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116 Communication Between BASIC and Assembly Language
The array information returned by the Get_info utility is used for accessing elements in arrays
passed as arguments. It is used by the element-retrieval utilities described in a later section of
this chapter. Once retrieved, the information is usable any number of times for accessing the
array associated with it. It is not necessary to retrieve the information every time you access an
array, as long as you have not altered the information (except the pointer) between accesses.
The seventeenth word of the array information (word 16 on the chart) is reserved to hold the
offset from the start of the array of the element to be accessed. Therefore, it is permissible
(indeed, it is necessary) to alter the contents of that location to indicate which element in the
array you wish to retrieve. None of the other words returned by the utility should be changed.
In making multiple accesses with the same information, caution should be taken when an array
is involved. If a REDIM statement is executed upon the array between accesses, the information
may not reflect the true structure of the array. This potentiality can be addressed in one of two
ways-
• Advise the BASIC user against using a REDIM on the array between executions of the
routine or routines involved .
• Call the Get_info utility each time the array is accessed.
Similar problems exist when a BASIC subprogram is called recursively, and the subprogram
uses a local array as an argument in an ICALL, or when a subprogram calls a routine and later
exits (causing its local arrays to disappear).
Retrieving the Value of an Argument
At some point during execution of your assembly language routine, you may want to retrieve
the value of an argument so that you can use it in your processing. By doing so, you accomplish
one of the methods of communicating with assembly language - namely, passing a value TO
the assembly language routine from BASIC.
There are a number of utilities for this purpose. The one to use is dependent upon the type of
argument passed. The utilities available are -
Name
Get value
Get element
Get_bytes
Get _ elem _ bytes
Used For
Simple variables, expressions, individual elements of
arrays passed as arguments, and file numbers
Elements (from arrays passed as arguments)
Substrings of strings passed as arguments either as
simple string variables, expressions, or individual
elements of arrays passed as arguments
Substrings of individual elements (from string arrays
passed as arguments)
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