Chapter SevenOnboard BIOS InterfaceIntroductionThe AHA-1740A/1742A/1744 BIOS operates in two different modes which correspondto the two possible firmware configurations. The two modes supported by the BIOSare 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 theSCSI bus. This support enables use of drives attached to the adapter underreal mode operating systems such as DOS without a device driver. The limitfor Int 13h supported drives is set at two (80h-81h). Future revisions of theBIOS will support more than two drives, a feature which can be used by DOS5.0 and later.• Provide capability for booting from a fixed disk installed on the AHA-1740A/1742A/1744.Operation with the Standard InterfaceThe 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. IfSCSI devices other than CCS disk drives, or if support for more than two disk drivesis required under DOS, the Adaptec DOS Driver, ASW-1410 must be used. The BIOSprovides 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 hostsystem to boot from a SCSI disk drive and to support standard DOS calls from anystandard program.The BIOS communicates with the host adapter through a special set of commandspassed 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. TheAdaptec 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 themotherboard. Any other drives must be managed through the Mailbox Interface us-ing an appropriate driver. The operation of the AHA-1740A/1742A/1744 BIOS on7-1
