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The most common solution to this problem is to include  a special SCSI BIOS ROM on the controller itself. This will usually  contain the  software  that identifies the device connected to  the  SCSI interface as a disk  drive and  will even boot the system from the drive. In this case the  usual system  BIOS has no part  to play in  handling  the disk drive and the usual setup procedure  should  be used to  set the drive type  to “none”. If an ST-506, IDE or ESDI drive is used at the same  time as a  SCSI drive then it  is usual for it not to be used to boot the system. The SCSI  BIOS usually scans the SCSI  bus when the   machine is first  switched  on by sending message asking each  device ID in turn to  identify itself. There is usually a  limit on the number of  hard disks supported typically two. If you need more  then you have to make use of additional software drivers. The devices are  scanned in order  of ID number, 0 to 6 and the first device  to identify  itself as a   hard disk  is  used to boot  the system. After  this  SCSI  hard disk behave  like normal hard  disks but low level formatting and advanced  diagnostic  programs generally  don’t work unless  you obtain a version  specifically written for SCSI drives.

The great  attraction of SCSI is the  idea of being  able to connect  other devices to the same controller. In practice this is more difficult  than  you would  imagine, again  because of the  lack of a standard software  interface. You may be able to connect a SCSI tape drive  to the system with out any fear of hardware  incompatibilities, but you also need a  device driver  to allow MS-DOS, or whatever  operating  system, to make use of  the device. The trouble is that this  driver needs to be written  specifically for the device  and the controller combination. device manufactures have often overcome this difficulty by bundling a SCSI controller and software drivers  with their device. So you get a  SCSL controller when you buy a hard  disk, another