Standard Mode Firmware; Mailboxes - Adaptec AHA-1740A Technical Reference Manual

EISA-to-Fast SCSI Host Adapter
systems with a large amount of bus latency, typically found in performance EISA
systems.

Standard Mode Firmware

The AHA-1740A/1742A/1744 support multithreaded SCSI initiator operation through
a simple mailbox protocol. The firmware accepts as many Command Control Blocks
(CCBs) as required and executes them from its local RAM. The firmware controls all
of the SCSI activity that a task may require, including:
Arbitration
Selection
Disconnection
Reconnection
Command completion
Using 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 SCSI
disks, allowing the SCSI subsystem to completely replace the usual internal disk
functions.
A multiuser, multitasking operating system issues a large number of I/O tasks in a
rapid sequence. The architecture of the AHA-1740A/1742A/1744 makes management
of 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 and
the AHA-1740A/1742A/1744 required to accomplish an I/O task in standard mode.

Mailboxes

The AHA-1740A/1742A/1744 Standard Mode uses a mailbox architecture for task
communication between the host and host adapter. This allows the host adapter to
perform 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 are
completed, the host issues an initialization command to inform the host adapter of
the mailbox location. There is always an equal number of Outgoing Mailboxes (MBO)
and Incoming Mailboxes (MBI). MBIs are located immediately after the MBOs.
Architecture
2-5