INT 21,44,4 / IOCTL,4 - Read from Block Device

	AH = 44h
	AL = 04
	BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)
	CX = number of bytes to read
	DS:DX = pointer to data buffer


	on return
	AX = number of bytes read if CF clear
	   = error code if CF set  (see DOS ERROR CODES)


	- block drivers are not required to support this function
	- returns AX = 01 (invalid function code) if not supported