The actual boundary limitation is somewhat less restrictive. If the binary values ofthe starting address of a segment, the byte count of that segment, and the startingaddress of the next segment are all exclusive OR’d together, and the result of that ex-clusive OR is an even number, the boundary between those two segments is valid. An-other way to express the limitation is to require that the ending boundary of onesegment must be the same as the beginning boundary of the next segment. If a seg-ment ends on a word boundary, the next segment must begin on a word boundary.Similarly, if a segment ends on an odd-byte boundary, the next segment must beginon an odd-byte boundary.Description of OperationThis section describes the interface operations required to invoke the desired SCSIbehavior. The AHA-1740A/1742A/1744 must be properly initialized before any ofthese operations can be performed.After system initialization is done, the I/O Command Port initialization commandsmust be executed. The Mailbox Initialization command must be executed to assignthe mailbox area. The Enable Target Mode command may optionally be executed ifTarget Mode is to be allowed.Execution of Initiator Mode OperationsTo begin an Initiator Mode SCSI command, the host first allocates a data buffer area.A CCB is then created to perform the desired operation to the correct peripheral de-vice and a pointer to the data buffer area is placed in the CCB. Once the CCB is com-pletely defined, the host places a pointer to the CCB in an empty MBO location,places an MBO full status in the MBO status byte, and transmits a Start SCSI com-mand to the I/O command port. If it is not already scanning the MBO for an activeMBO entry, the Start SCSI command to the I/O port causes the host adapter to beginscanning for such an entry.After finding an active MBO entry, the AHA-1740A/1742A/1744 copies the MBO Com-mand field contents and CCB pointer into its internal RAM and clears the MBO Com-mand byte, freeing the mailbox. Up to 16 initiator CCBs and 16 target CCBs can bestored in the host adapter’s internal RAM concurrently. The AHA-1740A/1742A/1744maximizes the SCSI bus utilization by starting the next available CCB as soon as thebus is free. Disconnection and reconnection on the bus are automatically taken careof by the AHA-1740A/1742A/1744. As the host adapter starts the SCSI operation, itwill first determine if the addressed target/LUN is busy. If the target/LUN is busy,the command will be placed in the task queue to be tried again later.Commands are removed from the queue in the order they were received, establishinga First In First Out (FIFO) command execution order. Of course, the order of taskcompletion may vary due to the different amounts of time required to process andcomplete different commands. If the target/LUN is not busy, the new command isstarted at the next bus free phase. If Busy status is received by the AHA-1740A/1742A/1744, the CCB is placed at the end of the FIFO queue to be restartedlater.EISA-to-Fast SCSI Host Adapter Standard Mode Firmware Description5-29
