Bitfields for AT fixed disk control byte:
Bit(s)	Description	)
 0	unused
 1	reserved (0)  (disable IRQ)
 2	reserved (0)  (no reset)
 3	set if more than 8 heads
 4	always 0
 5	set if manufacturer's defect map on max cylinder+1  (AT and later only)
 6	disable ECC retries
 7	disable access retries