Int 21H, AH=58H, AL=02H

5 and higher

GET UMB LINK STATE

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

 CF clear if successful
 AL = current link state
		    00h UMBs not part of DOS memory chain
		    01h UMBs in DOS memory chain
CF set on error
    AX = error code (01h) (see #01680)		

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
 

#01687,#02766 at INT 2F/AX=4310h

Text based on Ralf Brown Interrupt List Release 61