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Home > BIOS Systems > BIOS Index Pages > The Motherboard ROM Bios
These pages will provide an understanding of what the Basic Input Output System is and how it works in a personal computer.
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The Motherboard Rom BIOS

All motherboards have a special chip containing software called the ROM BIOS. This ROM chip contains the startup programs and drivers that get the system running and act as the interface to the basic hardware in the system. When you turn on a system, the POST in the BIOS also tests the major components in the system. Additionally, you can run a setup program to store system configuration data in the CMOS memory, which is powered by a battery on the motherboard. This CMOS RAM is often called NVRAM (nonvolatile RAM) because it runs on about 1 millionth of an amp of electrical current and can store data for years when powered by a tiny lithium battery.

The motherboard ROM contains a collection of programs embedded in one or more chips, depending on the design of your computer. That collection of programs is the first thing loaded when you start your computer, even before the OS. Simply put, the BIOS in most PCs has four main functions:

  • POST The POST tests your computer’s processor, memory, chipset, video adapter, disk controllers, disk drives, keyboard, and other crucial components.

  • Setup The system configuration and setup program is usually a menu-driven program activated by pressing a special key during the POST. It enables you to configure the motherboard and chipset settings along with the date and time, passwords, disk drives, and other basic system settings. You also can control the power-management settings and boot-drive sequence from the BIOS Setup, and on some systems, you can also configure CPU timing and clock-multiplier settings. Some older 286 and 386 systems did not have the Setup program in ROM and required that you boot from a special setup disk. Also, some newer systems use a Windows-based application to access BIOS Setup settings.

  • Bootstrap loader A routine that reads the first physical sector of various disk drives looking for a valid master boot record (MBR). If one meeting certain minimum criteria (ending in the signature bytes 55AAh) is found, the code within is executed. The MBR program code then continues the boot process by reading the first physical sector of the bootable volume, which is the start of the volume boot record (VBR). The VBR then loads the first OS startup file, which is usually IO.SYS (Windows 9x/Me), ntldr (Windows XP/2000/NT), or bootmgr (Windows 7/Vista). The OS is then in control and continues the boot process.

  • BIOS— This refers to the collection of actual drivers that act as a basic interface between the OS and your hardware when the system is booted and running. When running DOS or Windows in Safe mode, you are running almost solely on ROM-based BIOS drivers because none are loaded from disk.

ROM Hardware

Read-only memory (ROM) is a type of memory that can permanently or semi-permanently hold data. It is called read-onlybecause it is either impossible or difficult to write to. ROM is also often called nonvolatile memory because any data stored in ROM remains even if the power is turned off. As such, ROM is an ideal place to put the PC’s startup instructions—that is, the software that boots the system (the BIOS).

Note that ROM and RAM are not opposites, as some people seem to believe. In fact, ROM is technically a subset of the system’s RAM. In other words, a portion of the system’s random access memory address space is mapped into one or more ROM chips. This is necessary to contain the software that enables the PC to boot up; otherwise, the processor would have no program in memory to execute when it is powered on.

For example, when a PC is turned on, the processor automatically jumps to address FFFF0h, expecting to find instructions to tell the processor what to do. This location is exactly 16 bytes from the end of the first megabyte of RAM space, as well as the end of the ROM. If this location were mapped into regular RAM chips, any data stored there would have disappeared when the power was previously turned off, and the processor would subsequently find no instructions to run the next time the power was turned on. By placing a ROM chip at this address, a system startup program can be permanently loaded into the ROM and will be available every time the system is turned on.

Normally, the system ROM starts at address E0000h or F0000h, which is 128KB or 64KB prior to the end of the first megabyte. Because the ROM chip usually is up to 128KB in size, the ROM programs are allowed to occupy the entire last 128KB of the first megabyte, including the critical FFFF0h startup instruction address, which is located 16 bytes from the end of the BIOS space. Some motherboard ROM chips are larger, up to 256KB or 512KB in size. The additional code in these is configured to act as a video card ROM (addresses C0000h–C7FFFh) on motherboards with built-in video and might even contain additional ROM drivers configured anywhere from C8000h to DFFFFh to support additional onboard devices, such as RAID or network adapters.

Figure 5.2 shows a map of the first megabyte of memory in a PC; notice the upper memory areas reserved for adapter card and motherboard ROM BIOS at the end of the first megabyte.

Figure 5.2. PC memory map showing ROM BIOS.

Some think it is strange that the PC would start executing BIOS instructions 16 bytes from the end of the ROM, but this design is intentionally built in to Intel’s x86 processors. All the ROM programmer has to do is place a JMP (jump) instruction at that address that instructs the processor to jump to the actual beginning of the ROM—in most cases, close to F0000h—which is about 64KB earlier in the memory map. It’s like deciding to read every book starting 16 pages from the end and then having all book publishers agree to place an instruction there to jump back the necessary number of pages to get to page 1. By setting the processor startup location in this way, Intel enabled the ROM to grow to be any size, all the while keeping it at the upper end of addresses in the first megabyte of the memory address space.

Adapter card ROMs are automatically scanned and read by the motherboard ROM during the early part of the boot process—during the POST. The motherboard ROM scans a special area of RAM reserved for adapter ROMs (addresses C0000–DFFFFh) looking for 55AAh signature bytes. This area indicates the start of a ROM.

All adapter ROMs must start with 55AAh; otherwise, the motherboard won’t recognize them. The third byte indicates the size of the ROM in 512-byte units called paragraphs, and the fourth byte is the actual start of the driver programs. The motherboard ROM uses the size byte for testing purposes. The motherboard ROM adds all the bytes in the ROM and divides the sum by the number of bytes. The result should produce a remainder of 100h. Thus, when creating a ROM for an adapter, the programmer typically uses a “fill” byte at the end to get the checksum to come out right. Using this checksum, the motherboard tests each adapter ROM during the POST and flags any that appear to have been corrupted.

The motherboard ROM automatically runs the programs in any adapter ROMs it finds during the scan. You see this in most systems when you turn them on, and during the POST you see the video card BIOS initialize and announce its presence.

ROM Shadowing

ROM chips by nature are slow, with access times of 150ns (nanoseconds, or billionths of a second), compared to dynamic RAM (DRAM) access times of well under 10ns on most systems. Because of this, in virtually all systems the ROMs areshadowed, which means they are copied into RAM at startup to allow faster access during normal operation. The shadowing procedure copies the ROM into RAM and then assigns that RAM the same address as the ROM originally used, disabling the actual ROM in the process. This makes the system seem as though it has ROM running at the same speed as RAM.

The performance gain from shadowing adapter card ROMs is often slight, and it can cause problems if not set up properly. Therefore, in most cases, it is wise to shadow only the motherboard (and maybe the video card BIOS) and leave the others alone.

Typically, shadowing is useful only if you are running a 16-bit OS, such as DOS or Windows 3.x. If you are running a 32-bit or 64-bit OS, shadowing is virtually useless because those OSs do not use the 16-bit ROM code while running. Instead, those OSs load drivers into RAM, which replace the 16-bit ROM code used only during system startup.

If present, shadowing options are found in the CMOS Setup program in the motherboard ROM, which is covered in more detail in the section titled “CMOS Setup.”

ROM Chip Types

The four main types of ROM chips that have been used in PCs are as follows:

  • ROM— Read-only memory

  • PROM— Programmable ROM

  • EPROM— Erasable PROM

  • EEPROM— Electrically erasable PROM, also sometimes called a flash ROM

No matter which type of ROM your system uses, the data stored in a ROM chip is nonvolatile and remains indefinitely unless intentionally erased or overwritten (in those cases where that is possible).

ROM (True or Mask ROM)

Originally, most ROMs were manufactured with the binary data (0s and 1s) already “cast in” or integrated into the die. The die represents the actual silicon chip. These are called mask ROMs because the data is formed into the mask from which the ROM die is photolithographically produced. This type of manufacturing method is economical if you are making hundreds of thousands of ROMs with the same information. If you must change a single bit, however, you must remake the mask, which is an expensive proposition. Because of costs and inflexibility, nobody uses mask ROMs anymore.

Mask ROMs are exactly analogous to prerecorded CD-ROMs. Some people think a CD-ROM is first manufactured as a blank and then the data is written to it by a laser, but that is not true. A CD-ROM is literally a piece of plastic that is stamped in a press, and the data is directly molded in, not written. The only actual recording is done with the master disc from which the molds or stamps are made.


PROMs are a type of ROM that is blank when new and must be programmed with whatever data you want. The PROM was invented in the late 1970s by Texas Instruments and has been available in sizes from 1KB (8Kb) to 2MB (16Mb) or more. PROMs can be identified by their part numbers, which usually are 27nnnn—where the 27 indicates the TI type PROM and the nnnn indicates the size of the chip in kilobits (not bytes). For example, most PCs that used PROMs came with 27512 or 271000 chips, which indicate 512Kb (64KB) or 1Mb (128KB), respectively.


Since 1981, all cars sold in the United States have used onboard computers with some form of ROM containing the control software. For example, the 1989 Pontiac Turbo Trans Am I had came with an onboard computer containing a 2732 PROM, which was a 32Kb (4KB) chip in the ECM (electronic control module or vehicle computer) under the dash. This chip contained the vehicle operating software as well as all the data tables describing spark advance, fuel delivery, and other engine and vehicle operating parameters. Many devices with integrated computers use PROMs to store their operating programs.

Although we say these chips are blank when new, they are technically preloaded with binary 1s. In other words, a 1Mb ROM chip used in a PC would come with 1 million (actually 1,048,576) bit locations, each containing a binary 1. A blank PROM can then be programmed, which is the act of writing to it. This usually requires a special machine called a device programmer, ROM programmer, or ROM burner (see Figure 5.3).

Figure 5.3. Typical gang (multisocket) device programmer (PROM burner).

Programming the ROM is sometimes referred to as burning it because that is technically an apt description of the process. You can think of each binary 1 bit as a fuse, which is intact. Most chips run on 5 volts, but when a PROM is programmed, a higher voltage (normally 12 volts) is placed at the various addresses within the chip. This higher voltage actually blows or burns the fuses at the desired locations, thus turning any given 1 into a 0. Although you can turn a 1 into a 0, you should note that the process is irreversible; that is, you can’t turn a 0 back into a 1.

The device programmer examines the program you want to write into the chip and then selectively changes only the 1s to 0s where necessary in the chip.

PROM chips are often referred to as one-time programmable (OTP) chips for this reason. They can be programmed once and never erased. Most PROMs are inexpensive (about $3 for a typical PC motherboard PROM), so if you want to change the program in a PROM, you discard it and program a fresh one with the new data.

The act of programming a PROM takes anywhere from a few seconds to a few minutes, depending on the size of the chip and the algorithm the programming device uses. Figure 5.3 shows an illustration of a typical PROM programmer that has multiple sockets. This is called a gang programmer, and it can program several chips at once, saving time if you have several chips to write with the same data. Less expensive programmers are available with only one socket, which is fine for most individual use.

I use and recommend a programmer from a company called Andromeda Research Labs (

Custom Programming of PROM Chips

I even used a PROM programmer to reprogram the chip in an 1989 Turbo Trans Am, changing the factory preset speed and rpm limiters, turbocharger boost, torque converter lockup points, spark advance, fuel delivery, idle speed, and much more! I also incorporated a switch box under the dash that enables me to switch from among four different chips, even while the vehicle is running. One chip I created I called a “valet chip,” which, when engaged, cuts off the fuel injectors at 36 miles per hour and restarts them when the vehicle coasts down to 35 mph. Another chip I created cuts off fuel to the engine altogether, which I engage for security purposes when the vehicle is parked. If you are interested in such a chip-switching device or custom chips for your Turbo Trans Am or Buick Grand National, I recommend you contact Casper’s Electronics (


One variation of the PROM that has been popular is the EPROM. An EPROM is a PROM that is erasable. An EPROM chip can be easily recognized by the clear quartz crystal window set in the chip package directly over the die (see Figure 5.4). You can actually see the die through the window! EPROMs have the same 27xxxx part-numbering scheme as the standard PROM, and they are functionally and physically identical except for the clear quartz window above the die.

Figure 5.4. An EPROM showing the quartz window for ultraviolet erasing.

The purpose of the window is to allow ultraviolet light to reach the chip die because the EPROM is erased by exposure to intense UV light. The window is quartz crystal because regular glass blocks UV light. You can’t get a suntan through a glass window!


The quartz window makes the EPROMs more expensive than the OTP PROMs. This extra expense is needless if erasability is not important.

The UV light erases the chip by causing a chemical reaction, which essentially melts the fuses back together. Thus, any binary 0s in the chip become 1s, and the chip is restored to a new condition with binary 1s in all locations. To work, the UV exposure must be at a specific wavelength (2,537 angstroms), at a fairly high intensity (12,000 uw/cm2), in close proximity (2cm–3cm, or about 1 inch), and last for between 5 and 15 minutes. An EPROM eraser is a device that contains a UV light source (usually a sunlamp-type bulb) above a sealed compartment drawer in which you place the chip or chips (see Figure 5.5).

Figure 5.5. A professional EPROM eraser.

This figure shows a professional-type EPROM eraser that can handle up to 50 chips at a time. I use a much smaller and less expensive one that erases up to four chips at a time and is both economical and portable. Several low-cost EPROM erasers are sold by DigiKey ( and other sources of EPROM programming equipment.

The quartz crystal window on an EPROM typically is covered by tape, which prevents accidental exposure to UV light. UV light is present in sunlight, of course, and even in standard room lighting, so that over time a chip exposed to the light can begin to degrade. For this reason, after a chip is programmed, you should put a sticker over the window to protect it.


A newer type of ROM is the EEPROM, which stands for electrically erasable PROM. These chips are also called flash ROMsand are characterized by their capability to be erased and reprogrammed directly in the circuit board they are installed in, with no special equipment required. By using an EEPROM, or flash ROM, you can erase and reprogram the motherboard ROM in a PC without removing the chip from the system or even opening the system chassis.

With a flash ROM or EEPROM, you don’t need a UV eraser or device programmer to program or erase chips. Not only do virtually all PC motherboards built since 1994 use flash ROMs or EEPROMs, but most automobiles built since then use them as well.

The EEPROM or flash ROM can be identified by a 28xxxx or 29xxxx part number, as well as by the absence of a window on the chip. Having an EEPROM or a flash ROM in your PC motherboard means you now can easily upgrade the motherboard ROM without having to swap chips. In most cases, you download the updated ROM from the motherboard manufacturer’s website and then run a special program it provides to update the ROM. This procedure is described in more detail later in this chapter.

I recommend that you periodically check with your motherboard manufacturer to see whether an updated BIOS is available for your system. An updated BIOS might contain bug fixes or enable new features or device support not originally found in your system.

Non-PC ROM Upgrades

For the auto enthusiasts out there, you might want to do the same for your car; that is, check to see whether ROM upgrades are available for your vehicle’s computer. Now that updates are so easy and inexpensive, vehicle manufacturers are releasing bug-fix ROM upgrades that correct operational problems or improve vehicle performance. In most cases, you must check with your dealer to see whether any new vehicle ROMs are available. If you have a GM car, GM has a website where you can get information about the BIOS revisions available for your car, which it calls Vehicle Calibrations. The GM Vehicle Calibration Information site address is

Besides updates from the vehicle manufacturer, some aftermarket companies produce ROM upgrades for vehicles. If you are interested in having a custom program installed in your flash ROM–equipped vehicle, I recommend Fastchip (, Hypertech (, and Superchips ( If you want to develop your own vehicle calibrations, see for more information.

Flash ROM updates can also be used to add new capabilities to existing peripherals—for example, updating a modem to support newer communications standards or updating optical rewritable drives to support new media.

These days, many objects with embedded computers controlling them are using flash ROMs; for example, I have updated the flash ROM code (often called firmware) in hard drives, optical drives, network routers, wireless access points, network attached storage drives, Blu-ray players, digital cameras, and more. Installing flash ROM or firmware upgrades is as easy as downloading a file from the device manufacturer website and running the update program included in the file. Who knows, one day you might find yourself downloading flash ROM upgrades for your toaster!

ROM BIOS Manufacturers

Several popular BIOS manufacturers on the market today supply the majority of motherboard and system manufacturers with the code for their ROMs. This section discusses the various available versions.

Several companies have specialized in the development of a compatible ROM BIOS product. The three major companies that come to mind in discussing ROM BIOS software are American Megatrends, Inc. (AMI), Phoenix Technologies, and Award Software (now owned by Phoenix Technologies). Each company licenses its ROM BIOS to motherboard manufacturers so those manufacturers can worry about the hardware rather than the software. To obtain one of these ROMs for a motherboard, the original equipment manufacturer (OEM) must answer many questions about the design of the system so that the proper BIOS can be either developed or selected from those already designed. Combining a ROM BIOS and a motherboard is not a haphazard task. No single, generic, compatible ROM exists, either. AMI, Award, and Phoenix ship many variations of their BIOS code to different board manufacturers, each one custom-tailored to that specific motherboard.

Over the years, some major changes have occurred in the BIOS industry. Intel, perhaps the largest BIOS customer, has switched between Phoenix and AMI for most of its motherboards. Intel originally used a Phoenix BIOS core in its motherboards up through 1995, when it changed to an AMI core. It then used AMI until 1997, when it switched back to Phoenix. In 1999 Intel switched again, this time back to AMI. In each case note that while Intel gets the core BIOS from Phoenix or AMI, they are highly customized for the individual motherboards they are installed in.

Another major development occurred in late 1998, when Phoenix bought Award. Since then Phoenix has sold both the Phoenix and Award BIOS as different products. The Award BIOS is sold as its standard product, whereas the Phoenix BIOS is sold as a more feature-rich BIOS for high-end systems. Currently, the BIOS market is mostly divided between AMI and Phoenix; however, Phoenix not only develops the BIOS for many systems, but also is the primary BIOS developer responsible for new BIOS development and new BIOS standards.

Another development in recent years has been the creation of separate BIOS products for 32-bit and 64-bit desktop systems, mobile systems, 32-bit and 64-bit servers, and embedded devices. Although all BIOS chips must perform some of the same tasks, a BIOS product optimized for a mobile computer often needs additional support for features such as docking modules, advanced battery power management, as well as bootable USB and removable flash memory devices, whereas a BIOS optimized for a server needs support for features such as advanced hardware monitoring and 64-bit PCI slots. By creating customized BIOS versions for different platforms, BIOS vendors provide support for the features needed by a particular computing platform and provide better performance and stability.


Most OEMs contract with AMI or Phoenix for motherboard BIOS core and then perform their own customizations for specific chipsets and motherboards. Although most use either the AMI or Phoenix core offerings, some OEMs have developed their own compatible BIOS ROMs independently. Companies such as Dell, HP/Compaq, AT&T, and Acer have developed several of their own BIOS products. In some cases they may have started with an AMI or Phoenix core that they licensed years ago, and continued development in-house after that.

Even though AMI or Phoenix might have done the initial development, you still must get upgrades or fixes from the system or motherboard manufacturer. This is really true for all systems because the system or motherboard manufacturers customize the BIOS for their boards.


The AMI BIOS is one of the most popular BIOSs in PC systems. Some versions of the AMI BIOS are called Hi-Flex because of the high flexibility found in the BIOS configuration program. The AMI Hi-Flex BIOS products are used in Intel, AMI, and many other manufacturers’ motherboards. AMI is also the only third-party BIOS manufacturer to make its own motherboards and other hardware devices.

During power-up, the AMI BIOS ID string is displayed on the lower-left part of the screen. This string tells you valuable information about which BIOS version you have and about certain settings that the built-in setup program determines.


A good trick to help you view the BIOS ID string is to shut down and either unplug your keyboard or hold down a key as you power it back on. This causes a keyboard error, and the string remains displayed.

You also can download the AMI Motherboard ID Utility program (AMIMBID) from AMI’s website ( and run it to determine the contents of ID String 1.

The primary BIOS identification string (ID String 1) is displayed by any AMI BIOS during the POST in the bottom-left corner of the screen, below the copyright message. Two additional BIOS ID strings (ID Strings 2 and 3) can be displayed by the AMI Hi-Flex BIOS by pressing the Insert key during the POST. These additional ID strings display the options installed in the BIOS.

The AMI BIOS has many features, including a built-in setup program normally activated by pressing the Delete or Esc key within the first few seconds of powering on the system. In most cases the BIOS prompts you briefly on the screen as to which key to press and when to press it.

A unique feature of some of the AMI BIOS versions was that in addition to the setup, they had a built-in, menu-driven diagnostics package—essentially a limited version of the standalone AMIDIAG product. The internal diagnostics are not a replacement for more comprehensive disk-based programs, but they can help in a pinch. The menu-driven diagnostics do not do extensive memory testing, for example, and the hard disk low-level formatter works only at the BIOS level rather than at the controller register level. These limitations often have prevented it from being capable of formatting severely damaged disks. Most newer AMI BIOS versions no longer include the full diagnostics.

AMI produces BIOS upgrades only for its own motherboards. If you have a non-AMI motherboard with a customized AMI BIOS, you must contact the motherboard or system manufacturer for an upgrade.

Phoenix Award BIOS

Phoenix now refers to the family of products built on its Award BIOS as Phoenix Award BIOS. For several years it was also called the Phoenix FirstBIOS. The Award BIOS has all the features you expect, including a built-in setup program activated by pressing a particular key on startup (usually prompted on the screen). The POST is good, although the few beep codes supported means that a POST card may be helpful if you want to diagnose power-on fatal error problems. Phoenix provides technical support for the Award BIOS on its website at


The Phoenix BIOS is currently licensed by Phoenix under the SecureCore brand. For many years it has been a standard of compatibility by which others are judged. Phoenix was one of the first third-party companies to legally reverse-engineer the IBM BIOS using a clean-room approach. In this approach, a group of engineers studied the IBM BIOS and wrote a specification for how that BIOS should work and what features should be incorporated. This information then was passed to a second group of engineers who had never seen the IBM BIOS. They could then legally write a new BIOS to the specifications set forth by the first group. This work was then unique and not a copy of IBM’s BIOS; however, it functioned the same way.

The Phoenix BIOS has a built-in setup program that is normally activated by pressing F1 or F2 during the POST. The Phoenix BIOS includes a built-in setup program normally activated by pressing the F1 or F2 key within the first few seconds of powering on the system. In most cases the BIOS prompts you briefly on the screen as to which key to press and when to press it.

Phoenix has extensive technical support and documentation on its website at


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BIOS Hardware/Software


Some important drivers must be active during boot time. For example, how can you boot from a hard disk if the drivers necessary to make the disk interface work must be loaded from that disk? Obviously, at least a minimum level of hard disk drivers must be preloaded into ROM either on the motherboard or on an adapter card for the system to be able to boot.

As another example, how can you 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. Instead, when IBM designed the original PC, 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 OS from the hard disk.A few cards (adapter boards) almost always have a ROM onboard, including the following:

  • Video cards— All have an onboard BIOS.

  • RAID (Redundant Array of Independent Disks) cards— Adapters that enable you to attach multiple drives and array them in different ways to improve reliability, redundancy, and performance. These cards require an onboard BIOS to enable the array to be bootable.

  • 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 local area network (LAN) as diskless workstations—also called Net PCs, NCs (network computers), thin clients, or even smart terminals.

  • ATA/Serial ATA (SATA) 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 the BIOS that 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.

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