BIOS Hardware and Software

written by: Melinda Mirovich; article published: year 2006, month 09;


In: Root » Computers and technology » Software » BIOS Hardware and Software

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The BIOS itself is software running in memory that consists of all the various drivers that interface the hardware to the operating system. The BIOS is unique compared to normal software in that it doesn't all load from disk; some of it is preloaded into memory chips (read-only memory, or ROM) installed in the system or on adapter cards.

The BIOS in a PC comes from three possible sources:

  • Motherboard ROM

  • Adapter card ROM (such as that found on a video card)

  • Loaded into RAM from disk (device drivers)

The motherboard ROM BIOS is most often associated with hardware rather than software. This is because the BIOS on the motherboard is contained in a ROM chip on the board, which contains the initial software drivers needed to get the system running. Years ago, when only DOS was running on basic PCs, this was enough, so no other drivers were neededthe motherboard BIOS had everything that was necessary. The motherboard BIOS usually includes drivers for all the basic system components, including the keyboard, floppy drive, hard drive, serial and parallel ports, and more. As systems became more complex, new hardware was added for which no motherboard BIOS drivers existed. These included devices such as newer video adapters, CD-ROM drives, SCSI hard disks, USB ports, and so on.

Rather than requiring a new motherboard BIOS that would specifically support the new devices, it was far simpler and more practical to copy any new drivers that were necessary onto the system hard disk and configure the operating system to load them at boot time. This is how most CD-ROM drives, sound cards, scanners, printers, PC Card (PCMCIA) devices, and so on are supported. Because these devices don't need to be active during boot time, the system can boot up from the hard disk and wait to load the drivers during the initial operating system load.

Some drivers, however, must be active during boot time. For example, how could you boot from a hard disk if the drivers necessary to make the disk interface work must be loaded from that disk? Obviously, the hard disk drivers must be preloaded into ROM either on the motherboard or on an adapter card.

How will you be able to see anything onscreen if your video card doesn't have a set of drivers in a ROM? The solution to this could be to provide a motherboard ROM with the appropriate video drivers built in; however, this is impractical because of the variety of video cards, each needing its own drivers. You would end up with hundreds of different motherboard ROMs, depending on which video card you had. Instead, when IBM designed the original PC, it created a better solution. It designed the PC's motherboard ROM to scan the slots looking for adapter cards with ROMs on them. If a card was found with a ROM on it, the ROM was executed during the initial system startup phase, before the system began loading the operating system from the hard disk.

By putting the ROM-based drivers right on the card, you didn't have to change your motherboard ROM to have built-in support for new devices, especially those that needed to be active during boot time. A few cards (adapter boards) almost always have a ROM onboard, including the following:

  • Video cards. All have an onboard BIOS.

  • SCSI adapters. Those that support booting from SCSI hard drives or CD-ROMs have an onboard BIOS. Note that, in most cases, the SCSI BIOS does not support any SCSI devices other than a hard disk; if you use a SCSI CD-ROM, scanner, Zip drive, and so on, you still need to load the appropriate drivers for those devices from your hard disk. Most newer SCSI adapters support booting from a SCSI CD-ROM, but CD-ROM drivers are still necessary to access the CD-ROM when booting from another drive or device.

  • Network cards. Those that support booting directly from a file server have what is usually called a boot ROM or IPL (initial program load) ROM onboard. This enables PCs to be configured on a LAN as diskless workstationsalso called Net PCs, NCs (network computers), thin clients, or even smart terminals.

  • ATA or floppy upgrade boards. Boards that enable you to attach more or different types of drives than what is typically supported by the motherboard alone. These cards require an onboard BIOS to enable these drives to be bootable.

  • Y2K boards. Boards that incorporate BIOS fixes to update the century byte in the CMOS RAM. These boards have a small driver contained in a BIOS, which monitors the year byte for a change from 99 to 00. When this is detected, the driver updates the century byte from 19 to 20, correcting a flaw in some older motherboard ROM BIOS. Although it might seem strange to list Y2K boards because the century changed several years ago, if installed these boards often remain in use until the systems using them are retired.

BIOS and CMOS RAM

Some people confuse BIOS with the CMOS RAM in a system. This confusion is aided by the fact that the Setup program in the BIOS is used to set and store the configuration settings in the CMOS RAM. They are, in fact, two totally separate components.

The BIOS on the motherboard is stored in a fixed ROM chip. Also on the motherboard is a chip called the RTC/NVRAM chip, which stands for real-time clock/nonvolatile memory. This is where the BIOS Setup information is stored, and it is actually a digital clock chip with a few extra bytes of memory thrown in. It is usually called the CMOS chip because it is made using CMOS (complementary metal-oxide semiconductor) technology.

The first example of this ever used in a PC was the Motorola MC146818 chip, which had 64 bytes of storage, of which 14 bytes were dedicated to the clock function, leaving 50 bytes to store BIOS Setup settings. Although it is called nonvolatile, it is actually volatile, meaning that without power, the time/date settings and the data in the RAM portion will in fact be erased. It is considered nonvolatile by many because it is designed using CMOS technology, which results in a chip that still requires power but very little compared to other chips. A battery in the system, rather than the AC wall current, provides that power. This is also why most people call this chip the CMOS RAM chip; although not technically accurate (almost all modern chips use a form of CMOS technology), the term has stuck. Most RTC/NVRAM chips run on as little as 1 micro amp (millionth of an amp), so they use very little battery power to run. Most lithium coin cell batteries can last up to 5 years or more before the battery runs out and the information stored (as well as the date and time) is lost. Some systems use special versions of these chips made by Dallas Semiconductor, Benchmarq, or Odin (such as the DS12885 and DS12887) that include both the RTC/NVRAM chip and the battery in a single component.

When you enter your BIOS Setup, configure your hard disk parameters or other BIOS Setup settings, and save them, these settings are written to the storage area in the RTC/NVRAM (otherwise called CMOS RAM) chip. Every time your system boots up, it reads the parameters stored in the CMOS RAM chip to determine how the system should be configured. A relationship exists between the BIOS and CMOS RAM, but they are two distinctly different parts of the system.

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