Compatibility: 2.0+
Expects: AH    4aH
ES    segment of an allocated memory block
BX    desired size for the block, in paragraphs (16-byte chunks)
                                                                  
Returns: AX    error code if CF is set to CY
BX    largest block available (if a "grow" request fails)
                                                                  
Info: Changes the size of an existing memory block.

When a program receives control, the 4bH EXEC function has
allocated a memory block starting at the PSP that contains all of
available memory.  To make memory available to EXEC a child
process, shrink the block starting at your PSP paragraph.

Notes:   DOS Fn 31H and INT 27H shrink the block at the PSP.

  .COM programs: Move your stack down BEFORE shrinking your
block!

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DOS Fn 4aH: Shrink or Expand a Memory Block