Expects: AH    48H
          BX    requested amount of memory in paragraphs (16-byte chunks)
          ------------------------------------------------------------------
 Returns: AX    error code if CF is set to CY
                segment address of allocated block (if no error)
          BX    size of available RAM, in paragraphs (if allocation fails)
          ------------------------------------------------------------------
    Info: Allocates a block of memory BX paragraphs long, returning the
          segment address of that block in AX (block starts at AX:0000).

          If the allocation fails, the Carry Flag is set, an error code is
          returned in AX, and BX contains the maximum size allowable (in
          paragraphs) for an allocation.

          To determine the largest block available, it is a common practice
          to set BX=ffffH before the call.  The allocation will fail,
          returning the available allocation size in BX.