Data Or Scatter/Gather List Pointer - Adaptec AHA-1740A Technical Reference Manual

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Flag Name and Function
REC Error Recovery
This flag (bit 14) is used by the host to perform error recovery procedures while the
associated target is in a state of extended contingent allegiance. A state of extended
contingent allegiance exists as long as the host adapter returns this status (Status
Word 1, bit 14) in the status block. The condition is cleared by issuing a Resume
immediate command.
If this bit is set to one, and the target is in a state of extended contingent allegiance,
the host adapter will execute the command. If the target is not in a state of
extended contingent allegiance, the host adapter will reject the control block and
report a specification check. The control blocks will also be rejected with a
specification check if the Tagged Queuing flag (bit 3) is set.
If this bit is set to zero, and the target is in a state of extended contingent
allegiance, the host adapter will accept the command but not execute it until the
condition is cleared. If the target is not in a state of extended contingent allegiance,
the control block will be executed normally.
This option has been defined for future versions of SCSI-2 subsystems. For current
versions, this flag should always be set to zero.
NRB No Retry on Busy Status
This flag (bit 15) determines whether or not the host adapter will automatically
retry an Initiator SCSI Command if the target returns Busy status.
If this bit is set to one, the host adapter will terminate the command upon detecting
busy status from the target. The host will be notified that the command completed
with error. The target status field in the status block (byte 03) will reflect that a
Busy status code (08h) was returned.
If this bit is set to zero and the target returns busy status, the host adapter will
retry the command indefinitely until the target completes the command successfully
or until the command is aborted by the host via an Immediate command.

Data or Scatter/Gather List Pointer

This 32-bit field contains the physical memory address (least significant byte first) of
the first byte of the host memory data area to be used during the data phase of the
SCSI command.
If the Scatter/Gather flag (Flag Word 1, bit 12) is set to one, this field contains the
physical address of the first byte of list of data pointers and lengths to be used during
the data phase of the SCSI command. The number of 8-byte segments is limited to
128 (one pointer and one length), but the host adapter will only store a maximum of
17 segments in its internal memory at a time. The diagram below describes the struc-
ture of the Scatter/Gather list.
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AHA-1740A/1742A/1744