INT 21,E - Select Disk

	AH = 0E
	DL = zero based, drive number (0-25, A: - Z:)


	on return:
	AL = one based, total number of logical drives including
	     hardfiles (1-26)


	- for DOS 3.x+, this function returns the number of logical
	  drives or the value of LASTDRIVE (default of 5) in the
	  CONFIG.SYS file