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Function 5803h          Set Upper-Memory Link                        DOS 5.0+

    Links or unlinks the upper memory area. When this area is linked,
    a program can allocate memory from it.

        Entry   AX = 5803h
                BX = 00h | 01h   link | unlink upper memory area

        Return  Nothing
                or
                AX = Error code, if CF is set
                     | 01h Invalid function
                     | 07h Memory blocks destroyed

    ------------------------------------------------------------------

    If this function returns an error value of 1 (invalid function),
    DOS has been loaded without the command "DOS=UMB" having been
    specified in the CONFIG.SYS file.

    A program that changes the upper-memory link state should restore
    its original state before terminating.