Int 21 Fn 58  - Dos 5+ - Get Or Set Umb Link State                         [D]

   AH = 58h
   AL = subfunction
       02h get UMB link state

       Return: AL = current link state
               00h UMBs not part of DOS memory chain
               01h UMBs in DOS memory chain
       03h set UMB link state
       BX = new link state
           0000h remove UMBs from DOS memory chain
           0001h add UMBs to DOS memory chain

Return: CF clear if successful
   CF set on error
       AX = error code (01h) (see #1366)

Notes: a program which changes the UMB link state should restore it before
     terminating
   UMBs will only be available if CONFIG.SYS contains the line DOS=UMB,
     the UMBs have been linked into the memory chain with AX=5803h, and
     the allocation strategy has been set to include high memory with
     AX=5801h

See Also: #1373,#2453 at INT 2F/AX=4310h