread(s) will be dispatched (scheduled) next?
Or, stated another way, on what bases is the dispatching decision made?
There are two general scheduling methods and variations on each:
Priority-based
Real-time

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Systems Architecture, Seventh Edition

Priority-Based Scheduling
Priority-based scheduling determines which thread(s) in the running are most important and dispatches them next
The decision about which thread is most important can be based on multiple criteria:
Explicit priority
Each thread has a priority level stored in the thread list
The priority may be based on the thread owner, thread type, or other criteria
Dynamic priorities can change during thread life
First-come first-served (FCFS)
Whichever thread is “first in line” is dispatched next
FCFS can be combined with explicit priority to determine which thread among those with equal priority is next dispatched – this is the most common scheduling method in modern OSs
Shortest time remaining
Thread closest to completion is dispatched next
Difficult to implement since the OS must “guess” how long the thread will run and track its progress
Rarely implemented on modern OSs

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Systems Architecture, Seventh Edition

Windows Thread Priorities
Table 11.1 Thread priority classes and levels
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Exercise
On a Windows machine
Start task manager
Click the Processes tab
Click the button or check box to show processes from all users
Right-click on winlogin.exe, mouse over Set Priority, and view the current priority
Check a few other processes
Download PVIEW.EXE and start it
Select the process winlogon and examine the number and priority of each thread
Do the same check for a few other processes

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Systems Architecture, Seventh Edition

Real-Time Scheduling
Real-time scheduling guarantees that a process will receive whatever resources (including CPU cycles) are needed to “keep up with its workload”
A thread cycle is one unit of “work” performed by the thread (e.g., process one transaction or data sample)
OS must know the maximum number of CPU cycles required to complete one thread cycle and must track consumption
Few OSs support real-time scheduling due to the complexity involved

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Systems Architecture, Seventh Edition

Memory Allocation
Like CPU cycles, memory is a resource that must be allocated to each process
In multitasking OSs, memory is divided into fixed-size partitions and one or more partitions are allocated to each active process

FIGURE 11.15 Several processes and the OS loaded into 250 MB fixed-size memory partitions
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Memory Partition Tables
As with other resources, the OS must track which memory resources are allocated to which processes, using a partition table

Table 11.2 Fixed-size memory partitions maintained by the operating system
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Memory Fragmentation
Processes change over time:
Start, execute, terminate
Grow and shrink
As processes change over time the memory allocated to them becomes fragmented:
Memory partitions allocated to a process become scattered throughout physical memory
Fragmentation is a performance problem for OSs that allocate memory contiguously – corrected through compaction which is slow
OSs that allocate memory non-contiguously don’t need to compact memory

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Systems Architecture, Seventh Edition

Memory Fragmentation and Compaction
Figure 11.16 Memory fragments as processes execute and terminate
Courtesy of Course Technology/Cengage Learning
Figure 11.17 Compaction combines multiple free space fragments (a) into a single contiguous partition (b)
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Non-Contiguous Memory Allocation
Non-contiguous memory allocation divides processes into fixed-size chinks (same size as memory partitions) and allocates partitions to processes wherever they exist in memory
Process chunks may be scattered throughout memory, out-of-order, and intermixed with chunks of other processes
Each process chunk has its own memory offset to implement relative addressing

FIGURE 11.18 Process 6 is allocated to non-contiguous memory partitions
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Virtual Memory Management
A single-threaded process:
May be spread over many memory partitions
Only one process instruction at a time can be executed
Thus, only one of the process’ allocated memory partitions is “active” at any one time
Virtual memory management (VMM) is a memory allocation and management technique that takes advantage of the above fact:
Memory partitions are allocated to process chunks as before, but
Process chunks may be temporarily written to disk when not in “active” use

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Systems Architecture, Seventh Edition

VMM Terminology
Processes are divided into pages – fixed size chunks, e.6., 4 KB
Memory is divided into page-sized chunks called page frames – same size as process pages
A page table stores information about all pages of a single process

Table 11.3 Portion of a process’s page table
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Page Swapping
When a process references a memory address the OS “calculates” in which page the referenced location exists, for example
Loading a 4 byte instruction at addresses 500210-500510 is a reference to the 5th – 8th byte of page 2 if page size is 4KB (409610 bytes)
The OS then looks up the page in the process’ page table:
If memory status is “in memory” then the access is a page hit
If memory status is “on disk” then the access is a page miss

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Systems Architecture, Seventh Edition

Page Swapping – Continued
A page miss generates an interrupt which suspends the current process and moves it to the blocked state
The interrupt handler that is called performs a page swap:
A page in memory is removed to make room for the needed page
The chosen page is called the victim
The victim may be a page belonging to another process
Choice of the victim can be based on the least recently used or least frequently used rule
If the modification status of the victim is:
Yes – The page in memory is written to disk (an area called the swap space, swap file, or page file) and the corresponding page table is updated
No – No write to disk is required but the page table must still be updated
Once the victim has been dealt with:
The accessed page is copied into the victim’s former frame
The page table is updated accordingly
The process that made the access is moved from blocked to running

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Systems Architecture, Seventh Edition

Exercise
On a Windows machine:
Right-click My Computer and select Properties
Select Advanced system settings
Click the Advanced tab and click Settings in the Performance group
Click the Advanced tab
Click Change in the Virtual memory area
Examine but don’t change the settings

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Systems Architecture, Seventh Edition

Memory Protection
Memory protection refers to OS and/or hardware protection of memory allocated to one process from reads and (especially) writes from other processes
Memory protection is both a security and reliability issue:
Prevent processes from reading important data or overwriting data/instructions in other processes (e.g., a virus)
Prevent one process from …

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