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Request High Memory Area (Function 01h)

        ARGS:   AH = 01h
                If the caller is a TSR or device driver,
                DX = Space needed in the HMA by the caller in bytes
                If the caller is an application program,
                DX = FFFFh

        RETS:   AX = 0001h if the HMA is assigned to the caller,
                     0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = 90h if the HMA does not exist
                BL = 91h if the HMA is already in use
                BL = 92h if DX is less than the /HMAMIN= parameter

This function attempts to reserve the 64K-16 byte high memory area for
the caller.  If the HMA is currently unused, the caller's size parameter
is compared to the /HMAMIN= parameter on the driver's command line. If
the value passed by the caller is greater than or equal to the amount
specified by the driver's parameter, the request succeeds.  This
provides the ability to ensure that programs which use the HMA
efficiently have priority over those which do not.