)
Values keyboard filter function is called with when keyboard is in field mode:
	AL = character
	AH = 00h or extended ASCII code if AL = 00h
	BL = field number
	CH = cursor column
	CL = cursor row
	DL = field type modifier (sixth item in field table entry)
	DH = seventh item in field table entry
	ES:SI = window's handle
	DS:DI -> field table entry for field containing the cursor
Return: AH = action to take
	    00h use keystroke
	    01h ignore keystroke
	    FFh beep and ignore keystroke
Note: the filter function is not allowed to make INT 15, DOS, or BIOS calls