TOPS-20 PASCAL Language Manual - Bitsavers

INTRODUCTION
1.1 OVERVIEW OF PASCAL
A PASCAL program performs operations on data items known as constants,
variables, and function designators. A constant is a quantity with an
unchanging value. A constant to which you give a name is called a
symbolic constant. A variable is a quantity whose value can change
while the program executes. A function designator causes the
execution of a group of statements that is associated with an
identifier and returns a value. The function type is determined by
the type of the value it returns.
1.1.1 Data Types
Every PASCAL data item is associated with a data type. A data type,
which is usually indicated by a type identifier, determines both the
range of values a data item can assume and the operations that can be
performed upon it. In addition, the type implicitly indicates how
much storage space is required for all possible values of the data
item.
PASCAL provides identifiers for many predefined types. Thus, a
program's operations can involve integers, real numbers, Boolean and
character data, arrays, records, sets, and pointers to dynamic
variables. PASCAL also allows you to create your own types by
defining an identifier of your choice to represent a range of values.
The type of a constant is the type of its corresponding value. You
establish variable and function types when you declare them. In
general, they cannot change. Although variables and functions can
change in value any number of times, all the values they assume must
be within the range established by their type. A variable does not
assume a value until the program assigns it one. A function is
assigned a value during its execution.
PASCAL associates types not only with data items, but also with
expressions. An expression is the computation of a value resulting
from a combination of variables, constants, function designators, and
operators. In PASCAL, you can form expressions using arithmetic,
relational, logical, string, and set operators. Arithmetic
expressions produce integer or real number values. Relational,
logical, string, and most set expressions yield Boolean results.
Other set expressions form the union, intersection, and differences of
two sets.
1.1.2 Structure of a PASCAL Program
A PASCAL program consists of a heading and a block. The heading
specifies the name of the program and the names of any external files
the program may use. The block is divided into two parts: the
declaration section, which contains data declarations; and the
executable section, which contains executable statements. Figure 1-1
points out each part of a sample PASCAL program.
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