SOLUTION MANUAL OF OPERATING
SYSTEM By ABRAHAM SILBERSCHATZ,
PETER BAER GALVIN GREG GAGNE
C P TER
Introduction
Practice Exercises
1.1 What are the three main purposes of an operating system?
Answer:
The three main puropses are:
• To provide an environment for a computer user to execute programs on computer hardware in a
convenient and efficient manner.
• To allocate the separate resources of the computer as needed to solve the problem given. The
allocation process should be as fair and efficient as possible.
• As a control program it serves two major functions: (1) supervision of the execution of user
programs to prevent errors and improper use of the computer, and (2) management of the
operation and control of I/O devices.
1.2 We have stressed the need for an operating system to make efficient use of the computing
hardware. When is it appropriate for the operating system to forsake this principle and to
“waste” resources? Why is such a system not really wasteful?
Answer:
Single-user systems should maximize use of the system for the user. A GUI might “waste” CPU
cycles, but it optimizes the user’s interaction with the system.
1.3 What is the main difficulty that a programmer must overcome in writing an operating system for a
real-time environment?
Answer:
The main difficulty is keeping the operating system within the fixed time constraints of a real-time
system. If the system does not complete a task in a certain time frame, it may cause a breakdown of
the entire system it is running. Therefore when writing an operating system for a real-time system,
the writer must be sure that his scheduling schemes don’t allow response time to exceed the time
, constraint.
Chapter 1 Introduction
1.4 Keeping in mind the various definitions of operating system, consider whether the operating
system should include applications such as Web browsers and mail programs. Argue both that it
should and that it should not, and support your answers.
Answer:
An argument in favor of including popular applications with the operating system is that if the
application is embedded within the operating system, it is likely to be better able to take
advantage of features in the kernel and therefore have performance advantages over an
application that runs outside of the kernel. Arguments against embedding applications within
the operating system typically dominate however: (1) the applications are applications - and not
part of an operating system, (2) any performance benefits of running within the kernel are
offset by security vulnerabilities, (3) it leads to a bloated operating system.
1.5 How does the distinction between kernel mode and user mode function as a rudimentary form
of protection (security) system?
Answer:
The distinction between kernel mode and user mode provides a rudi- mentary form of
protection in the following manner. Certain instructions could be executed only when the CPU is
in kernel mode. Similarly, hard- ware devices could be accessed only when the program is
executing in kernel mode. Control over when interrupts could be enabled or disabled is also
possible only when the CPU is in kernel mode. Consequently, the CPU has very limited capability
when executing in user mode, thereby enforcing protection of critical resources.
1.6 Which of the following instructions should be privileged?
a. Set value of timer.
b. Read the clock.
c. Clear memory.
d. Issue a trap instruction.
e. Turn off interrupts.
f. Modify entries in device-status table.
g. Switch from user to kernel mode.
h. Access I/O device.
Answer:
The following operations need to be privileged: Set value of timer, clear memory, turn off
interrupts, modify entries in device-status table, access I/O device. The rest can be performed in
user mode.
1.7 Some early computers protected the operating system by placing it in a memory partition that
could not be modified by either the user job or the operating system itself. Describe two
difficulties that you think could arise with such a scheme.
Answer:
Practice Exercises
The data required by the operating system (passwords, access controls, accounting information,
and so on) would have to be stored in or passed through unprotected memory and thus be
accessible to unauthorized users.
1.8 Some CPUs provide for more than two modes of operation. What are two possible uses of these
multiple modes?
Answer:
, Although most systems only distinguish between user and kernel modes, some CPUs have
supported multiple modes. Multiple modes could be used to provide a finer-grained security
policy. For example, rather than distinguishing between just user and kernel mode, you could
distinguish between different types of user mode. Perhaps users belonging to the same group
could execute each other’s code. The machine would go into a specified mode when one of these
users was running code. When the machine was in this mode, a member of the group could run
code belonging to anyone else in the group.
Another possibility would be to provide different distinctions within kernel code. For example, a
specific mode could allow USB device drivers to run. This would mean that USB devices could be
serviced without having to switch to kernel mode, thereby essentially allowing USB device drivers
to run in a quasi-user/kernel mode.
1.9 Timers could be used to compute the current time. Provide a short description of how this
could be accomplished.
Answer:
A program could use the following approach to compute the current time using timer interrupts.
The program could set a timer for some time in the future and go to sleep. When it is awakened by
the interrupt, it could update its local state, which it is using to keep track of the number of
interrupts it has received thus far. It could then repeat this process of continually setting timer
interrupts and updating its local state when the interrupts are actually raised.
1.10 Give two reasons why caches are useful. What problems do they solve? What problems do they
cause? If a cache can be made as large as the device for which it is caching (for instance, a cache as
large as a disk), why not make it that large and eliminate the device?
Answer:
Caches are useful when two or more components need to exchange data, and the components
perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of
intermediate speed between the components. If the fast device finds the data it needs in the cache,
it need not wait for the slower device. The data in the cache must be kept consistent with the data
in the components. If a component has a data value change, and the datum is also in the cache, the
cache must also be updated. This is especially a problem on multiprocessor systems where more
than one process may be accessing a datum. A component may be eliminated by an equal-sized
cache, but only if: (a) the cache and the component have equivalent state-saving capacity (that is,
if the component retains its data when electricity is removed, the cache must
Chapter 1 Introduction
retain data as well), and (b) the cache is affordable, because faster storage tends to be more
expensive.
1.11 Distinguish between the client– server and peer-to-peer models of distributed systems.
Answer:
The client-server model firmly distinguishes the roles of the client and server. Under this
model, the client requests services that are provided by the server. The peer-to-peer model
doesn’t have such strict roles. In fact, all nodes in the system are considered peers and thus may
act as either clients or servers— or both. A node may request a service from another peer, or the
node may in fact provide such a service to other peers in the system.
For example, let’s consider a system of nodes that share cooking recipes. Under the client-
server model, all recipes are stored with the server. If a client wishes to access a recipe, it must
request the recipe from the specified server. Using the peer-to-peer model, a peer node could ask
other peer nodes for the specified recipe. The node (or perhaps nodes) with the requested recipe
could provide it to the requesting node. Notice how each peer may act as both a client (it may
request recipes) and as a server (it may provide recipes).
, Operating- System
Structures
Practice Exercises
2.1 What is the purpose of system calls?
Answer:
System calls allow user-level processes to request services of the operat- ing system.
2.2 What are the five major activities of an operating system with regard to process management?
Answer:
The five major activities are:
a. The creation and deletion of both user and system processes
b. The suspension and resumption of processes
c. The provision of mechanisms for process synchronization
d. The provision of mechanisms for process communication
e. The provision of mechanisms for deadlock handling
2.3 What are the three major activities of an operating system with regard to memory management?
Answer:
The three major activities are:
a. Keep track of which parts of memory are currently being used and by whom.
b. Decide which processes are to be loaded into memory when memory space becomes
available.
c. Allocate and deallocate memory space as needed.
2.4 What are the three major activities of an operating system with regard to secondary-storage
management?
Answer:
The three major activities are:
SYSTEM By ABRAHAM SILBERSCHATZ,
PETER BAER GALVIN GREG GAGNE
C P TER
Introduction
Practice Exercises
1.1 What are the three main purposes of an operating system?
Answer:
The three main puropses are:
• To provide an environment for a computer user to execute programs on computer hardware in a
convenient and efficient manner.
• To allocate the separate resources of the computer as needed to solve the problem given. The
allocation process should be as fair and efficient as possible.
• As a control program it serves two major functions: (1) supervision of the execution of user
programs to prevent errors and improper use of the computer, and (2) management of the
operation and control of I/O devices.
1.2 We have stressed the need for an operating system to make efficient use of the computing
hardware. When is it appropriate for the operating system to forsake this principle and to
“waste” resources? Why is such a system not really wasteful?
Answer:
Single-user systems should maximize use of the system for the user. A GUI might “waste” CPU
cycles, but it optimizes the user’s interaction with the system.
1.3 What is the main difficulty that a programmer must overcome in writing an operating system for a
real-time environment?
Answer:
The main difficulty is keeping the operating system within the fixed time constraints of a real-time
system. If the system does not complete a task in a certain time frame, it may cause a breakdown of
the entire system it is running. Therefore when writing an operating system for a real-time system,
the writer must be sure that his scheduling schemes don’t allow response time to exceed the time
, constraint.
Chapter 1 Introduction
1.4 Keeping in mind the various definitions of operating system, consider whether the operating
system should include applications such as Web browsers and mail programs. Argue both that it
should and that it should not, and support your answers.
Answer:
An argument in favor of including popular applications with the operating system is that if the
application is embedded within the operating system, it is likely to be better able to take
advantage of features in the kernel and therefore have performance advantages over an
application that runs outside of the kernel. Arguments against embedding applications within
the operating system typically dominate however: (1) the applications are applications - and not
part of an operating system, (2) any performance benefits of running within the kernel are
offset by security vulnerabilities, (3) it leads to a bloated operating system.
1.5 How does the distinction between kernel mode and user mode function as a rudimentary form
of protection (security) system?
Answer:
The distinction between kernel mode and user mode provides a rudi- mentary form of
protection in the following manner. Certain instructions could be executed only when the CPU is
in kernel mode. Similarly, hard- ware devices could be accessed only when the program is
executing in kernel mode. Control over when interrupts could be enabled or disabled is also
possible only when the CPU is in kernel mode. Consequently, the CPU has very limited capability
when executing in user mode, thereby enforcing protection of critical resources.
1.6 Which of the following instructions should be privileged?
a. Set value of timer.
b. Read the clock.
c. Clear memory.
d. Issue a trap instruction.
e. Turn off interrupts.
f. Modify entries in device-status table.
g. Switch from user to kernel mode.
h. Access I/O device.
Answer:
The following operations need to be privileged: Set value of timer, clear memory, turn off
interrupts, modify entries in device-status table, access I/O device. The rest can be performed in
user mode.
1.7 Some early computers protected the operating system by placing it in a memory partition that
could not be modified by either the user job or the operating system itself. Describe two
difficulties that you think could arise with such a scheme.
Answer:
Practice Exercises
The data required by the operating system (passwords, access controls, accounting information,
and so on) would have to be stored in or passed through unprotected memory and thus be
accessible to unauthorized users.
1.8 Some CPUs provide for more than two modes of operation. What are two possible uses of these
multiple modes?
Answer:
, Although most systems only distinguish between user and kernel modes, some CPUs have
supported multiple modes. Multiple modes could be used to provide a finer-grained security
policy. For example, rather than distinguishing between just user and kernel mode, you could
distinguish between different types of user mode. Perhaps users belonging to the same group
could execute each other’s code. The machine would go into a specified mode when one of these
users was running code. When the machine was in this mode, a member of the group could run
code belonging to anyone else in the group.
Another possibility would be to provide different distinctions within kernel code. For example, a
specific mode could allow USB device drivers to run. This would mean that USB devices could be
serviced without having to switch to kernel mode, thereby essentially allowing USB device drivers
to run in a quasi-user/kernel mode.
1.9 Timers could be used to compute the current time. Provide a short description of how this
could be accomplished.
Answer:
A program could use the following approach to compute the current time using timer interrupts.
The program could set a timer for some time in the future and go to sleep. When it is awakened by
the interrupt, it could update its local state, which it is using to keep track of the number of
interrupts it has received thus far. It could then repeat this process of continually setting timer
interrupts and updating its local state when the interrupts are actually raised.
1.10 Give two reasons why caches are useful. What problems do they solve? What problems do they
cause? If a cache can be made as large as the device for which it is caching (for instance, a cache as
large as a disk), why not make it that large and eliminate the device?
Answer:
Caches are useful when two or more components need to exchange data, and the components
perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of
intermediate speed between the components. If the fast device finds the data it needs in the cache,
it need not wait for the slower device. The data in the cache must be kept consistent with the data
in the components. If a component has a data value change, and the datum is also in the cache, the
cache must also be updated. This is especially a problem on multiprocessor systems where more
than one process may be accessing a datum. A component may be eliminated by an equal-sized
cache, but only if: (a) the cache and the component have equivalent state-saving capacity (that is,
if the component retains its data when electricity is removed, the cache must
Chapter 1 Introduction
retain data as well), and (b) the cache is affordable, because faster storage tends to be more
expensive.
1.11 Distinguish between the client– server and peer-to-peer models of distributed systems.
Answer:
The client-server model firmly distinguishes the roles of the client and server. Under this
model, the client requests services that are provided by the server. The peer-to-peer model
doesn’t have such strict roles. In fact, all nodes in the system are considered peers and thus may
act as either clients or servers— or both. A node may request a service from another peer, or the
node may in fact provide such a service to other peers in the system.
For example, let’s consider a system of nodes that share cooking recipes. Under the client-
server model, all recipes are stored with the server. If a client wishes to access a recipe, it must
request the recipe from the specified server. Using the peer-to-peer model, a peer node could ask
other peer nodes for the specified recipe. The node (or perhaps nodes) with the requested recipe
could provide it to the requesting node. Notice how each peer may act as both a client (it may
request recipes) and as a server (it may provide recipes).
, Operating- System
Structures
Practice Exercises
2.1 What is the purpose of system calls?
Answer:
System calls allow user-level processes to request services of the operat- ing system.
2.2 What are the five major activities of an operating system with regard to process management?
Answer:
The five major activities are:
a. The creation and deletion of both user and system processes
b. The suspension and resumption of processes
c. The provision of mechanisms for process synchronization
d. The provision of mechanisms for process communication
e. The provision of mechanisms for deadlock handling
2.3 What are the three major activities of an operating system with regard to memory management?
Answer:
The three major activities are:
a. Keep track of which parts of memory are currently being used and by whom.
b. Decide which processes are to be loaded into memory when memory space becomes
available.
c. Allocate and deallocate memory space as needed.
2.4 What are the three major activities of an operating system with regard to secondary-storage
management?
Answer:
The three major activities are:
