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   There are four library routines available for handling heaps. A heap is
   a byte-stream file which is buffered in available memory. Each heap is
   associated with a status word specified when the heap is created, which
   is written to when an operation on the heap fails. Each heap is thereby
   attached to a particular program, namely the program which contains the
   heap's status word, and is automatically deallocated (unless it has
   already been explicitly deallocated by a call to CBL_CLOSE_VFILE) when
   that program is canceled.
 
   Each heap is paged, if necessary, onto a separate disk-file with name
   progname.Vnn, where progname is the root name of the program to which
   the heap is attached and 'nn' is an integer from 1 to 99. Once nn
   reaches 99, the V will be overwritten. The maximum number allowed for
   the extension is 384. Each program is limited to 384 local heaps.
 
   Heaps are accessed by means of heap identifier words, which may be
   passed around between programs so that any heap may be read or written
   by any program. However, in order to check for failure, the program
   needs to have access (using a pointer variable, for example, or via
   normal linkage section mapping) to the associated status word.
   Alternatively the RETURN-CODE register can be examined to catch a
   general heap function failure, or the ON OVERFLOW/EXCEPTION syntax
   can also be used on the CALL statement to trap any error. In these two
   cases, the specific error can then be determined by examining the heap
   status word.
 
 
   General Notes on Virtual Heaps
 
    o  You can use reference modification to declare a buffer in the
       middle of a COBOL record. This is compiled efficiently provided
       the length is given as fixed; since the length is ignored by the
       call interface, you can give it as one.
 
    o  If the read/write succeeds, the return code is zero. If it fails,
       the return code is non zero and the heap status byte is 9, with
       detail in the second byte.
 
    o  There is a general restriction that the heap status word used by
       CBL_READ_VFILE/CBL_WRITE_VFILE, must reside within the first 64K of
       some COBOL program: note that it can be in the Linkage Section of
       the current program, as long as it is within the first 64K of some
       other COBOL program.
 
    o  The CBL_xxxxx_VFILE routines have been introduced to replace
       existing _HPxxxxx routines. The _HPxxxxx routines are documented
       in the on-disk document MFCOMPAT.DOC.
 
    o  The general restriction on the buffer for both COBOL_READ_VFILE and
       COBOL_WRITE_VFILE in previous versions of this COBOL system no longer
       exists. The buffer can reside anywhere within the Data or Linkage
       Section, and it can be dynamically allocated using the CBL_ALLOC_MEM
       routine. The buffer must not cross a 64K boundary; use of the
       FLAG-CHIP directive will highlight those data items that do cross a
       64K boundary.
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