ERROR HANDLING

            Most Int  31h calls  can fail.   The  DPMI 0.9 specification
            does not  specify error return codes for most calls.  When a
            call fails  it will  set the  carry flag and return with the
            value in AX unmodified unless otherwise specified.  However,
            future DPMI  implementations will  return error codes in the
            AX register.   All  specific error  codes will have the high
            bit (bit  15) set.  If a function returns with carry set and
            the high  bit of AX clear, it should be treated as a general
            failure.   Specific error  codes will allow programs running
            under  future   DPMI  implementations  to  take  appropriate
            corrective action in some cases.