systems with a large amount of bus latency, typically found in performance EISAsystems.Standard Mode FirmwareThe AHA-1740A/1742A/1744 support multithreaded SCSI initiator operation througha simple mailbox protocol. The firmware accepts as many Command Control Blocks(CCBs) as required and executes them from its local RAM. The firmware controls allof the SCSI activity that a task may require, including:• Arbitration• Selection• Disconnection• Reconnection• Command completionUsing the same mailbox protocol, the AHA-1740A/1742A/1744 can operate as a proc-essor-type device serving as a multitasking target to other initiators. This feature al-lows high bandwidth communication between multiple hosts.In addition, the AHA-1740A/1742A/1744 firmware cooperates with the BIOS in-stalled on the host adapter to emulate the standard DOS BIOS calls. This allows boot-ing operations and the execution of standard DOS operations from attached SCSIdisks, allowing the SCSI subsystem to completely replace the usual internal diskfunctions.A multiuser, multitasking operating system issues a large number of I/O tasks in arapid sequence. The architecture of the AHA-1740A/1742A/1744 makes managementof this activity very easy and straightforward for the operating system and its associ-ated I/O drivers. This section briefly explains the interaction between the system andthe AHA-1740A/1742A/1744 required to accomplish an I/O task in standard mode.MailboxesThe AHA-1740A/1742A/1744 Standard Mode uses a mailbox architecture for taskcommunication between the host and host adapter. This allows the host adapter toperform multithreaded operations with a minimum of host intervention. The mail-boxes are located in main system memory. Each mailbox entry is four bytes long. Af-ter power-up sequencing, host initialization procedures, and the boot procedure arecompleted, the host issues an initialization command to inform the host adapter ofthe mailbox location. There is always an equal number of Outgoing Mailboxes (MBO)and Incoming Mailboxes (MBI). MBIs are located immediately after the MBOs.EISA-to-Fast SCSI Host Adapter Architecture2-5
