SIMSCRIPT II.5 Programming Language

6. Advanced Topics
6.1 Introduction
This chapter describes a variety of SIMSCRIPT II.5 features which need not concern the first-time
user, but which an experienced programmer may find of interest.
6.2 Programmer-Defined Array Structures: Pointer Variables
We stated previously that the allocation of storage space to an array is determined from the number
of elements, or in the case of multidimensional arrays, from the product of the array dimension
bounds. Although true in principle, this statement is a simplification. The storage allocation for the
elements of a one-dimensional array, for example, is determined from both the number of elements
and the declared mode of the array. Associated with this storage allocation is an array base pointer,
which holds the address in the computer memory of the allocated storage space. In the case of a
one-dimensional array named X, the associated base pointer is named X(*). Recall this usage in
the reserve statement. The function of the reserve statement is to allocate computer storage to
an array and assign the internal location of this storage as the value of the base pointer. An array
base pointer is internally represented in a way similar to an integer variable. It can be manipulated
as an integer variable. However, this is generally unnecessary and should be done with great
care and full appreciation of the internal array representation.
A one-dimensional array X, allocated storage by the statement:
reserve X(*) as 10
is structured as a contiguous group of memory locations, pointed to by the array base pointer, as
shown in figure 6-1. The base pointer itself is not contiguous with the array storage, but is allocated
storage in the same way as an integer variable.
A two-dimensional array was introduced conceptually as an array of one-dimensional arrays. In
fact, this is precisely how a two-dimensional array is internally represented. The base pointer of a
two-dimensional array points not directly to the doubly subscripted variable elements, but to a onedimensional
array of pointers. Each element of this pointer array serves as a base pointer for the
one-dimensional array representing an entire row of the two-dimensional array. Note that the base
pointer itself and the array of row pointers are all pointer type variables, and therefore stored in an
integer-like form. The representation of the fully subscripted variable elements depends on the
declared mode of the array. A two-dimensional array X, which has been allocated storage by the
statement:
reserve X(*,*) as 5 by 3
is stored as shown in figure 6-2.
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