CDB Control Byte, bits 6&7

John Lohmeyer jlohmeye at ncr-mpd.FtCollinsCO.NCR.COM
Fri Oct 7 08:14:53 PDT 1994

"Duncan Penman wrote:"
>   In SCSI-2, these 2 bits are defined as vendor-specific.  Quite reasonable 
> for the CD-ROM drive manufacturers to use in this way.  The ATAPI mapping 
> is direct..byte 9 of the CDB to byte 9 of the Command Packet.  The 
> question in this is, Can we define a SCSI vendor-specific field to 
> have a specific meaning for one device type?
>   Would appreciate a bit of strategic guidance here before getting too 
> deep into implementation alternatives.

We have a bit of a conundrum here.  The Control Byte is really intended for
command- and device-independent control functions such as linking and, more
recently, ACA control.  However, these two bits have been designated vendor
specific since SCSI-1 because some SASI devices had used these bits for
retry control.

To define these bits specifically for the READ TOC command at least bends
the architecture.  However, the CD-ROM vendors didn't have any other VU
bits to use and, for whatever reason, did not to approach the committee
to define this function in a reserved field.

We have solved such conundrums in the past by defining two new bits in a
reserved field (for new designs), while permitting old designs to continue 
using the VU fields.  Eventually, people can stop supporting the VU field.


John Lohmeyer                      E-Mail:  John.Lohmeyer at FtCollinsCO.NCR.COM
NCR Microelectronics                Voice:  719-573-3362
1635 Aeroplaza Dr.                    Fax:  719-597-8225
Colo Spgs, CO 80916              SCSI BBS:  719-574-0424 300--14400 baud

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