操作系统概念课后答案

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> chapter 1

1.1 in a multiprogramming and time-sharing environment,

several users share the system simultaneously. this situation

can result in various security problems. a. what are two such

problems?

b. can we ensure the same degree of security in a time-shared

machine as in a dedicated machine? explain your answer.

answer:

a. stealing or copying one’s programs or data; using system

resources (cpu, memory, disk space, peripherals) without

proper accounting.

b. probably not, since any protection scheme devised by

humans can inevitably be broken by a human, and the more

complex the scheme, the more difficult it is to feel confident of

its correct implementation.

1.2 the issue of resource utilization shows up in different

forms in different types of operating systems. list what

resources must be managed

carefully in the following settings: a. mainframe or

minicomputer systems b. workstations connected to serversc.

handheld computers

answer:

a. mainframes:memory and cpu resources, storage, network

bandwidth.

b. workstations: memory and cpu resouces

c. handheld computers: power consumption, memory

resources.

1.3 under what circumstances would a user be better off

using a timesharing system rather than a pc or single-user

workstation?

answer: when there are few other users, the task is large, and

the hardware is fast, time-sharingmakes sense. the full power

of the system can be brought to bear on the user’s problem.

the problemcan be solved faster than on a personal computer.

another case occurs when lots of other users need resources

at the same time.

a personal computer is best when the job is small enough to

be executed reasonably on it and when performance is

sufficient to execute the prog

ram to the user’s satisfaction.

1.4 which of the functionalities listed below need to be

supported by the operating system for the following two

settings: (a) handheld devices and (b) real-time systems. a.

batch programmingb. virtual memoryc. time sharing

answer: for real-time systems, the operating system needs to

support virtual memory

and time sharing in a fair manner. for handheld systems,the

operating system needs to provide virtual memory, but does

not need to provide time-sharing. batch programming is not

necessary in both settings.

1.5 describe the differences between symmetric and

asymmetric are three advantages and

one disadvantage of multiprocessor systems?

answer: symmetric multiprocessing treats all processors as

equals, and i/o can be processed on any cpu. asymmetric

multiprocessing has one master cpu and the remainder cpus

are slaves. the master distributes tasks among the slaves, and

i/o is usually done by the master only.

multiprocessors can save money by not duplicating power

supplies,housings, and peripherals. they can execute

programs more quickly and can have increased reliability. they

are also more complex in both hardware and software than

uniprocessor systems.

1.6 how do clustered systems differ from multiprocessor

systems? what is required for two machines belonging to a

cluster to cooperate to provide a highly available service?

answer: clustered systems are typically constructed by

combining multiple computers into a single system to perform

a computational task distributed across the cluster.

multiprocessor systems on the other hand could be a single

physical entity comprising of multiple cpus. a clustered

system is less tightly coupled than a multiprocessor

red systems communicate using messages,

while processors in a multiprocessor system could

communicate using shared memory.

in order for twomachines to provide a highly available service,

the state on the two machines should be replicated and should

be consistently updated. when one of the machines fail, the

other could then take-over the functionality of the failed

machine.

1.7 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 tha

t share

cooking 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 (i.e. it may request recipes) and as a server

(it may provide recipes.)

1.8 consider a computing cluster consisting of twonodes

running be two ways in which the cluster

software can manage access to the data on the disk. discuss

the benefits and disadvantages of each.

answer: consider the following two alternatives: asymmetric

clustering and parallel clustering. with asymmetric clustering,

one host runs the database application with the other host

simply monitoring it. if the server fails, the monitoring host

becomes the active server. this is appropriate for providing

redundancy. however, it does not utilize the potential

processing power of both hosts. with parallel clustering, the

database application can run in parallel on both hosts. the

difficulty implementing parallel clusters is providing some

form of distributed locking mechanism for files on the shared

disk.

1.9 how are network computers different from traditional

personal computers? describe some usage scenarios in which

it is advantageous to use network computers.

answer: a network computer relies on a centralized computer

for most of its services. it can therefore have a minimal

operating system to manage its resources. a personal

computer on the other hand has to be capable of providing all

of the required functionality in a standalonemanner without

relying on a centralized manner. scenarios where

administrative costs are high and where sharing leads to more

efficient use of resources are precisely those settings where

network computers are preferred.

1.10 what is the purpose of interrupts? what are the

differences between a trap and an interrupt? can traps be

generated intentionally by a user program? if so, for what

purpose?

answer: an interrupt is a hardware-generated change-of-flow

within the system. an interrupt handler is summoned to deal

with the cause of the interrupt; control is then returned to the

interrupted context and instruction. a trap is a software-

generated interrupt. an interrupt can be used to signal the

completion of an i/o to obviate the need for device polling. a

trap can be used to call operating system routines or to catch

arithmetic errors.

1.11 direct memory access is used for high-speed i/o devices

in order to avoid increasing the cpu′s execution load.

a. how does the cpu interface with the device to coordinate

the transfer? b. how does the cpu know when the memory

operations are complete?

c. the cpu is allowed to execute other programs while the dma

controller is

transferring data. does this process interfere with the

execution of the user programs? if so, describe what forms of

interference are caused.

answer: the cpu can initiate a dma operation by writing values

into special registers that can be independently accessed by

the device initiates the corresponding operation

once it receives a command from the cpu. when the device is

finished with its operation, it interrupts the cpu to indicate the

completion of the operation.

both the device and the cpu can be accessing memory

memory controller provides access to the

memory bus in a fair manner to these two entities.

a cpu might therefore be unable to issue memory operations

at peak speeds since it has to compete with the device in order

to obtain access to the memory bus.

1.12 some computer systems do not provide a privileged

mode of operation in hardware. is it possible to construct a

secure operating system for these computer systems? give

arguments both that it is and that it is not possible.

answer: an operating system for a machine of this type would

need to remain in control (or monitor mode) at all times. this

could be accomplished by two methods:

a. software interpretation of all user programs (like some

basic,java, and lisp systems, for example). the software

interpreter would provide, in software, what the hardware does

not provide.

b. require meant that all programs be written in high-level

languages so that all object code is compiler-produced. the

compiler would generate (either in-line or by function calls) the

protection checks that the hardware is missing.

1.13 give two reasons why caches are 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 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 omponent 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 systemswhere

more than one process may be accessing a

nent 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 retain data as

well), and (b) the cache is affordable, because faster storage

tends to be more expensive.

1.14 discuss, with examples, how the problem of maintaining

coherence of cached data manifests itself in the following

processing environments:

a. single- processor systems

b. multiprocessor systems

c. distributed systems

answer: in single-processor systems, the memory needs to

be updated when a processor issues updates to cached values.

these updates can be performed immediately or in a lazy

manner. in amultiprocessor system,different processors might

be caching the same memory location in its local caches. when

updates are made, the other cached locations need to be

invalidated or updated. in distributed systems, consistency of

cached memory values is not an issue. however, consistency

problems might arise when a client caches file data.

1.15 describe a mechanism for enforcing memory protection

in order to prevent a program from modifying the memory

associated with other programs.

answer: the processor could keep track of what locations are

associated with each process and limit access to locations

that are outside of a program’s extent. information regarding

the exten

t of a program’s memory could be maintained by

using base and limits registers and by performing a check for

every memory access.

1.16 what network configuration would best suit the following

environments? a. a dormitory floor

b. a university campus

c. a state

d. a nation

answer:

a. a dormitory floor - a lan.

b. a university campus - a lan, possible a wan for very large

campuses. c. a state - awan.

d. a nation - a wan.

1.17 define the essential properties of the following types of

operating systems: a. batch

b. interactive

c. time sharing

d. real time

e. network

f. parallel

g. distributed

h. clustered

i. handheld

【篇二：操作系统概念

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第六版

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重点部分

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中文答案

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urposes of an operating system?

1 to provide an environment for a computer user to execute

programs on computer hardware in a convenient and ef?cient

manner.

2 to allocate the separate resources of the computer as

needed to solve the problem given. the allocation process

should be as fair and ef?cient as possible.

3 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) manage-

ment of the operation and control of i/o devices.

?

环境提供者，为计算机用户提供一个环境，使得能够在计 算机硬

件上方便、高效的执行程序

?

资源分配者，为解决问题按需分配计

算机的资源，资源分配需尽可能公平、高效

?

控制程序

监控用户程序的执行，防止出错和对计算机的不正当使用管理

i /o

设

备的运行和控制

1.6 define the essential properties of the following types of

operating systems: a. batch

b. interactive c. time sharing d. real time e. network

f. distributed

a. batch. jobs with similar needs are batched together and run

through the computer as a group by an operator or automatic

job sequencer. performance is increased by attempting to keep

cpu and i/o devices busy at all times through buffering, off-line

operation, spooling, and multiprogramming. batch is good for

executing large jobs that need little interaction; it can be

submitted and picked up later.

b. interactive. this system is composed of many short

transactions where the results of the next transactionmay be

unpredictable. response time needs to be short (seconds)

since the user submits and waits for the result.

c. time stemsuses cpu scheduling and

multiprogramming to provide economical interactive use of a

system. the cpu switches rapidly from one user to

another. instead of having a job de?ned by spooled card

images, each program readsits next control card from the

terminal, and output is normally printed immediately to the

screen.

d. real time. often used in a dedicated application, this system

reads information from sensors and must respond within

a ?xed amount of time to ensure correct perfor- mance.

e. network.

f. system distributes computation among

several physical processors. the processors do not share

memory or a clock. instead, each processor has its own local

memory. they communicate with each other through various

communication lines, such as a high- speed bus or telephone

line.

a. batch

相似需求的

job

分批、成组的在计算机上执行，< /p>

job

由操作员或自

动

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程序装置装载；

可以通过采用

buffering, off-line operation, spooling,

multiprogramming

等技术使

cpu

和

i/o

不停忙来提高性能