INT 21,F8 - Set OEM Int 21 Handler (functions F9-FF, Undocumented)
AH = F8 DS:DX = pointer to OEM handler for INT 21H calls F9 through FF = FFFF:FFFF to reset to original handlers - DOS is set up to allow ONE handler for all 7 function calls. Any call to these handlers will set the CF and AX will contain 1 if they are not initialized. The handling routine is passed all registers just as the user set them. The handler should exit with an IRET.