Objectives of an operating system

Answer 1

1 Objectives and functions

1.1 OS as a user/computer interface - Usability

The reason for an operation system to exist is to make computers more convenient to use. An

OS aims to wrap the underneath hardware resources and provides services to end users in a

systematic way. These services may be divided into two types: services directly available for

end users through all kinds of I/O devices, such as mouse, keyboard, monitor, printer, and so

on; and services for application programs, which in turn provides services for end users.

If we look on these services as interfaces among different components of a computer system,

then the following hierarchical architecture may be obtained:

Users Programmer

| | |

v | |

+---------------+ | |

| Applications | v |

+-------------------------+ | OS designer

| Utilities | v |

+------------------------------+ |

| OS | V

+-----------------------------------+

| Hardware |

+-----------------------------------+

It is also common to consider Utilities and Applications that are distributed together

with an OS parts of the OS, but obviously they are not essential. Utilities are usually called

libraries or APIs, providing frequently used functions for the upper-level applications.

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From the point of view of end users, a computer system consists of a variety of applications

they may use. An application is developed by programmers in a programming language. The

higher level the utilities are, the easier and more comfortable it is for programmers to code in

the corresponding programming language; or the lower, the harder. In an extreme case, the

assembly language is almost the same as machine instructions except that mnemonic symbols

are used to replace binary strings or opcodes. In this kind of language, programmers have to

deal with an overwhelmingly complexity of manipulating computer hardware. On the contrary,

in a higher-level language, more user-friendly APIs are available, e.g. opening a file by calling

open("C:/filename.txt", "rw")

1.2 OS as resource manager - Efficiency

It is not the OS itself but the hardware that makes all kinds of services possible and available

to application programs. An OS merely exploits the hardware to provide easily accessible

interfaces. Exploitation means management upon the hardware resources, and thus also imposes

control upon or manages the entities that use the services so that the resources are used

efficiently. In the classes later on, we will discuss this aspect, including process scheduling,

memory management, I/O device management, etc.

One thing worth mentioning here is that, different from other control systems where the controlling

facility, the controller, is distinct and external to the controlled parts, the OS has to

depend on the hardware resources it manages to work.

+-------------------+

+------------+ | |

| | | Controlled |

| Controller |-------->| components |

| | | |

+------------+ +-------------------+

As we know, an OS is in nature a program, consisting instructions, thus it also needs CPU to

execute instructions so as to function as a controller, and main memory to hold instructions

for CPU to fetch. At the same time, the OS has to be able to relinquish and regain later

the control of CPU so that other programs can get chance to run but still under the control

of the OS (An analogy to this is that an administrator of an apartment building might live

in the same building himself). By utilizing the facilities provided by hardware, the OS may

schedule different processes to run at different moments and exchange the instructions and

data of programs between external storage devices, like hard disks, and main memory. These

topics will be covered as the course proceeds.

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1.3 Evolution of OS - Maintainability

It does not suffice to simply consider an operating system an unvariable unit. An OS may

evolve while time elapses due to the following reasons:

• hardware upgrades or new types of hardware: With hardware technologies development,

the OS also needs to upgrade so as to utilize the new mechanisms introduced

by new hardware. For example, Pentium IV extended instruction set of Pentium III

for multimedia applications and internet transmission. An OS designed for the previous

versions of Intel x86 series will have to be upgraded to be able to accommodate these

new instructions.

• new services: An OS may also expand to include more services in response to user

demand.

• fixes: No software is perfect, and any program may contain more or less bugs or defects,

thus fixes should be made from time to time. Microsoft Windows is a vivid example of

this kind.

These situations all require OS designers to build an OS in the way that the system can be

maintained and upgraded easily. All the common software design techniques may be applied

to an OS, such as modularization. With modularization, the OS is split into multiple modules

with clearly defined interfaces between them. Thus, as long as the interfaces are left untouched,

each single module may be upgraded independently.