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INT 21 - DOS v??? - SET OEM INT 21 HANDLER
        AH = F8h
        DS:DX -> OEM INT 21 handler for functions F9h to FFh
                 FFFFh:FFFFh resets to original handlers

Notes:  calls to AH=F9h through AH=FFH will return CF set and AX=1 (invalid
          function) if no handler set
        handler is called with all registers exactly as set by caller, and
          should exit with IRET