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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

  ◄Variables►   ◄Variable Storage Allocation►   ◄Contents►   ◄Index►
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Variable Storage Allocation
 
BASIC stores different kinds of variables in different areas in memory. You
only need to worry about where variables are stored if you are doing mixed-
language programming or you are using one of the following BASIC statements
or functions:
 
  CALL, CALLS (non-BASIC procedures)
  DECLARE (non-BASIC procedures)
  SADD
  SETMEM
  VARPTR
  VARSEG
  VARPTR$
 
BASIC stores variables either in an area called DGROUP or as far objects.
(DGROUP is the name of the default data segment, the segment referenced
when DEF SEG is used without an address.) Variables stored in DGROUP
can be referenced by using near addresses or pointers. A near address
consists of a single value or offset from the beginning of a segment
or block of memory. Far objects are referenced by using far addresses
or pointers. A far address consists of two parts: the starting address
of a segment or block of memory, and the offset within the segment.
 
Whether a variable is stored in DGROUP or as a far object depends
first on whether it is a simple variable or an array. All simple
variables are stored in DGROUP. Array storage is a little more complex
and is slightly different between programs run as .EXE files and
programs run within the QuickBASIC environment.
 
When doing mixed-language programming, far addresses are preferred. This
ensures that all generated code is independent of the BASIC implementation.
 
In programs run as .EXE files, array variables are stored as follows:
 
  ■ All static arrays are stored in DGROUP and can be referenced
    with near addresses.
  ■ All dynamic arrays of variable-length strings are also stored
    in DGROUP and can also be referenced with near addresses.
  ■ All other dynamic arrays are stored as far objects and
    require far addresses.
 
In programs run within the QuickBASIC environment, array variable
storage follows these rules:
 
  ■ All static arrays in COMMON are stored in DGROUP and can be
    referenced with near addresses.
  ■ All arrays of variable-length strings are also stored in DGROUP
    and can also be referenced with near addresses.
  ■ All other arrays are stored as far objects and require far
    addresses.
 
Because BASIC attempts to make the most efficient use of memory possible,
several different things may cause variables to move in memory:
 
  ■ A reference to a string literal or expression
  ■ A DEF FN or FUNCTION invocation
  ■ The use of a BASIC string or memory-related function
 
Because BASIC variables may move, use the results of a VARPTR, VARSEG,
VARPTR$, or SADD function call immediately after the function call.