INT 21 - DOS v2.11 - SET OEM INT 21 HANDLER
        AH = F8h
        DS:DX -> OEM INT 21 handler for functions F9h to FFh
                 FFFFh:FFFFh disables OEM handler

Notes:  this function is supported by Toshiba T1000 ROM MSDOS 2.11
        calls to AH=F9h through AH=FFH will return AL=00h if no handler set
        handler is called with all registers exactly as set by caller, and
          should exit with IRET