7-2 CHARACTERISTICS OF OPERATING SYSTEM
As we have stated, operating systems are normally unique to their manufacturers
and the hardware in which they are run.Generally, when a new computer system
is installed, operational software suitable to that hardware is purchased.Users
want reliable operational software that can effectively support their processing
Though operational software varies between manufacturers,it has similar characteristics.Modern
hardware, because of its sophistication, requires that operating systems meet
certain specific standards.For example, considering the present state of the
field, an operating system must support some form of online processing.Functions
normally associated with operational software are:
1. Job management
2. Resource management
3. Control of I/O operations
5. Memory management
A very important responsibility of any operational software is the scheduling of jobs to be handled by a computer system.This is one of the main tasks of the job management function.The operating system sets up the order in which programs are processed, and defines the sequence in which particular jobs are executed.The term job queue is often used to describe the series of jobs awaiting execution.The operating system weighs a variety of factors in creating the job queue.These include which jobs are currently being processed, the system's resources being used,which resources will be needed to handle upcoming programs, the priority of the job compared to other tasks,and any special processing requirements to which the system must respond.
The operational software must be able to assess these factors and control the order in which jobs are processed.
The management of resources in a computer system is another major concern of the operating system.Obviously, a program cannot use a device if that hardware is unavailable.As we have seen, the operational software oversees the execution of all programs.It also monitors the devices being used.To accomplish this, it establishes a table in which programs are matched against the devices they are using or will use.The operating system checks this table to approve or deny use of a specific device.
CONTROL OF I/O OPERATIONS
Allocation of a system's resources is closely tied to the operational software's control of I/O operations.As access is often necessary to a particular device before I/O operations may begin, the operating system must coordinate I/O operations and the devices on which they are performed.In effect, it sets up a directory of programs undergoing execution and the devices they must use in completing I/O operations.Using control statements, jobs may call for specific devices.This lets users read data from specific sites or print information at selected offices.Taking advantage of this facility, data read from one location may be distributed throughout computerized system.
To facilitate execution of I/O operations,most operating systems have a standard
set of control instructions to handle the processing of all input and output
instructions.These standard instructions, referred to as the input/output control
system (IOCS), are an integral part of most operating systems.They simplify
the means by which all programs being processed may undertake I/O operations.
In effect, the program undergoing execution signals the operating system that
an I/O operation is desired, using a specific I/O device.The controlling software
calls on the IOCS software to actually complete the I/O operation.Considering
the level of I/O activity in most programs, the IOCS instructions are extremely