Bitfields for critical error type and processing flags:
Bit(s)	Description	)
 7	class
	=0  disk I/O error
	=1  -- if block device, bad FAT image in memory
	    -- if char device, error code in DI
 6	unused
 5	Ignore allowed (DOS 3.0+)
 4	Retry allowed (DOS 3.0+)
 3	Fail allowed (DOS 3.0+)
 2-1	disk area of error
	00 = DOS area	01 = FAT
	10 = root dir	11 = data area
 0	set if write, clear if read
Note:	Abort is always allowed
SeeAlso: #02545,#02546