Bitfields for SFS drive number:
Bit(s)	Description	)
 15-12	drive access mode
	0000 BIOS access
	0001 direct IDE access
	0010 direct SCSI access
---if BIOS access---
 11-8	unused (0)
 7-0	BIOS drive number
---if IDE access---
 11-8	unused (0)
 7-0	IDE drive number
---if SCSI access---
 11-8	SCSI host number
 7-4	SCSI target ID
 3-0	SCSI logical unit number