CDB Control Byte, bits 6&7
Devon Worrell Worrell x7042
worrell at st9.dt.wdc.com
Fri Nov 4 09:31:52 PST 1994
I would make a recommendation that a new 12 byte opcode be assigned for
the READ TOC, and the format kept the same. This would allow migration
to the new command for SCSI and not break any of the ATAPI devices. The
ATAPI drives could also move to support both opcodes.
On Oct 7, 9:14am, John Lohmeyer wrote:
> Subject: Re: CDB Control Byte, bits 6&7
> "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
>-- End of excerpt from John Lohmeyer
"Every truth passes through three stages before it is recognized.
In the first it is ridiculed, in the second it is opposed, in the
third it is regarded as self-evident!"
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