ACA Recovery Methods Supported by a Logical Unit(X3T10/94-177R0)

Charles Monia, SHR3-2/W3, 237-6757 07-Sep-1994 1630 monia at
Wed Sep 7 13:28:45 PDT 1994

From:     Charles Monia
     Digital Equipment Corporation

To:  Members of X3T10

Subject:  ACA Recovery Methods Supported by a
          Logical Unit (X3T10/94-177R0)


As the result of proposal X3T10/93-065, the committee approved the
addition of the ACA bit to the CDB control byte. If this bit is set to one, the
SCSI-3 rules apply to an ACA condition caused by the command. If set
to zero, the SCSI-2 rules apply.

In a proposed response to written comments on SAM (see X3T10/94-
173R0, comment HP 043) support for an ACA bit value of one or zero is
defined as a logical unit option. A logical unit must, of course, support at
least one value and may support both. Since that is the case, there needs
to be some explicit way for an initiator to determine which values are
supported. This proposal's intent is to define a way for an initiator to
inquire about the ACA recovery methods supported by a logical unit.


The data returned in response to an INQUIRY command shall contain two
bits which define the rules for ACA handling that are implemented by the
logical unit. The assignment of specific bits is left to the discretion of the
SPC technical editor. For the purposes of this discussion, the bits are
identified as ACA-2 and ACA-3. They shall have the values and significance
specified below. For an SCSI-3 initiator to work with an SCSI-2 target
device, the bit values for ACA-2 and ACA-3 are defined so that zero represents 
normal, SCSI-2 behavior.

ACA-2 = 0, SCSI-2 rules implemented.
ACA-2 = 1, SCSI-2 rules unimplemented.

ACA-3 = 0, SCSI-3 rules unimplemented.
ACA-3 = 1, SCSI-3 rules implemented.

To avoid terninating the INQUIRY command with a CHECK CONDITION
status before the initiator has determined which ACA recovery rules are
implemented, the logical unit shall observe the following:

1.   The logical unit shall not complete an INQUIRY command with
     CHECK CONDITION status because the ACA bit was set to an
     unsupported value.

2.   As described in SAM clause 6.6.5, if the logical unit receives an
     INQUIRY command while a unit attention is pending, the logical
     unit shall perform the INQUIRY command and shall not clear the
     unit attention condition. 

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