Download Operating System Architecture

計算機概論
作業系統
(Operating Systems)
楊舜仁
清大資工系
References
• J. Glenn Brookshear, "Computer Science - An
Overview", Addison-Wesley
• Abraham Silberschatz, Peter B. Galvin, and Greg
Gagne, "Operating System Concepts", Wiley
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
What is an Operating System?
An operating system is the software that
controls the overall operation of a computer.
It:
– provides the means by which a user can store
and retrieve files,
– provides the interface by which a user can
request the execution of programs, and
– provides the environment necessary to
execute the programs requested.
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
Batch Processing (1/2)
• The computers of the 1940s and 1950s were not
very flexible or efficient.
• Anyone wanting a program run was required to
submit it, along with
– any required data and
– special directions about the program’s requirements
to the computer and return later for the results.
• The programs were collected in a single batch,
and then executed without further interaction with
the user.
Batch Processing (2/2)
The Drawback of Batch Processing
• A major drawback of batch processing is that
– the users have no interaction with their jobs once they
are submitted to the computer.
• This approach is acceptable for applications,
such as payroll processing,
– in which the data and all processing decisions are
established in advance.
• However, it is not acceptable when the user must
interact with a program during its execution.
– Examples include word processing systems and
computer games.
Interactive Processing (1/2)
• To allow a program being executed to carry on a
dialogue with the user—a feature known as
interactive processing,
– the actions of the computer should be sufficiently fast
to coordinate with the needs of the user
– rather than forcing the user to conform to the
machine’s timetable
• In a sense, the computer is forced to execute
tasks under a deadline,
– a process that became known as real-time
processing
• in which the actions performed are said to occur in real-time.
Interactive Processing (2/2)
Time-sharing/Multitasking (1/2)
• When several users seek interactive service from
a machine at the same time, real-time
considerations presented obstacles.
– If the operating system insisted on executing only one
job at a time,
• only one user would receive satisfactory real-time service.
• The solution to this problem was
– to design operating systems that provided service to
multiple users at the same time:
• a feature called time-sharing, applying the technique called
multiprogramming.
Time-sharing/Multitasking (2/2)
• In multiprogramming:
– time is divided into intervals and then the execution of
each job is restricted to only one interval at a time;
– at the end of each interval, the current job is
temporarily set aside and another is allowed to
execute during the next interval;
– by rapidly shuffling the jobs back and forth in this
manner, the illusion of several jobs executing
simultaneously is created.
• Today, multiprogramming techniques are used in
single-user as well, called multitasking:
– referring to one user executing numerous tasks
simultaneously
Multiprocessor Machines
• The development of multiprocessor machines
has led to operating systems that provide timesharing/multitasking capabilities by
– assigning different tasks to different processors
– sharing the time of each single processor
• These operating systems must wrestle with such
problems as
– load balancing: dynamically allocating tasks to the
various processors
– scaling: breaking tasks into a number of subtasks
compatible with the number of processors available
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
Software Classification
Types of Software
• Application software
– Performs specific tasks for users
– Examples: spreadsheets, database systems,
accounting systems, program development software,
and games.
• System software
– Performs those tasks that are common to computer
systems in general
• Provides infrastructure for application software
– Consists of the operating system and utility software
(e.g., software for playing multimedia presentations
and handling network communication)
• Unfortunately, the distinction between software
can be vague.
Operating System Components
• Shell or User Interface: Communicates with users
– Text based
– Graphical user interface (GUI)
• Kernel: Performs basic required functions
– File manager
– Memory manager
– Device drivers
– Scheduler and dispatcher
• Scheduler: determines which activities are to be
considered for execution;
• Dispatcher: controls the allocation of time to the activities
The shell as an interface between
users and the operating system
File Manager
• Its job is to coordinate the use of the machine’s
mass storage facilities. It “records”
– Where each file is located
– Which users are allowed to access the various files
– Which portions of mass storage are available for new files
or extensions to existing files
• The procedure of any access to a file by other
software units:
– requesting that the file manager grant access to the file
through a procedure known as opening the file
– if the file manager approves the requested access, it
provides:
• the information needed to find and to manipulate the file
Memory Manager (1/2)
• For simple batch processing:
– the program for performing the current task is placed
at a predetermined location in main memory, executed,
– and then replaced by the program for performing the
next task
• However, for multiuser or multitasking
– many programs and blocks of data must reside in
main memory concurrently
– the memory manager must
• find and assign memory space for these needs and
• ensure that the actions of each program are restricted to the
program’s allotted space
– the memory manager must also keep track of those
memory areas no longer occupied
Memory Manager (2/2)
• The total main memory space required may exceed
the space actually available in the computer.
• In this case, the memory manager may create the
illusion of a large virtual memory space by using a
technique called paging:
– the memory manager reserves some storage space on a
magnetic disk
– the data is divided into uniform sized units called pages,
which are typically a few KB in size
– then, the memory manager shuffles these pages back
and forth between main memory and mass storage
• so that the pages that are needed at any given time are actually
present in the “real” main memory
Device Drivers (1/2)
• Device drivers are the software units that
communicate with the controllers to carry out
operations on the peripheral devices.
• Each device driver is uniquely designed for its
particular type of device (such as a printer, disk
drive, or monitor):
– translates generic requests into the more technical steps
required by the device assigned to that driver
– e.g., reading and decoding a printer’s status word as well
as all the other handshaking details
Communicating with Other Devices
The Role of Controllers (1/2)
• Communication between a computer and other
devices is normally handled
– through an intermediary apparatus known as a controller.
• A controller may
– consist of circuitry permanently mounted on the
computer’s motherboard or,
– it may take the form of a circuit board that plugs into a slot
on the motherboard
• The controller connects via cables
– to peripheral devices within the computer case or
– perhaps to a connector, called a port, on the back of the
computer where external devices can be attached.
The Role of Controllers (2/2)
• These controllers are sometimes small computers
themselves,
– each with its own memory circuitry and simple CPU
• A controller translates messages and data back and
forth between forms compatible with
– the internal characteristics of the computer and
– those of the peripheral device to which it is attached
• Each controller communicates with the computer
itself
– by means of connections to the same bus that connects
the computer’s CPU and main memory
Handshaking (1/2)
• Handshaking: a constant two-way dialogue,
in which
– the computer and the peripheral device
exchange information about the device’s status
and coordinate their activities
• Handshaking often involves a status word:
– which is a bit pattern that is generated by the
peripheral device and sent to the controller;
– the bits reflect the conditions of the device
Handshaking (2/2)
• For example, in the case of a printer,
– the least significant bit of the status word may
indicate whether the printer is out of paper
– while the next bit may indicate whether the
printer is ready for additional data
• The controller may respond to this status
information itself or make it available to the
CPU.
Device Drivers (1/2)
• Device drivers are the software units that
communicate with the controllers to carry out
operations on the peripheral devices.
• Each device driver is uniquely designed for its
particular type of device (such as a printer, disk
drive, or monitor):
– translates generic requests into the more technical steps
required by the device assigned to that driver
– e.g., reading and decoding a printer’s status word as well
as all the other handshaking details
Device Drivers (2/2)
• The other software components can merely rely on
the device drivers to take care of the details.
– In this manner, the design of the other software units can
be independent of the unique characteristics of particular
devices.
Getting it Started
• How does the operating system get started?
– through a procedure known as boot strapping
(often shortened to booting)
• that is performed by a computer each time it is turned
on
• To understand the boot strap process and
the reason it is necessary, we begin by
considering the machine’s CPU:
– A CPU is designed so that its program counter
starts with a particular predetermined address
each time the CPU is turned on.
The 1st Approach
• How about storing the operating system at
this predetermined memory address the
program counter points to?
• Unfortunately, for technical reasons, a
computer’s main memory (RAM) is typically
constructed from volatile technologies:
– the memory loses the data stored in it when the
computer is turned off
– the contents of main memory must be
replenished each time the computer is restarted
The 2nd Approach
• How about storing the operating system at
nonvolatile memory cells
– known as read-only memory (ROM) because its
contents can be read but not altered
• However, devoting large blocks of main
memory to nonvolatile storage is not efficient
with today’s technology.
• Moreover, computer operating systems
undergo frequent updates
– for security and device drivers of the latest
hardware.
The 3rd Approach: Boot Strapping
• A program called the boot loader is permanently
stored in the machine’s ROM.
– the program that is initially executed when the machine is
turned on
• The boot loader:
1. first directs the CPU to transfer the operating system
from a predetermined location (e.g., disk storage) into
the main memory, in particular, the RAM;
2. then, directs the CPU to execute a jump instruction to
that area of memory to launch the operating system.
• The overall process of executing the boot loader
and thus starting the operating system is called
booting the computer.
The booting process
Firmware
• The boot loader is a type of firmware.
• The term firmware was coined to describe the
middle ground between hardware and software.
– Firmware is the software stored in nonvolatile memory
such as FlashROM.
– not immutably etched into the silicon of the machine—the
hardware
– also not as readily changeable as the rest of the
programs in mass storage—the software
• BIOS (Basic Input/Output System) is also a long
used firmware system
– used by the boot loader to perform I/O activities before
the operating system becomes functional
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
The Concept of a Process (1/2)
• Program vs. Process:
– Program: a static set of directions
– Process: the activity of executing a program
• a dynamic activity whose properties change as time
progresses
– Analogy: a piece of sheet music vs. a musician
performing that piece
The Concept of a Process (2/2)
• Process State: Current status of the activity
– the value of the program counter
• the current position in the program being executed
– the values in the other CPU registers
– the associated memory cells
The process state is a snapshot of the
machine at a particular time.
Process Competition/Administration
• Typical time-sharing/multitasking computers are
running many processes
– all competing for the computer’s resources
• It is the task of the operating system (the
scheduler and dispatcher) to manage these
processes so that:
– each process has the resources (peripheral devices,
space in main memory, access to files, and access to
a CPU) that it needs;
– independent processes do not interfere with one
another;
– processes that need to exchange information are able
to do so
The Scheduler (1/2)
• To keep track of all the processes:
– the scheduler maintains a block of information in main
memory called the process table.
• Each time the execution of a program is
requested
– the scheduler creates a new entry for that process in
the process table
• This table entry contains such information as
– the memory area assigned to the process (obtained
from the memory manager),
– the priority of the process, and
– whether the process is ready or waiting.
The Scheduler (2/2)
• Ready vs. waiting
– A process is ready if it is in a state in which its
progress can continue.
– A process is waiting if its progress is currently
delayed until some external event occurs, e.g.,
• the completion of a mass storage operation,
• the pressing of a key at the keyboard, or
• the arrival of a message from another process.
The Dispatcher
• The dispatcher is the component that controls
the allocation of time slices to the processes in
the process table.
• In a time-sharing/multitasking system, this task is
accomplished by multiprogramming:
1. dividing time into short segments, each called a time
slice
2. then switching the CPU’s attention among the
processes as each is allowed to execute for one
time slice
The procedure of changing from one process to
another is called a process switch (or a
context switch).
Time-sharing between process A and
process B
Interrupt Handling (1/2)
• Each time the dispatcher awards a time slice to a
process,
– it initiates a timer circuit that will indicate the end of the
slice by generating a signal called an interrupt.
• An interrupt is a signal to the processor emitted by
hardware or software indicating an event that needs
immediate attention.
– Other applications of a computer’s interrupt system:
• clicking a mouse
• pressing a key on the keyboard
• if the computer’s power is unexpectedly disrupted
– All these cases generate interrupt signals that cause the
CPU to set aside its current activity and address the cause
of the interrupt.
Interrupt Handling (2/2)
• Interrupt handling:
– When receiving an interrupt signal, the CPU
1. completes its current machine cycle
2. saves the current process’s state (for later context
switch-back)
3. begins execution of a program, called an interrupt
handler, which preempts the current process and
transfers control back to the dispatcher
– The dispatcher then
1. selects the highest-priority ready process from the
process table
2. restarts the timer circuit and begin the next time slice
Performance of Multiprogramming
• The use of multiprogramming has been found
to increase the overall efficiency of a machine
– although the shuffling of processes required by
multiprogramming introduces an overhead.
• If a process executes an I/O request
 identified as “waiting” in the process table
 the dispatcher will cease to award time slices to that
process
 progress on other tasks will be made while the I/O
request is being performed
 the entire collection of tasks will be completed in
less time
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
Allocation of a Machine’s
Resources
• An important task of an operating system
is the allocation of the machine’s resources
to the processes in the system.
– the file manager allocates access to files and
allocates mass storage space for the
construction of new files;
– the memory manager allocates memory space;
– the scheduler allocates space in the process
table;
– the dispatcher allocates time slices.
Access to a Printer
• If a process needs to print its results, it must
request that the operating system give it access
to the printer’s device driver.
• The operating system must decide whether to
grant this request:
– depending on whether the printer is already being
used by another process.
– If it is not, the operating system should
• grant the request and allow the process to continue;
– otherwise, the operating system should
• deny the request
• classify the process as a waiting process until the printer
becomes available
A Simple Flag System (1/2)
• To control access to the printer,
– the operating system must keep track of
whether the printer has been allocated.
• One approach is to use a flag, a bit in
memory whose states are referred to as
set and clear:
– a clear flag (value 0) indicates that
• the printer is available
– a set flag (value 1) indicates that
• the printer is currently allocated
A Simple Flag System (2/2)
• The operating system merely checks the
flag each time a request for printer access
is made.
– If it is clear, the request is granted and the
operating system sets the flag.
– If the flag is set, the operating system makes
the requesting process wait.
• Each time a process finishes with the printer, the
operating system
– either allocates the printer to a waiting process
– or, if no process is waiting, merely clears the flag
A Problem of this Simple Flag
System (1/4)
• The task of testing and possibly setting the
flag may require several machine
instructions.
– The value of the flag must be
• retrieved from main memory,
• manipulated within the CPU, and
• finally stored back in memory.
• It is therefore possible for a task to be
interrupted
– after a clear flag has been detected but
– before the flag has been set.
A Problem of this Simple Flag
System (2/4)
• In particular, suppose the printer is
currently available, and a process requests
use of it.
– The flag is retrieved from main memory and
found to be clear.
• However, at this point,
– the process is interrupted and
– another process begins its time slice, who too
requests the use of the printer.
– Again, the flag is retrieved from main memory
and found still clear.
A Problem of this Simple Flag
System (3/4)
• Consequently, the operating system allows the
second process to begin using the printer.
• Later, the original process resumes execution
where it left off,
– which is immediately after the operating system found
the flag to be clear.
• Thus the operating system continues by
– setting the flag in main memory and
– granting the original process access to the printer.
=> Two processes are now using the same
printer!
A Problem of this Simple Flag
System (4/4)
• The solution to this problem is to insist that
– the task of testing and possibly setting the flag
be completed without interruption.
The 1st Approach: Interrupt
Enable/Disable Instructions
• Using the interrupt disable and interrupt enable
instructions provided in most machine languages.
– an interrupt disable instruction causes future interrupts
to be blocked,
– an interrupt enable instruction causes the CPU to
resume responding to interrupt signals.
• Thus, if the operating system
1. starts the flag-testing routine with a disable interrupt
instruction and
2. ends it with an enable interrupt instruction
no other activity can interrupt the routine once it
starts.
The 2nd Approach: test-and-set
Instruction
• Using the test-and-set instruction that is
available in many machine languages.
• This instruction directs the CPU to
1. retrieve the value of a flag,
2. note the value received, and then
3. set the flag
all within a single machine instruction.
• Because the CPU always completes an
instruction before recognizing an interrupt,
– the task of testing and setting the flag cannot be split
when it is implemented as a single instruction.
Semaphore
• A properly implemented flag, as just described, is
called a semaphore,
– in reference to the railroad signals used to control
access to sections of track.
• Semaphores are used in software systems in
much the same way as they are in railway
systems:
– In railway systems: the section of track that can
contain only one train at a time
– In software systems: a sequence of instructions that
should be executed by only one process at a time.
Critical Region and Mutual
Exclusion (1/2)
• In software systems, a sequence of instructions
that
– should be executed by only one process at a time
is called a critical region.
• The requirement that
– only one process at a time be allowed to execute a
critical region
is known as mutual exclusion.
• A common way of obtaining mutual exclusion to a
critical region is to
– guard the critical region with a semaphore.
Critical Region and Mutual
Exclusion (2/2)
1. To enter the critical region, a process must
– find the semaphore clear and
– then set the semaphore before entering the
critical region;
2. Upon exiting the critical region, the
process must clear the semaphore.
3. If the semaphore is found in its set state,
the process trying to enter the critical
region must
– wait until the semaphore has been cleared.
Deadlock
• Deadlock: the condition in which two or more
processes are blocked from progressing
because
– each is waiting for a resource that is allocated to
another.
• For example: in systems in which processes are
allowed to create new processes to perform
subtasks, if
– the scheduler has no space left in the process table and
– each process in the system must create an additional process
before it can complete its task,
then no process can continue.
A deadlock resulting from competition for
nonshareable railroad intersections
Deadlock Conditions
• Deadlock cannot occur unless all three of
the following conditions are satisfied:
1. There is competition for nonshareable
resources.
2. The resources are requested on a partial
basis; that is,
•
having received some resources, a process will
return later to request more.
3. Once a resource has been allocated, it
cannot be forcibly retrieved.
Deadlock Detection and Correction
• Use of deadlock detection and correction
schemes to attack the third condition
– the occurrence of such deadlock is considered
so remote
– no effort is made to avoid the problem
• The approach is to detect it should it occur
and then correct it by forcibly retrieving
some of the allocated resources
– If deadlock should occur due to a full process
table, administrator can use his/her powers as
“super-user” to “kill” some of the processes.
Deadlock Avoidance
• Use of deadlock avoidance schemes to
attack the first two conditions.
– Condition 2: Requiring each process to
request all its resources at one time
– Condition 1: Converting nonshareable
resources into shareable ones
• e.g., the “spooling” technique of holding data for
output at a later but more convenient time
An Example of Spooling
• Suppose the resource is a printer and a
variety of processes require its use.
• Instead of connecting the process to the
printer’s device driver,
– connect it to a device driver that stores the
information to be printed in mass storage
– the nonshareable resource appears shareable
under the illusion of more than one printer
• Later, when the printer is available,
– the operating system could transfer the data
from mass storage to the printer
Outline
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The History of Operating Systems
Operating System Architecture
Coordinating the Machine’s Activities
Handling Competition Among Processes
Security
Security
• Since the operating system oversees the
activities in a computer, it is natural for it to
play a vital role in maintaining security.
• This responsibility manifests itself in
multiple forms, one of which is reliability.
– a flaw in the file manager causes the loss of
part of a file
– a defect in the dispatcher leads to a system
failure causing the loss of an hour’s worth of
typing
Attacks from outside (2/2)
• Access by unauthorized personnel
– establishing “accounts” for the authorized users
• an account: being essentially a record containing the
user’s name, password, and privileges
– login procedure to control access to the system
• Auditing software: record and analyze the
activities taking place within the system
– attempts to login using incorrect passwords
– activities within a user’s account that do not
conform to that user’s past behavior
– the presence of sniffing software
Attacks from outside (2/2)
– the presence of sniffing software
• e.g., a program that simulates the operating system’s
login procedure and trick authorized users into
thinking they are communicating with the operating
system
• The carelessness of the users themselves
– easy to guess passwords
– share their passwords with friends
– fail to change their passwords on a timely basis
– import unapproved software
– transfer mass storage devices between
machines
Attacks from within
• If an intruder gains access to a general
user’s account,
– to trick the operating system into allowing the
intruder to reach beyond the privileges granted
to that user
– allowing a process to access main memory
cells outside its allotted area (memory
manager)
– retrieving files whose access should be denied
(file manager)
– etc.
An Example: Memory Access
Restriction (1/2)
• Restrict a process to the area of main
memory assigned to it by the memory
manager
– special-purpose registers that store the upper
and lower limits of a process’s allotted memory
area.
• If the reference is found to be outside the
process’s designated area,
– the CPU automatically transfers control back to
the operating system
• by performing an interrupt sequence
An Example: Memory Access
Restriction (2/2)
• Unfortunately, without further security
features, a process could still gain access
to memory cells outside of its designated
area merely
– by changing the special-purpose registers that
contain its memory limits.
Privilege Levels and Privileged
Instructions (1/2)
• To protect against such actions, CPUs are
designed to operate in one of two privilege levels:
– in privileged mode:
• the CPU is able to execute all the instructions in its machine
language
– in nonprivileged mode:
• the list of acceptable instructions is limited
• The instructions that are available only in
privileged mode are called privileged instructions.
Examples include instructions that
– change the contents of memory limit registers;
– change the current privilege mode of the CPU;
Privilege Levels and Privileged
Instructions (2/2)
• When first turned on, the CPU is in privileged
mode.
– Thus, when the operating system starts at the end
of the boot process, all instructions are executable.
• Each time the operating system allows a
process to start a time slice,
– it switches the CPU to nonprivileged mode by
executing a “change privilege mode” instruction
– the operating system will be notified if the process
attempts to execute a privileged instruction