attached SCSI devices depends on the number of standard hard disks installed di-rectly on the system.No Standard Hard Disks InstalledIn this case, two SCSI drives can be supported by the AHA-1740A/1742A/1744 BIOS.SCSI target 0 Logical Unit Number (LUN) 0 is designated as drive 0 (drive C). EitherSCSI target 0 LUN 1 or SCSI target 1 LUN 0 may be designated as drive 1 (drive D).This set of addresses allows the use of two drives with either embedded controllers orbridge controllers. Booting is only performed from SCSI Target 0 LUN 0. When nostandard hard disks are installed, the AHA-1740A/1742A/1744 BIOS is operating innonconcurrent mode.One Standard Hard Disk InstalledIn this case, the internally installed hard disk is designated as drive 0 (drive C).SCSI device 0 LUN 0 is designated as drive 1 (drive D). Booting is performed fromthe internally installed hard disk. This mode of operation is called concurrent opera-tion, indicating that the internally installed drive and one SCSI drive can operate to-gether under the BIOS.Two Standard Internal Hard Disks InstalledIn this case, all the disks that can be supported by the BIOS are directly installed.The AHA-1740A/1742A/1744 BIOS will not support any SCSI drives. A device drivermust be installed to access the SCSI drives.Enhanced Interface OperationHardwareThe BIOS occupies 16KBytes of system ROM space at one of a number of optionalphysical locations. These hex locations are CC000, D0000, D4000, D8000, DC000,E0000, E4000, E8000 and EC000. 2KBytes of this space is R/W shadow RAM, ena-bling the BIOS to store critical information without risking compatibility by usingsystem data areas. The BIOS is coded using the 386 instruction set.InitializationThe system BIOS recognizes the AHA-1740A/1742A BIOS by the standard ROM sig-nature information in the first four bytes of the EPROM. This signature includes anoffset to an initialization routine which is subsequently called by the system BIOS.When the AHA-1740A/1742A BIOS gains control, the following steps will be taken:1. The BIOS will search the EISA slots for the adapter onto which it is installed. Thecorrect adapter can be identified by matching the current code segment with theadaptec AHA-1740A/1742A/17447-2
