CDB Control Byte, bits 6&7
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
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
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