1 CHAPTERIntroductionPractice Exerc

1 CHAPTER
Introduction
Practice Exercises
1.1 What are the three main purposes of an operating system?
Answer:
• 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 What are themain differences between operating systems formainframe
computers and personal computers?
Answer: Generally, operating systems for batch systems have simpler
requirements than for personal computers. Batch systems do not have
to be concerned with interacting with a user as much as a personal
computer. As a result, an operating system for a PC must be concerned
with response time for an interactive user. Batch systems do not have
such requirements. A pure batch system also may have not to handle
time sharing,whereas an operating systemmust switch rapidly between
different jobs.
1.3 List the four steps that are necessary to run a program on a completely
dedicated machine.
Answer:
12 Chapter 1 Introduction
a. Reserve machine time.
b. Manually load program into memory.
c. Load starting address and begin execution.
d. Monitor and control execution of program from console.
1.4 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.
1.5 What is themain difficulty that a programmermust overcome inwriting
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
systemfor a real-time system, thewritermust be sure that his scheduling
schemes don’t allow response time to exceed the time constraint.
1.6 Consider the various definitions of operating system.Considerwhether
the operating systemshould include applications such asWeb browsers
andmail programs. Argue both that it should and that it should not, and
support your answer.
Answer: Point. Applications such as web browsers and email tools are
performing an increasingly important role inmodern desktop computer
systems. To fulfill this role, they should be incorporated as part of the
operating system. By doing so, they can provide better performance
and better integration with the rest of the system. In addition, these
important applications can have the same look-and-feel as the operating
system software.
Counterpoint. The fundamental role of the operating system is to man-
age system resources such as the CPU,memory, I/O devices, etc. In ad-
dition, it’s role is to run software applications such as web browsers and
email applications. By incorporating such applications into the operating
system, we burden the operating system with additional functionality.
Such a burdenmay result in the operating systemperforming a less-than-
satisfactory job at managing system resources. In addition, we increase
the size of the operating system thereby increasing the likelihood of
system crashes and security violations.
1.7 How does the distinction between kernelmode and usermode function
as a rudimentary form of protection (security) system?
Answer: The distinction between kernel mode and user mode pro-
vides a rudimentary formof protection in the followingmanner. Certain
instructions could be executed only when the CPU is in kernel mode.
Similarly, hardware devices could be accessed only when the program
is executing in kernel mode. Control over when interrupts could be en-Practice Exercises 3
abled 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.
1.8 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.
1.9 Some early computers protected the operating system by placing it in
a memory partition th