Host Adapter Selected By Another Initiator; Immediate Commands - Adaptec AHA-1740A Technical Reference Manual

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Host Adapter Selected by Another Initiator

If the host adapter is selected by another initiator on the SCSI subsystem, the host
adapter will respond as a processor-type target. If the command received by the in-
itiator requests a data transfer (Send or receive command) and the host adapter does
not already have a prepared Target SCSI command from the host, the host adapter
will disconnect from the selecting initiator and interrupt the host with an Asynchro-
nous event notification. The SCSI ID in the Interrupt Status register will be the ID of
the selecting initiator. The Mailbox In registers will contain the following informa-
tion:
F E D C B A
D LUN
The first three bytes of the Mailbox In registers contain the number of bytes (least
significant byte first) that was requested to be transferred by the selecting initiator.
These bytes rflect the values of bytes 2-4 of the command descriptor block that was
sent by the selecting initiator.
The most significant byte of the Mailbox In registers, bits 2-0, contains the logical
unit number of the host adapter that was specified in the identify message received
from the selecting initiator.
The most significant byte of the Mailbox In registers, bit 7, contains the direction of
the requested data transfer. If this bit is set to one, then a Send command was issued
by the selecting initiator (data transfer from initiator to host memory). If this bit is
set to zero, then a Receive command was issued by the selecting initiator (data trans-
fer from host memory to initiator).
The remaining bits in the most significant byte of the Mailbox In registers will be set
to zero.
Upon detecting the interrupt, the host can use the information in the Interrupt
Status and Mailbox In registers to create a Target SCSI command control block that
will enable the host adapter to complete the data transfer.

Immediate Commands

Immediate commands are non-control block operations that will be processed by the
host adapter immediately. They use the same delivery interface as control block com-
mands. Instead of a control block address, a 32-bit immediate command is stored into
the Mailbox Out registers, least significant byte first. The Attention register is then
stored with the following value:
6-42
9 8 7 6 5 4 3 2 1
CDB Byte 2
AHA-1740A/1742A/1744
0 F E D C B A 9 8
CDB Byte 3
7 6 5 4 3 2 1 0
CDB Byte 4