INT 40 - Acorn BBC Master 512 - "OSFIND" - OPEN FILE

	AL = operation
	    00h close file
	    40h open file for reading
	    80h open file for writing
	    C0h open file for random access
	DS:BX -> CR-terminated filename
Return: AL = file handle (00h if file closed or could not be opened)
Note:	the Acorn BBC Master 512 is an 80186-based add-on board for the
	  6502-based Master 128 which uses the original CPU as an I/O processor
SeeAlso: INT 41"Acorn",INT 42"Acorn",INT 43"Acorn",INT 44"Acorn",INT 4C"Acorn"