INT 21 - DOS 5.0 - GET OR SET UMB LINK STATE
AH = 58h
AL = subfunction
02h get UMB link state
Return: AL = 00h UMBs not part of DOS memory chain
= 01h UMBs in DOS memory chain
03h set UMB link state
BX = 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 AH=59h)
Note:   a program which changes the UMB link state should restore it before
terminating


2158 - DOS 5.0 - GET OR SET UMB LINK STATE