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Reallocate Upper Memory Block (Function 12h)

        ARGS:   AH = 12h
                BX = New size for UMB in paragraphs
                DX = Segment number of the UMB to reallocate

        RETS:   AX = 1 if the block was reallocated, 0 otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = B0h if no UMB large enough to satisfy the request
                         is available.
                         In this event, DX is returned with the size of
                         the largest UMB that is available.
                BL = B2h if the UMB segment number is invalid

This function attempts to reallocate an Upper Memory Block to a newly
specified size. If the new size is smaller than the old block's size,
all data at the upper end of the block is lost.

Note: This function is new to version 3.0