- The x86 BIOS: Its Limits and Capabilities
- 16- and 32-Bit Code
- The BIOS as Driver for DOS
- Modern Uses of the BIOS
- What BIOS Do You Have?
- Add-On Card BIOSs
- Video BIOSs
- Boot BIOSs for SCSI and Networking
- Additional BIOSs
- BIOS Updates
- EIDE and SCSI Hard Disk Handling
- Understanding CHS Geometry Limits
- Getting Around the 1024-Cylinder Limit
- Common BIOS Disk Utilities
- The Handoff to the OS
Boot BIOSs for SCSI and Networking
Standard BIOSs support booting a PC from a floppy disk or an integrated device electronics (IDE) or enhanced IDE (EIDE) hard disk. These disk types are controlled in the same way as many older drive types, so they can all use the same basic BIOS support to operate. If you want to boot from a SCSI hard disk or run a diskless workstation and boot from a network, though, you must have an appropriate SCSI host adapter or network card with a boot BIOS. This BIOS extends the PC's standard BIOS so that the PC can use the add-on device in place of the standard device for booting the computer. In most cases, you must also provide a driver for each OS to use the new device because the OS won't use the BIOS to access the drive after the OS is running the computer.
For both SCSI host adapters and network cards, the least expensive models often lack boot BIOSs. These cards are perfectly good for using non-boot SCSI devices and for networking a computer that boots from a hard disk, respectively, so if you have such a card, the lack of a boot BIOS might not be sufficient cause to ditch the card.
Some midrange SCSI host adapters, particularly those based on some SCSI chips from Symbios, lack their own BIOSs, but BIOSs are available and can be installed in the motherboard's BIOS. In fact, some motherboards ship with this Symbios SCSI BIOS support. If you have such a motherboard, you can boot even from a BIOS-less Symbios-based SCSI host adapter. The Symbios BIOS won't do you any good with BIOS-less host adapters based on other manufacturers' chipsets, however, because the Symbios BIOS is specific to Symbios products.
Watch your computer screen carefully as it boots. Many boot BIOSs for SCSI host adapters include setup utilities you can use to configure the hardware. When present, the SCSI BIOS displays a prompt informing you of a key sequence to press to access the utility. You can use these utilities to alter the SCSI termination, adjust the order in which the SCSI BIOS attempts to boot from devices, and so on.
Normally, a PC attempts to boot first from an EIDE drive and only then from a SCSI disk or network device. Therefore, if you have both SCSI and EIDE disks but want to boot from the SCSI disk, you might have difficulties. Most BIOSs produced since 1997, though, include an option to change the boot order so that SCSI hard disks can be tried first. Such computers can also often boot from CD-ROM drives, LS-120 disks, and so on. You control these features from the motherboard BIOS's CMOS setup utility, not from the SCSI host adapter's BIOS. If you select such an option, be aware that you might need to make adjustments in your OS to tell it what the boot drive is. In OS/2, for example, you must ensure that your SCSI host adapter's driver appears before the IBM1S506.ADD IDE/EIDE driver in OS/2's CONFIG.SYS file.
With many more advanced OSs, you can boot from a SCSI device even if the motherboard's BIOS doesn't support this feature. You do so by telling the motherboard BIOS that you have no EIDE devices, even when you do. The BIOS will then fail to detect the EIDE drives and will boot from the SCSI drive. The drawback to this is that, if your OS relies on the BIOS to handle disk accesses, the EIDE drives will be unavailable. If your OS includes its own EIDE drivers, however, you'll regain access to the EIDE drives as soon as the OS has booted.