Introduction; Operation With The Standard Interface - Adaptec AHA-1740A Technical Reference Manual

Chapter Seven
Onboard BIOS Interface

Introduction

The AHA-1740A/1742A/1744 BIOS operates in two different modes which correspond
to the two possible firmware configurations. The two modes supported by the BIOS
are Standard Mode (communicates with the adapter via the standard compatible in-
terface) and the Enhanced (EISA) Mode (communicates with the adapter via the en-
hanced interface). In each mode the responsibilities of the BIOS are the same:
Perform any power-up initialization needed for either the AHA-
1740A/1742A/1744 or SCSI devices connected to it.
Support the standard Interrupt 13h interface for SCSI disk drives on the
SCSI bus. This support enables use of drives attached to the adapter under
real mode operating systems such as DOS without a device driver. The limit
for Int 13h supported drives is set at two (80h-81h). Future revisions of the
BIOS will support more than two drives, a feature which can be used by DOS
5.0 and later.
Provide capability for booting from a fixed disk installed on the AHA-
1740A/1742A/1744.

Operation with the Standard Interface

The AHA-1740A/1742A/1744 BIOS resides on the host adapter board. It provides sup-
port for up to two SCSI Common Command Set (CCS) disk drives under DOS. If
SCSI devices other than CCS disk drives, or if support for more than two disk drives
is required under DOS, the Adaptec DOS Driver, ASW-1410 must be used. The BIOS
provides a very simple single-threaded capability that does not make use of the Mail-
box Interface used by more sophisticated programs. This capability allows the host
system to boot from a SCSI disk drive and to support standard DOS calls from any
standard program.
The BIOS communicates with the host adapter through a special set of commands
passed under the I/O Command Port command 03 (Start PC AT BIOS Command).
These commands are not available to any programs except the Adaptec BIOS. The
Adaptec BIOS provides a standard BIOS interface. The DOS interface to the stand-
ard BIOS is described in this section.
In many machines, up to two hard disks are supported by the BIOS resident on the
motherboard. Any other drives must be managed through the Mailbox Interface us-
ing an appropriate driver. The operation of the AHA-1740A/1742A/1744 BIOS on
7-1