Bitfields for request flags:
Bit(s)	Description	)
 7	reserved (0)
 6	"ByteXfer"
	=0 use word transfer mode
	=1 use byte transfer mode
 5	(ATAPI device) DSC unavailable action (DUA)
	=0 queue the request and service it when DSC bit is set
	=1 return with status 83h
 4-3	direction
	00 direction determined by device
	01 data in
	10 data out
	11 no data transfer
 2	request type
	=0 ATAPI Packet Command
	=1 AT Task File Structure
 1	reserved (0)
 0	"Post"
	=0 disable posting
	=1 enable  posting
SeeAlso: #90003