Int 21 Fn 49  - DOS 2+ - Free Memory                                       [D]

   AH = 49h
   ES = segment of block to free

Return: CF clear if successful
   CF set on error
       AX = error code (07h,09h) (see AH=59h)

Notes: apparently never returns an error 07h, despite official docs; DOS 2.1+
     code contains only an error 09h exit
   DOS 2.1-6.0 does not coalesce adjacent free blocks when a block is
     freed, only when a block is allocated or resized
   the code for this function is identical in DOS 2.1-6.0 except for
     calls to start/end a critical section in DOS 3+

See Also: AH=48h,AH=4Ah