CDB Control Byte, bits 6&7

Jeff Williams jlw at
Sat Oct 8 08:32:44 PDT 1994

> 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.

HP uses the VU bits in the control bytes for certain special sets of 
firmware.  We would object to using these bits as non-VU bits.

Jeffrey L. Williams                                HP TelNet: 396-5030
Disk Controllers Lab             	 Telephone: (1) (208) 396-5030
Disk Memory Division                     Facsimile: (1) (208) 396-6858
Hewlett-Packard Co.                    ARPANET: jlw at

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