Device Status Word

	The device status word is set to zero on entry and is set by
	the driver interrupt routine on return.

	|15|14-10|9|8|7-0| STATUS word
	 |    |   | |  `---- Error return code (if bit 15=1)
	 |    |   | `------ Done bit, function completed
	 |    |   `------- Busy bit
	 |    `---------- Reserved
	 `------------- Error flag bits 0-7 have error code

	Error return codes

	00  Write protect violation	01  Unknown unit
	02  Device not ready		03  Unknown command
	04  CRC error			05  Bad drive request structure length
	06  Seek error			07  Unknown media
	08  Sector not found		09  Printer out of paper
	0A  Write fault 		0B  Read fault
	0C  General failure		0D  Reserved
	0E  Reserved			0F  Invalid disk change