Checking BIOS Settings

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The Basic Input/Output System (BIOS) is the lowest-level software component in a computer. The CPU runs BIOS code as part of its startup procedure. As a result, the BIOS configures many fundamental aspects of the computer before Linux has a chance to boot. The BIOS also provides tools that the computer uses to load the Linux kernel into memory.

Although the x86 BIOS provides some standard features, it's not entirely standardized. In particular, modern BIOSes provide a setup tool, often referred to as the Complementary Metal Oxide Semiconductor (CMOS) setup utility, that you can use to set various low-level options. The options available in a computer's CMOS setup utility differ from one computer to another, both because of differences in hardware and because of different BIOS designs.

Most computers display a prompt at boot time that tells you how to get into the CMOS setup utility. This is usually done by hitting a key, such as Delete or F2, at a critical point during the boot process. Once you've done this, you'll see a BIOS setup screen, such as the one shown in Figure 1.3. This screen allows you to select and set various options, typically by moving through menus by pressing the arrow keys on the keyboard.

SCSI host adapters often include their own BIOSes and setup utilities, which are separate from the motherboard BIOS. The SCSI setup utilities usually have setup options you can adjust by pressing a key sequence at a particular point in the boot process. Watch your boot displays or consult your SCSI adapter's documentation for details.

FIGURE 1.3 CMOS setup utilities use menu-driven displays to let you adjust a computer's built-in hardware.

PhoenixBIOS Setup Utilitu

Aduanced Security Power Boot Exit

System T ime:

[Eg: IB: 261

System Date:


Legacy Diskette A:

[1.44/1.25 MB 3n

Legacy Diskette B:


Primary Master


Primary Slave


Secondary Master


Secondary Slaue


Keyboard Features

System Memory:

640 KB

Extended Memory:

64512 KB


[English (US)]

Item Specific Help

<Tab>, <Shift-Tab>, or <Enter> selects Field.

Help m Select Item Exit ? Select Menu

Change Ualues Select ► Sub-Menu

Setup Defaults Save and Exit

Most systems come with reasonable default BIOS settings, but you may want to check, and possibly adjust, a few. These include the following:

Disk settings There are two common hard disk settings you may need to adjust. The first specifies the size of the disk. An auto-detection feature normally works well for this. The second setting determines how the BIOS interprets the disk's cylinder/head/sector (CHS) addresses. On most BIOSes, a linear block addressing (LBA) mode is the best choice. If you use SCSI hard disks, the main motherboard BIOS won't detect them. This is normal; the SCSI BIOS provides the necessary support.

On-board ports Modern motherboards include RS-232 serial, parallel, USB, EIDE, and frequently other types of ports. You can enable or disable these or change their settings (for instance, you can change the IRQs used by the devices). Disabling unused ports can free up resources for other devices.

PCI settings Some BIOSes allow you to specify how the system treats PCI devices. Most commonly, you can choose from two or more rules for how the BIOS assigns IRQs to PCI devices. Sometimes, one rule results in IRQ conflicts and another doesn't, so such a setting is worth investigating if you have problems booting and suspect IRQ conflicts.

Passwords In a high-security environment, you may want to set a BIOS password. This prevents the system from booting unless the correct password is entered. It can slow down intruders who have physical access to the computer and boot with their own boot disk, but if intruders have physical access to the computer, they can bypass this feature in various ways. Setting a BIOS password also prevents automatic reboots in the event of a power failure. Nonetheless, slowing down an intruder may be worthwhile in some environments.

Memory settings BIOSes can be configured to copy parts of themselves, or of BIOSes stored on other devices, to RAM. This practice, which is known as shadowing, speeds up access to the BIOS, and it is useful in DOS, which relies on the BIOS for input/output. Linux doesn't use the BIOS as much, so it's generally best to disable all shadowing in Linux, which can result in slightly more memory available in Linux. Some BIOSes also allow you to control one or more memory holes—regions of the CPU's memory map that are unusable. These sometimes cause Linux to misdetect the amount of RAM installed in the computer, so you may want to experiment with different memory hole settings.

Boot Devices Modern BIOSes support booting from a wide variety of disk and disk-like devices, including floppy disks, EIDE disks, SCSI disks, CD-ROM drives, and high-capacity removable disks like Zip or LS-120 disks. You can usually set the system to boot from some subset of these devices in any order you like. The BIOS tries each medium in turn, and if it's not present or isn't bootable, it tries the next one. For highest security, set the system to boot from your EIDE or SCSI hard disk first; for convenient booting of installation or emergency media, set it to boot from a CD-ROM, floppy, or other removable media drive first.

In practice, you may need to experiment with a particular computer's CMOS settings to determine which work best. It's generally not a good idea to try random changes on a working system, though; experiment with these settings only if you're having trouble. Making changes without cause can produce an unbootable system, although if you remember what you changed, you can usually recover your system to a working state.

Most CMOS setup utilities include an option to restore the settings to the factory default values. These may not always produce optimal results, but they'll usually work

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