Host Adapter Selected by Another InitiatorIf the host adapter is selected by another initiator on the SCSI subsystem, the hostadapter 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 doesnot already have a prepared Target SCSI command from the host, the host adapterwill 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 ofthe selecting initiator. The Mailbox In registers will contain the following informa-tion:F E D C B A 9 8 7 6 5 4 3 2 1 0 F E D C B A 9 8 7 6 5 4 3 2 1 0D LUN CDB Byte 2 CDB Byte 3 CDB Byte 4The first three bytes of the Mailbox In registers contain the number of bytes (leastsignificant 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 wassent by the selecting initiator.The most significant byte of the Mailbox In registers, bits 2-0, contains the logicalunit number of the host adapter that was specified in the identify message receivedfrom the selecting initiator.The most significant byte of the Mailbox In registers, bit 7, contains the direction ofthe requested data transfer. If this bit is set to one, then a Send command was issuedby the selecting initiator (data transfer from initiator to host memory). If this bit isset 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 setto zero.Upon detecting the interrupt, the host can use the information in the InterruptStatus and Mailbox In registers to create a Target SCSI command control block thatwill enable the host adapter to complete the data transfer.Immediate CommandsImmediate commands are non-control block operations that will be processed by thehost 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 intothe Mailbox Out registers, least significant byte first. The Attention register is thenstored with the following value:adaptec AHA-1740A/1742A/17446-42
