SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
This can provide a second method of equivalencing attributes: If two attributes are explicitly assigned
to the same word number within an entity structure, they are equivalenced. Care should be
taken that this is not done inadvertently. Some implementations insist that such equivalencing be
recognized by enclosing the attribute names in parentheses, as is done when explicitly equivalencing
an attribute group. In addition, there are certain restrictions on equivalencing attributes of different
modes. In general, for example, text mode variables may not be equivalenced with other
modes because of the way text is implemented through pointer values. Any inconsistent usage
of these pointers could give rise to serious errors.
Attributes of permanent entities and system attributes are not assigned to words, but a similar effect
is achieved by using the clause in array i, where i is an integer constant. This array may be
thought of as comprising the structure of the system entity.
In certain implementations, such explicit assignment, either for temporary or permanent entities,
may be recognized by the compilation process, leading to efficiencies at program execution time.
It may also affect the requirements for independent compilation of the routines in a program. Consult
the appropriate user manual for implementing specific details of packing and equivalencing.
Obviously, the use of these features may give rise to problems if a program is to be transported
across different systems.
The results of packing, equivalence, and word and array specification are shown in table 6-3.
Table 6-3. Attribute Specifications
Specification
No packing, equivalence, or word or array specification
Word or array specification
Assignment
Attributes assigned to separate words or arrays in
the order of their appearance in preamble
Attributes are assigned to specified words or array
locations. Remaining attributes assigned as above.
Equivalence specification
Packing specification
Specified attributes assigned to the same word or
array
Field and bit packing used to place more than one
attribute within a computer word
Intrapacking used to compress storage for arrays
The every statement optionally permits detailed specification of the attributes of an entity. The
use of packing factors, equivalence parentheses, and word and array clauses gives a programmer
a good deal of control over the allocation of computer storage.
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