Software of Safety Important I&C Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 154

are normally given as those pertain to processes
and products. Functional requirements as a rule
pertain to software and to the product, although
they can determine some of the requirements for
the process of software creation with consideration
of specific features of the design, the tools used
and so forth.
Below we examine the classification and
perform an analysis of the general requirements.
Results of the classification of general require-
ments for software characteristics (software as a
product) and processes of its creation are given in
Figures 3 and 4 respectively. In the classification
of requirements for software characteristics two
groups of requirements are delineated: for struc-
ture and for properties. The first group includes
requirements for features of the construction and
functioning of software. The second of these
groups brings together requirements for software
properties such as requirements for its sufficiency
and adequacy for functions execution, monitoring
and diagnosis, reliability and stability.

1. Requirements to modularity.
2. Requirements to use of off-the-shelf com-
ponents (pre-developed software).
3. Requirements to interfaces.
4. Restrictions for use of the operating system
and interrupts.

The first subgroup of the requirements is due
to the need to present software in the form of a
modular structure. In doing so the source code
of one module must contain a limited number
of operators, and the modules must have a clear
structure, be easily modifiable and tested.
The second subgroup determines the preference
of the use of previously developed software. Using
OTS software components one must: evaluate its
conformity to the functions and characteristics of
I&C system, where the use of OTS components is
preferred for which one should determine the func-
tions and characteristics of the OTS components
and correlate them with specifications for I&C
software; analyze the results of OTS components
operation from the standpoint of its conformity
to the adopted criteria, norms and rules of safety;
develop, if necessary, a list of the required modi-
fications for adaptation of the OTS components
to conditions of its use in I&C system; execute
such adaptation and perform testing; develop and

Requirements for Software
Characteristics

Requirements for structure and components in-
cludes the following requirements:

Figure 3. Classification of requirements for products (software characteristics)