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~.