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Easily the most important component in a PC system is the main board or motherboard. Some companies refer to the motherboard as a system board or planar. The terms motherboard, main board, system board, and planar are interchangeable. In this chapter, we will examine the different types of motherboards available, as well as those components usually contained on the motherboard and motherboard interface connectors.

Replacement Motherboards

Some manufacturers go out of their way to make their systems as physically incompatible as possible with any other system. Then replacement parts, repairs, and upgrades are virtually impossible to find--except, of course, from the original system manufacturer, at a significantly higher price than the equivalent part would cost to fit a standard PC-compatible system.

For example, if the motherboard in an AT-chassis system dies, you can find any number of replacement boards that will bolt directly in, with a large choice of processors and clock speeds, at very good prices. If the motherboard dies in a newer IBM, Compaq, Hewlett-Packard, Packard Bell, Gateway, AST, or other proprietary shaped system, you'll pay for a replacement available only from the original manufacturer, and you have little or no opportunity to select a board with a faster or better processor than the one that failed. In other words, upgrading or repairing one of these systems via a motherboard replacement is difficult and usually not cost-effective.

Knowing What to Look for (Selection Criteria)

You can ask a consultant to make a recommendation for purchases. Making these types of recommendations is one of the most frequent tasks a consultant performs. Many consultants charge a large fee for this advice. Without guidance, many individuals don't have any rhyme or reason to their selections and instead base their choices solely on magazine reviews or, even worse, on some personal bias. To help eliminate this haphazard selection process, here is a simple checklist that will help you select a system. This list takes into consideration several important system aspects overlooked by most checklists. The goal is to ensure that the selected system truly is compatible and has a long life of service and upgrades ahead.

It helps to think like an engineer when you make your selection. Consider every aspect and detail of the motherboards in question. For instance, you should consider any future uses and upgrades. Technical support at a professional (as opposed to a user) level is extremely important. What support will be provided? Is there documentation, and does it cover everything else?

In short, a checklist is a good idea. You can use the following check list to evaluate any PC-compatible system. You might not have to meet every one of these criteria to consider a particular system, but if you miss more than a few of these checks, consider staying away from that system. The items at the top of the list are the most important, and the items at the bottom are perhaps of lesser importance. The rest of this chapter discusses in detail the criteria in this checklist:

  • Processor. The processor is normally the most expensive chip in a system. Because the processor is the brain of the system, it has a large influence on the performance of the total system. In Chapter 6 - The Microprocessor, you can find the differences between all types of processors. When you are planning to assemble your own system, you have to realize that the motherboard also has a large part in the performance of the system. Notice that the processor has to fit on the motherboard (check for the right socket). Also notice that the operating speed of the CPU clock often depends on the clock speed of the motherboard.

  • Motherboard Speed. When you choose a motherboard, first ensure that your processor fits on it. Notice that most processors run at a multiple of the motherboard speed. For example, Pentium 75 runs at a motherboard speed of 50MHz; Pentium 60, 90, 120, 150, and 180MHz chips run at a 60MHz base motherboard speed; and the Pentium 66, 100, 133, 166, and 200 run at a 66MHz motherboard speed setting. The Pentium Pro 150, 180, and 200 run at 50, 60, and 66MHz speeds, respectively. All components on the motherboard (especially cache memory) should be rated to run at the maximum allowable motherboard speed.

  • Memory. Most motherboards use normal types of memory modules, for example 36-pin SIMMs, 72-pin SIMMs or 168-pin DIMMs (Dual In-line Memory Modules). Ensure that your memory fits on the motherboard. On some motherboards, the memory modules must be installed in matched pairs (depends on the bus width of the motherboard and the memory modules). For example, on a 64-bit design Pentium board, the 72-pin SIMMs must be installed in matched pairs, while DIMMs are installed one at a time (one per 64-bit bank). Carefully consider the total amount of memory that the board supports. The more memory sockets a motherboard has, the better. Mission-critical systems ideally should use Parity RAM and ensure that the motherboard fully supports parity checking or even ECC (Error Correcting Code) as well.

  • Cache Memory. The cache memory is mostly located on the motherboard. Some boards however, have a special socket to expand the cache memory by placing an extra cache module. Cache memory often is a combination of built-in and on-board memory. For example: most Pentium Pro processors have a built-in 256K or 512K Level 2 cache, but may also have more Level 2 cache on the motherboard for even better performance.

  • Bus Type. Be sure the motherboard has the right bus slots for your expansion cards (for example, ISA, MCA or PCI). Also take a look at the layout of the bus slots to ensure that cards inserted in them will not block access to memory sockets, or be blocked by other components in the case. Check for extra bus slots to enable expansion of the system in the future.

  • BIOS. If you build a new system, the motherboard should use an industry-standard BIOS such as those from AMI, Phoenix, Microid Research, or Award. For easy updating, the BIOS should be of a Flash ROM or EEPROM (Electrically Erasable Programmable Read Only Memory) design. If you use Plug and Play components, the BIOS should support the PnP specification.

  • Form Factor. Check for the right form factor of the motherboard and the case, to ensure that the board will fit in the case. For an explanation of all different types of form factors, see the section 'Motherboard Form Factors' later in this chapter.

  • Built-in interfaces. Ideally, a motherboard should contain as many built-in standard controllers and interfaces as possible. However, when you need a special type of interface, for example a special video card, it's better to choose a motherboard without a built-in video interface. In most cases the built-in interfaces can be disabled, but not all motherboards have this feature. Generally there are better choices in external local bus video adapters. Another problem with built-in interfaces is the driver support. Usually a slot card adapter is more easily supported via standard drivers and is more easily upgraded as well.

  • Plug and Play (PnP). If you use Plug and Play components in your system, the motherboard should fully support the Intel PnP specification. This will allow automatic configuration of PCI adapters as well as PnP ISA adapters.

TIP: Even if a motherboard doesn't list that it's PnP-compatible, it may be. PCI motherboards are required to be PnP-compatible, as it is a part of the PCI standard.

  • Power Management. To minimize the power usage of the system, the motherboard should fully support the APM (Advanced Power Management) and SMM (System Management Mode) protocols that allow for powering down various system components to different levels of readiness and power consumption.

  • Motherboard Chipset. There are a lot of different motherboard chipsets on the market. A lot of motherboard features are based on the chipset. For example, the Intel Triton II (430HX) chipset, allows parity checking. The popular original Intel Triton (430FX) chipset, along with the newer 430TX and 430VX chipsets, does not support parity-checked memory. For critical applications where accuracy and data integrity is important, it's recommended that you use a board that support ECC memory using true parity memory modules.

  • Documentation. Good technical documentation is a requirement. Documents should include information on any and all jumpers and switches found on the board, connector pinouts for all connectors, specifications for cache RAM chips, memory modules, and other plug-in components, and any other applicable technical information. You can acquire separate documentation from the BIOS manufacturer covering the specific BIOS used in the system, as well as the data books covering the specific chipset used in the motherboard. Additional data books for any other controller or I/O chips on-board are a bonus, and may be acquired from the respective chip manufacturers. Another nice thing to have is available online support and documentation updates, although this should not be accepted in place of good hardcopy manuals.

You may notice that these selection criteria seem fairly strict and may disqualify many motherboards on the market, including what you already have in your system! These criteria will, however, guarantee you the highest quality motherboard offering the latest in PC technology that will be upgradable, expandable, and provide good service for many years. It's most recommended to purchase boards from better-known motherboard manufacturers such as Intel, SuperMicro, Micronics, AMI, Biostar, Tyan, Asus, and so on. These boards might cost a little more than others that you have never heard of, but there is some safety in the more well-known brands; that is, the more boards that they sell, the more likely that any problems will have been discovered by others and solved long before you get yours. Also, if service or support are needed, the larger vendors are more likely to be around in the long run.


As mentioned, extensive documentation is an important factor to consider when you're planning to purchase a motherboard. Most motherboard manufacturers design their boards around a particular chipset, which actually counts as the bulk of the motherboard circuitry. There are a number of manufacturers offering chipsets, such as Intel, Opti, VIA, SiS, and others. It's recommended to obtain the data book or other technical documentation on the chipset directly from the chipset manufacturer.

One of the more common questions of people about a system relates to the BIOS Setup program. People want to know what the "Advanced Chipset Setup" features mean and what the effects of changing them will be. Often they go to the BIOS manufacturer thinking that the BIOS documentation will offer help. Usually, however, people find that there is no real coverage of what the chipset setup features are in the BIOS documentation. You will find this information in the data book provided by the chipset manufacturer. Although these books are meant to be read by the engineers who design the boards, they contain all the detailed information about the chipset's features, especially those that might be adjustable. With the chipset data book, you will have an explanation of all the controls in the Advanced Chipset Setup section of the BIOS Setup program.

Besides the main chipset data books, it's also recommended to collect any data books on the other major chips in the system. This would include any floppy or IDE controller chips, Super I/O chips, and of course the main processor. You will find an incredible amount of information on these components in the data books.

CAUTION: Most chipset manufacturers only make a particular chip for a short time, rapidly superseding it with an improved or changed version. The data books are only available during the time the chip is being manufactured, so if you wait too long, you will find that such documents may no longer be available. The time to collect documentation on your new motherboard is now!

ROM BIOS Compatibility

The issue of ROM BIOS compatibility is important. If the BIOS is not compatible, any number of problems can result. Several reputable companies that produce compatibles have developed their own proprietary ROM BIOS that works just like IBM's. Also, many of the compatibles' OEMs have designed ROMs that work specifically with additional features in their systems while effectively masking the effects of these improvements from any software that would "balk" at the differences.


Many OEMs (Original Equipment Manufacturers) have developed their own compatible ROMs independently. Companies such as Compaq and AT&T have developed their own BIOS products which are comparable to those offered by AMI, Phoenix, and others. These companies also offer upgrades to newer versions that often can offer more features and improvements or fix problems with the older versions. If you use a system with a proprietary ROM, make sure that it is from a larger company with a track record and one that will provide updates and fixes as necessary. Ideally, upgrades should be available for download from the Internet.

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), Award Software, and Phoenix Software. Each company licenses its ROM BIOS to a motherboard manufacturer so that the manufacturer can worry about the hardware rather than the software. To obtain one of these ROMs for a motherboard, the 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, Microid Research, and Phoenix ship to different manufacturers many variations of their BIOS code, each one custom-tailored to that specific system.


Although AMI customizes the ROM code for a particular system, it does not sell the ROM source code to the OEM. An OEM must obtain each new release as it becomes available. Because many OEMs don't need or want every new version developed, they might skip several version changes before licensing a new one.

The AMI BIOS is probably the most used BIOS in PC systems. Newer versions of the AMI BIOS are called Hi-Flex due to the high flexibility found in the BIOS configuration program. The AMI Hi-Flex BIOS is used in Intel, AMI, and many other manufacturers' motherboards. One special AMI feature is that it is a third-party BIOS manufacturer that makes its own motherboard.

During powerup, the BIOS ID string is displayed on the lower-left of the screen. This string tells you valuable information about which BIOS version you have, as well as certain settings which are determined by the built-in setup program.

TIP: 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 back on. This will cause a keyboard error, and the string will remained displayed.

The primary BIOS Identification string (ID String 1) is displayed by any AMI BIOS during the POST (Power On Self-Test) at 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 POST. These additional ID strings display the options that are installed in the BIOS.

The general BIOS ID String 1 format for older AMI BIOS versions is shown in Table 4.1.

Table 4.1  ABBB-NNNN-mmddyy-KK

Position Description
A BIOS Options:
D = Diagnostics built-in
S = Setup built-in
E = Extended Setup built-in
BBB Chipset or Motherboard Identifier:
C&T = Chips & Technologies chipset
NET = C&T NEAT 286 chipset
286 = Standard 286 motherboard
SUN = Suntac chipset
PAQ = Compaq motherboard
INT = Intel motherboard
AMI = AMI motherboard
G23 = G2 chipset 386 motherboard
NNNN The manufacturer license code reference number
mmddyy The BIOS release date, mm/dd/yy
KK The AMI keyboard BIOS version number

The BIOS ID String 1 format for AMI Hi-Flex BIOS versions is shown in Table 4.2.


Position Description
A Processor Type:
0 = 8086 or 8088
2 = 286
3 = 386
4 = 486
5 = Pentium
6 = Pentium Pro
B Size of BIOS:
0 = 64K BIOS
1 = 128K BIOS
CCcc Major and Minor BIOS version number
DDDDDD Manufacturer license code reference number
0036xx = AMI 386 motherboard, xx = Series #
0046xx = AMI 486 motherboard, xx = Series #
0056xx = AMI Pentium motherboard, xx = Series #
0066xx = AMI Pentium Pro motherboard, xx = Series #
E 1 = Halt on POST Error
F 1 = Initialize CMOS every boot
G 1 = Block pins 22 and 23 of the keyboard controller
H 1 = Mouse support in BIOS/keyboard controller
I 1 = Wait for <F1> key on POST errors
J 1 = Display floppy error during POST
K 1 = Display video error during POST
L 1 = Display keyboard error during POST
mmddyy BIOS Date, mm/dd/yy
MMMMMMMM Chipset identifier or BIOS name
N Keyboard controller version number

AMI Hi-Flex BIOS ID String 2 is shown in Table 4.3.


Position Description
AA Keyboard controller pin number for clock switching
B Keyboard controller clock switching pin function:
H = High signal switches clock to high speed
L = High signal switches clock to low speed
C Clock switching through chip set registers:
0 = Disable
1 = Enable
DDDD Port address to switch clock high
EE Data value to switch clock high
FF Mask value to switch clock high
GGGG Port Address to switch clock low
HH Data value to switch clock low
II Mask value to switch clock low
JJJ Pin number for Turbo Switch Input

AMI Hi-Flex BIOS ID String 3 is shown in Table 4.4.


Position Description
AA Keyboard controller pin number for cache control
B Keyboard controller cache control pin function:
H = High signal enables the cache
L = High signal disables the cache
C 1 = High signal is used on the keyboard controller pin
DDD Cache control through Chipset registers:
0 = Cache control off
1 = Cache control on
EE Port address to enable cache
FF Data value to enable cache
GGGG Mask value to enable cache
HH Port address to disable cache
II Data value to disable cache
JJ Mask value to disable cache
K Pin number for resetting the 82335 memory controller
L BIOS Modification Flag:
0 = The BIOS has not been modified
1-9, A-Z = Number of times the BIOS has been modified

The AMI BIOS has many features, including a built-in setup program activated by pressing the Delete or Esc key in the first few seconds of booting up your computer. The BIOS will prompt you briefly as to which key to press and when to press it. The AMI BIOS offers user-definable hard disk types, essential for optimal use of many IDE or ESDI drives. The newer BIOS versions also support Enhanced IDE drives and will auto-configure the drive parameters.

A unique AMI BIOS feature is that, in addition to the setup, it has a built-in, menu-driven, diagnostics package--essentially a very limited version of the stand-alone 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 does 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.

The AMI BIOS is sold through distributors. However, keep in mind that you cannot buy upgrades and replacements direct from AMI.


Award is unique among BIOS manufacturers because it sells its BIOS code to the OEM and allows the OEM to customize the BIOS. Of course, then the BIOS no longer is Award BIOS, but rather a highly customized version. AST uses this approach on its systems, as do other manufacturers, for total control over the BIOS code, without having to write it from scratch. Although AMI and Phoenix customize the ROM code for a particular system, they do not sell the ROM's source code to the OEM. Some OEMs that seem to have developed their own ROM code started with a base of source code licensed to them by Award or some other company.

The Award BIOS has all the normal features you expect, including a built-in setup program activated by pressing Ctrl+Alt+Esc. This setup offers user-definable drive types, required in order to fully use IDE or ESDI hard disks.

In all, the Award BIOS is high quality, has minimal compatibility problems, and offers a high level of support.


The Phoenix BIOS for many years has been a standard of compatibility by which others are judged. It 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 would then be unique and not a copy of IBM's BIOS; however, it would function the same way. This code has been refined over the years and has very few compatibility problems compared to some of the other BIOS vendors.

The Phoenix BIOS excels in two areas that put it high on the list of recommendations. One is that the POST is excellent. The BIOS outputs an extensive set of beep codes that can be used to diagnose severe motherboard problems which would prevent normal operation of the system. In fact, this POST can isolate memory failures in Bank 0 right down to the individual chip with beep codes alone. The Phoenix BIOS also has an excellent setup program free from unnecessary frills, but that offers all the features one would expect, such as user-definable drive types, and so on. The built-in setup is activated by pressing either Ctrl+Alt+S or Ctrl+Alt+Esc, depending on the version of BIOS you have.

The second area in which Phoenix excels is the documentation. Not only are the manuals that you get with the system detailed, but also Phoenix has written a set of BIOS technical-reference manuals that are a standard in the industry. The set consists of three books, titled System BIOS for IBM PC/XT/AT Computers and Compatibles, CBIOS for IBM PS/2 Computers and Compatibles, and ABIOS for IBM PS/2 Computers and Compatibles. Phoenix is one of few vendors who has done extensive research on the PS/2 BIOS and produced virtually all the ROMs in the PS/2 Micro Channel clones on the market. In addition to being an excellent reference for the Phoenix BIOS, these books serve as an outstanding overall reference to any company's IBM-compatible BIOS. Even if you never have a system with a Phoenix BIOS, these books are highly recommended.

Micronics motherboards have always used the Phoenix BIOS, and these motherboards are used in many of the popular name-brand compatible systems. Phoenix has been one of the largest OEMs of Microsoft MS-DOS. If you have MS-DOS, you also have the Phoenix OEM version. Phoenix licenses its DOS to other computer manufacturers so long as they use the Phoenix BIOS. Because of its close relationship with Microsoft, it has had access to the DOS source code, which helps in eliminating compatibility problems.

Unless the ROM BIOS is a truly compatible, custom OEM version such as Compaq's, you might want to install in the system the ROM BIOS from one of the known quantities, such as AMI, Award, or Phoenix. These companies' products are established as ROM BIOS standards in the industry, and frequent updates and improvements ensure that a system containing these ROMs will have a long life of upgrades and service.

Using Correct Speed-Rated Parts

Some compatible vendors use substandard parts in their systems to save money. Because the CPU is one of the most expensive components on the motherboard, and many motherboards are sold to system assemblers without the CPU installed, it is tempting for the assembler to install a CPU rated for less than the actual operating speed. A system could be sold as a 100MHz system, for example, but when you look "under the hood," you may find a CPU rated for only 90MHz. The system does appear to work correctly, but for how long? If the company that manufactures the CPU chip installed in this system had tested the chip to run reliably at 100MHz, it would have labeled the part accordingly. After all, the company could sell the chip for more money if it worked at the higher clock speed.

When a chip is run at a speed higher than it is rated for, it will run hotter than it would normally. This may cause the chip to overheat occasionally, which would appear as random lockups, glitches, and frustration. It's highly recommended that you avoid systems whose operation speed exceeds the design of the respective parts.

This practice is easy to fall into because the faster rated chips cost more money, and Intel and other chip manufacturers usually rate their chips very conservatively. People have taken several 25MHz 486 processors and run them at 33MHz, and they seemed to work fine. The Pentium 90 chips which are tested seem to run fine at 100MHz. If you purchase a 100MHz system from a vendor, you would fully expect it to have 100MHz parts, not 90MHz parts running past their rated speed! Many chips have some form of heat sink on them, which helps to prevent overheating, but which can also sometimes cover up for a "pushed" chip. If the price is too good to be true, ask before you buy: "Are the parts really manufacturer-rated for the system speed?"

To determine the rated speed of a CPU chip, look at the writing on the chip. Most of the time, the part number will end in a suffix of -xxx where the xxx is a number indicating the maximum speed. For example, -100 indicates that the chip is rated for 100MHz operation.

CAUTION: Be careful when running software to detect processor speed. Such programs can only tell you what speed the chip is currently running at, not what the true rating is. Also ignore the speed indicator lights on the front of some cases. These digital displays can literally be set via jumpers to read any speed you desire! They have no true relation to actual system speed.

Motherboard Form Factors

There are several compatible form factors used for motherboards. The form factor refers to the physical dimensions and size of the board, and dictates what type of case the board will fit into. The types of motherboard form factors generally available are the following:

  • Backplane Systems

  • Full-Size AT

  • Baby-AT

  • LPX

  • ATX

  • NLX

Backplane Systems

Not all systems have a motherboard in the true sense of the word. In some systems, the components normally found on a motherboard are located instead on an expansion adapter card plugged into a slot. In these systems, the board with the slots is called a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems.

Backplane systems come in two main types: passive and active. A passive backplane means the main backplane board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane. Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard is essentially a complete motherboard that is designed to plug into a slot in the passive backplane. The passive backplane/mothercard concept allows the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high function mothercard, this type of system design is rarely found in PC systems. The passive backplane design does enjoy popularity in industrial systems, which are often rack-mounted. Some high-end file servers also feature this design.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active backplane systems contain all the circuitry found on a typical motherboard except for the processor complex. The processor complex is the name of the circuit board that contains the main system processor and any other circuitry directly related to it, such as clock control, cache, and so forth. The processor complex design allows the user to easily upgrade the system later to a new processor type by changing one card. In effect, it amounts to a modular motherboard with a replaceable processor section. Most PC systems that use a backplane design use an active backplane/processor complex. Both IBM and Compaq have used this type of design in some of their high-end (server class) systems, for example. This allows an easier and generally more affordable upgrade than the passive backplane/mothercard design since the processor complex board is usually much cheaper than a mothercard. Unfortunately, because there are no standards for the processor complex interface to the system, these boards are proprietary and can only be purchased from the system manufacturer. This limited market and availability causes the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have both advantages and disadvantages. Most original personal computers were designed as backplanes in the late 1970s. Apple and IBM shifted the market to the traditional motherboard with a slot-type design because this type of system generally is cheaper to mass-produce than one with the backplane design. The theoretical advantage of a backplane system, however, is that you can upgrade it easily to a new processor and new level of performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design system, you often must change the motherboard itself, a seemingly more formidable task. Unfortunately, the reality of the situation is that a backplane design is often much more expensive to upgrade, and because the bus remains fixed on the backplane, the backplane design precludes more comprehensive upgrades that involve adding local bus slots, for example.

Another nail in the coffin of backplane designs is the upgradable processor. Intel has designed all 486 and newer processors to be upgradable to faster (sometimes called OverDrive) processors in the future by simply swapping (or adding) the new processor chip. Changing only the processor chip for a faster one is the easiest and generally most cost-effective way to upgrade without changing the entire motherboard. Because of the limited availability of the processor complex boards or mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry standard form factor.

Full-Size AT

The full-size AT motherboard is so named because it matches the original IBM AT motherboard design. This allows for a very large board of up to 12 inches wide by 13.8 inches deep. The keyboard connector and slot connectors must conform to specific placement requirements to fit the holes in the case. This type of board will fit into full-size AT or Tower cases only. Because these motherboards did not fit into the newer Baby-AT or Mini-Tower cases, and because of advances in component miniaturization, they were no longer being produced when the Baby-AT boards became popular.


The Baby-AT form factor is essentially the same as the original IBM XT motherboard, with modifications in screw hole positions to fit into an AT-style case (see Figure 4.1). These motherboards also have specific placement of the keyboard connector and slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard 5-pin DIN type connector for the keyboard. Baby-AT motherboards will fit into every type of case except the Low Profile or Slimline cases. Figure 4.1 shows the dimensions and layout of a Baby-AT motherboard.

FIG. 4.1  Baby-AT motherboard form factor.


Another popular form factor used in motherboards is the LPX (and Mini-LPX) form factor. This form factor was first developed by Western Digital for some of their motherboards. Although they no longer produce PC motherboards, the form factor lives on and has been duplicated by many other motherboard manufacturers. These are used in most of the Low Profile or Slimline case systems. It should be noted that systems using LPX boards may have other differences which can cause compatibility problems similar to those of proprietary systems.

The LPX boards are characterized by several distinctive features. The most noticeable is that the expansion slots are mounted on a bus riser card that plugs into the motherboard. Expansion cards must plug sideways into the riser card. This sideways placement allows for the Low Profile case design. Slots are located on one or both sides of the riser card depending on the system and case design.

Another distinguishing feature of the LPX design is the standard placement of connectors on the back of the board. An LPX board has a row of connectors for video (VGA 15-pin), parallel (25-pin), two serial ports (9-pin each), and mini-DIN PS/2 style Mouse and Keyboard connectors. All of these connectors are mounted across the rear of the motherboard and protrude through a slot in the case. Some LPX motherboards may have additional connectors for other internal ports such as Network or SCSI adapters. Figure 4.2 shows the standard form factors for the LPX and Mini-LPX motherboards.

FIG. 4.2  LPX and Mini-LPX motherboard form factors.


The ATX form factor is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with a modified power supply location and connector. The most important thing to know initially about the ATX form factor is that it is physically incompatible with either the previous Baby-AT or LPX designs. In other words, a different case and power supply are required to match the ATX motherboard.

The official ATX specification was released by Intel in July 1995, and has been written as an open specification for the industry. Intel has published detailed specifications so other manufacturers can use the ATX design in their systems.

ATX improves on the Baby-AT and LPX motherboard designs in several major areas:

  • Built-in double high external I/O connector panel. The rear portion of the motherboard includes a stacked I/O connector area, which is 6.25 inches wide by 1.75 inches tall. This allows external connectors to be located directly on the board and negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

  • Single keyed internal power supply connector. This is a boon for the average end user, who always had to worry about interchanging the Baby-AT power supply connectors and subsequently blowing the motherboard! The ATX specification includes a single keyed and shrouded power connector that is easy to plug in, and which cannot be installed incorrectly. This connector also features pins for supplying 3.3v to the motherboard, which means that ATX motherboards will not require built-in voltage regulators that are susceptible to failure.

  • Relocated CPU and memory. The CPU and memory modules are relocated so they cannot interfere with any bus expansion cards, and they can easily be accessed for upgrade without removing any of the installed bus adapters. The CPU and memory are relocated next to the power supply, which has a single fan blowing air across them, thus eliminating the need for inefficient and failure-prone CPU cooling fans. There is room for a large passive heat sink above the CPU as well.

  • Relocated internal I/O connectors. The internal I/O connectors for the floppy and hard disk drives are relocated to be near the drive bays and out from under the expansion board slot and drive bay areas. This means that internal cables to the drives can be much shorter, and accessing the connectors will not require card or drive removal.

  • Improved cooling. The CPU and main memory are cooled directly by the power supply fan, eliminating the need for separate case or CPU cooling fans. Also, the ATX power supply fan blows into the system chassis, thus pressurizing it which greatly minimizes dust and dirt intrusion into the system. If desired, an air filter can be easily added to the air intake vents on the power supply, creating a system that is even more immune to dirt or dust in the environment.

  • Lower cost to manufacture. The ATX specifications eliminate the need for the rats nest of cables to external port connectors found on Baby-AT motherboards, eliminates the need for additional CPU or chassis cooling fans, eliminates the need for on-board 3.3v voltage regulators, uses a single power supply connector, and allows for shorter internal drive cables. These all conspire to greatly reduce not only the cost of the motherboard, but also significantly reduces the cost of a complete system including the case and power supply.

Figure 4.3 shows the ATX system layout and chassis features. Notice how the entire motherboard is virtually clear of the drive bays, and how the devices like CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots. Also notice the power supply orientation and the single power supply fan that blows into the case directly over the high heat, generating items like the CPU and memory.

FIG. 4.3  ATX system chassis layout and features.

The ATX motherboard is basically a Baby-AT design rotated sideways. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets. In addition to a full-sized ATX layout, Intel also has specified a mini-ATX design as well, which will fit into the same case. Although the case holes are similar to the Baby-AT case, cases for the two formats are generally not compatible. The power supplies would require a connector adapter to be interchangeable, but the basic ATX power supply design is similar to the standard Slimline power supply. The ATX and mini-ATX motherboard dimensions are shown in Figure 4.4.

FIG. 4.4  ATX and Mini-ATX motherboard form factors.


NLX is a Low Profile form factor similar in appearance to LPX, but with a number of improvements designed to allow full integration of the latest technologies. Whereas the primary limitation of LPX boards includes an inability to handle the physical size of newer processors, as well as their higher thermal characteristics, the NLX form factor has been designed specifically to address these problems.

Specific advantages of the NLX form factor include:

  • Support for newer processor technologies. This is especially important in Pentium II systems because the size of the Single Edge Contact cartridge this processor uses can limit its use on older motherboard sizes.

  • Flexibility in the face of rapidly changing processor technologies. Backplane-like flexibility has been built into the form by allowing a new motherboard to be easily and quickly installed without tearing your entire system to pieces. But unlike traditional backplane systems, many industry leaders are putting their support behind NLX, including AST, Digital, Gateway, Hewlett-Packard, IBM, Micron, NEC, and others.

  • Support for other emerging technologies. This includes Accelerated Graphics Port (AGP) high-performance graphic solutions, Universal Serial Bus (USB), and tall memory modules and DIMM technology. Furthermore, with the emerging importance of multimedia applications, connectivity support for such things as video playback, enhanced graphics, and extended audio have been built into the motherboard. This should represent a good cost savings over expensive daughterboard arrangements, which have been necessary for many advanced multimedia uses in the past.

Figure 4.5 shows the basic NLX system layout. Notice that, like ATX, the system is clear of the drive bays and other chassis-mounted components. Also, the motherboard and I/O cards (which, like the LPX form factor, are mounted parallel to the motherboard) can easily be slid in and out of the side of the chassis, leaving the riser card and other cards in place. The processor itself can be easily accessed and enjoys greater cooling than in a more closed in layout.

FIG. 4.5 NLX system chassis layout and features.

As you can see, the NLX form factor has been designed for maximum flexibility and space efficiency. Even extremely long I/O cards will fit easily, without fouling on other system components as has been such a problem with Baby-AT form factor systems.

Motherboard Interface Connectors

There are a variety of different connectors you can find on a motherboard. Most newer motherboards do not only contain the microprocessor, memory and expansion slots, but also the multi-I/O ports (serial, parallel, harddisk and floppy connectors), sound system and video adapter. Also Infrared, USB and FireWire connectors can be part of the mainboard.

Generic Connectors

Every motherboard has a standard set of I/O connectors, to connect the power supply, battery, power LED, keylock and internal speaker. Tables 4.5 through 4.12 contain the pinouts of these generic interface and I/O connectors.

Table 4.5  ATX Motherboard Power Connector

Pin Signal Name Pin Signal Name
1 +3.3v 11 +3.3v
2 +3.3v 12 -12v
3 Ground 13 Ground
4 +5v 14 -PS-ON (Power Supply Remote On/Off Control)
5 Ground 15 Ground
6 +5v 16 Ground
7 Ground 17 Ground
8 PWRGD (Power Good) 18 -5v
9 +5v SB (Standby) 19 +5v
10 +12v 20 +5v

Table 4.6  Baby-AT Motherboard Power Connectors

Pin Signal Name Pin Signal Name
1 PWRGD (Power Good) 7 Ground
2 +5v 8 Ground
3 +12v 9 -5v
4 -12v 10 +5v
5 Ground 11 +5v
6 Ground 12 +5v

Table 4.7a  Serial Port Pin-Header Connectors (Straight Cable Based Type)

Pin Signal Name Pin Signal Name
1 CD (Carrier Detect) 6 CTS (Clear To Send)
2 DSR (Data Set Ready) 7 DTR (Data Terminal Ready)
3 RD (Receive Data) 8 RI (Ring Indicator)
4 RTS (Request To Send) 9 SG (Signal Ground)
5 TD (Transmit Data) 10 Reserved / Access Key

Table 4.7b  Serial Port Pin-Header Connectors (Pin Number Based Type)

Pin Signal Name Pin Signal Name
1 CD (Carrier Detect) 6 DSR (Data Set Ready)
2 RD (Receive Data) 7 RTS (Request To Send)
3 TD (Transmit Data) 8 CTS (Clear To Send)
4 DTR (Data Terminal Ready) 9 RI (Ring Indicator)
5 SG (Signal Ground) 10 Reserved / Access Key

Table 4.8  Parallel Port Pin-Header Connector

Pin Signal Name Pin Signal Name
1 -Strobe 2 -Auto Feed
3 Data 0 4 -Error
5 Data 1 6 -Initialize Printer
7 Data 2 8 -Select Input
9 Data 3 10 Data 0 Ground
11 Data 4 12 Data 1 Ground
13 Data 5 14 Data 2 Ground
15 Data 6 16 Data 3 Ground
17 Data 7 18 Data 4 Ground
19 -Acknowledge 20 Data 5 Ground
21 Busy 22 Data 6 Ground
23 Paper End 24 Data 7 Ground
25 Select 26 Reserved

Table 4.9a  Battery Connector Type 1

Pin Signal Name
1 Ground
2 Reserved
3 Access key
4 +6v

Table 4.9b  Battery Connector Type 2

Pin Signal Name
1 +6v
2 Reserved
3 Ground
4 Ground

Table 4.10  Power LED and Keylock Connector

Pin Signal Name
1 Power LED
2 Reserved / Access key
3 Ground
4 Keyboard Inhibit
5 Ground

Table 4.11a  Speaker Connector Type 1

Pin Signal Name
1 Speaker Output
2 Reserved / Access key
3 Ground / Board-Mounted Speaker
4 +5v

Table 4.11b  Speaker Connector Type 2

Pin Signal Name
1 Ground
2 Reserved / Access key
3 Board-Mounted Speaker *
4 Speaker Output

Some boards have a board mounted piezo speaker. It is enabled by placing a jumper over pins 3 and 4, which routes the speaker output to the board mounted speaker. Removing the jumper allows a conventional speaker to be plugged in.

Table 4.12  Microprocessor Fan Power Connector

Pin Signal Name
1 Ground
2 +12v
3 Ground / Sense tachometer

CAUTION: Do not place a jumper on this connector; serious board damage will result if the +12v is shorted to ground.

Motherboard Mouse (PS/2)

Newer motherboards also can have a motherboard mouse (PS/2) connector. The motherboard mouse port normally uses either a 5-pin (one of two types) or a 10-pin connector. Tables 4.13a, 4.13b and 4.13c show the pinouts of these three different motherboard mouse connectors.

Table 4.13a  Motherboard Mouse 5-Pin-Header Connector Type 1

Pin Signal Name
1 +5v
2 Reserved
3 Ground
5 Data

Table 4.13b  Motherboard Mouse 5-Pin-Header Connector Type 2

Pin Signal Name
1 Data
2 Reserved
3 Ground
4 +5v

Table 4.13c  Motherboard Mouse 10-Pin-Header Connector

Pin Signal Name Pin Signal Name
1 Ground 6 Reserved
2 Reserved 7 Data
3 Reserved 8 +5v
4 Reserved 9 Reserved
5 Access key 10 Reserved

NOTE: If you don't know which of these connectors is used, check the motherboard documentation. For the PS/2 bracket, usually the wires are colored as follows: red = Data, green = Ground, yellow = +5v and blue = CLK.

Infrared Data (IrDA)

The infrared I/O port can use either a 4-pin, 5-pin, 6-pin or 7-pin connector. Note that only the 5-pin connector is the official IrDA standard. Tables 4.14a through 4.14d show the pinouts of the four different infrared connectors.

Table 4.14a  Infrared 4-Pin-Header Connector

Pin Signal Name
1 +5v
2 IrRX
3 Ground
4 IrTX

Table 4.14b  Standard Infrared Data (IrDA) 5-Pin-Header Connector

Pin Signal Name
1 +5v
2 Access key
3 IrRX
4 Ground
5 IrTX

Table 4.14c  Infrared 6-Pin-Header Connector

Pin Signal Name
1 +5v
2 Access key
3 IrRX
4 Ground
5 IrTX
6 Reserved

Table 4.14d  Infrared 7-Pin-Header Connector

Pin Signal Name
1 IrRX
2 Ground
3 IrTX
4 +5v
6 +5v
7 Ground

Video Feature Connectors

When the VGA chipset is embedded in the motherboard, in most cases one type of feature connector is available. This is an additional connector that is used to connect the video chipset to other video devices, such as 3D accelerators, MPEG decoders and video capture cards. The reason that these connectors are used is that they permit the direct transfer of video information from these devices to the video card, without having to use the main system bus. Even high-performance local buses can get slowed down when trying to deal with the enormous amount of information that, for example, a full-motion video stream represents.

The VESA Feature Connector (VFC)

The VESA Feature Connector (VFC) is the first standard in video connectors. The VFC interface is 8-bit wide, resulting in a maximum resolution of 640x480 with a maximum of 256 colors (standard VGA). Table 4.15 shows the pinouts of the 26-pin-header VESA Feature Connector.

Table 4.15  VESA Feature Connector

Pin Signal Name Pin Signal Name
1 DAC Pixel Data 0 / PB 14 Ground
2 DAC Pixel Data 1 / PG 15 Ground
3 DAC Pixel Data 2 / PR 16 Ground
4 DAC Pixel Data 3 / PI 17 Select Internal Video
5 DAC Pixel Data 4 / SB 18 Select Internal Sync
6 DAC Pixel Data 5 / SG 19 Select Internal Dot Clock
7 DAC Pixel Data 6 / SR 20 Reserved
8 DAC Pixel Data 7 / SI 21 Ground
9 DAC Clock 22 Ground
10 DAC Blanking 23 Ground
11 Horizontal Sync 24 Ground
12 Vertical Sync 25 Reserved
13 Ground 26 Reserved

The VESA Advanced Feature Connector (VAFC)

The VESA Advanced Feature Connector (VAFC) is developed as an extension to the VESA Feature Connector. Version 1.0 of the VAFC was released in December 1995. The VAFC interface widens the port from 8 bits to 16 or 32, and provides improved signaling for more reliability. The maximum clock rate is 37.5MHz, resulting in a maximum data transfer rate of 150M/sec in 32-bit mode. An 80-pin connector is used. Table 4.16 shows the pinouts of the 80-pin VESA Advanced Feature Connector.

Table 4.16  VESA Advanced Feature Connector

Pin Signal Name Pin Signal Name
1 RSRV 0 41 Ground
2 RSRV 1 42 Ground
3 GENCLK 43 Ground
4 OFFSET 0 44 Ground
5 OFFSET 1 45 Ground
6 FSTAT 46 Ground
7 VRDY 47 Ground
8 GRDY 48 Ground
9 DAC Blanking 49 Ground
10 Vertical Sync 50 Ground
11 Horizontal Sync 51 Ground
12 EGEN 52 Ground
13 VCLK 53 Ground
14 RSRV 2 54 Ground
15 DAC Clock 55 Ground
16 External Video Select 56 Ground
17 DAC Pixel Data 0 57 DAC Pixel Data 1
18 Ground 58 DAC Pixel Data 2
19 DAC Pixel Data 3 59 Ground
20 DAC Pixel Data 4 60 DAC Pixel Data 5
21 Ground 61 DAC Pixel Data 6
22 DAC Pixel Data 7 62 Ground
23 DAC Pixel Data 8 63 DAC Pixel Data 9
24 Ground 64 DAC Pixel Data 10
25 DAC Pixel Data 11 65 Ground
26 DAC Pixel Data 12 66 DAC Pixel Data 13
27 Ground 67 DAC Pixel Data 14
28 DAC Pixel Data 15 68 Ground
29 DAC Pixel Data 16 69 DAC Pixel Data 17
30 Ground 70 DAC Pixel Data 18
31 DAC Pixel Data 19 71 Ground
32 DAC Pixel Data 20 72 DAC Pixel Data 21
33 Ground 73 DAC Pixel Data 22
34 DAC Pixel Data 23 74 Ground
35 DAC Pixel Data 24 75 DAC Pixel Data 25
36 Ground 76 DAC Pixel Data 26
37 DAC Pixel Data 27 77 Ground
38 DAC Pixel Data 28 78 DAC Pixel Data 29
39 Ground 79 DAC Pixel Data 30
40 DAC Pixel Data 31 80 Ground

The VESA Media Channel (VMC)

The VESA Media Channel (VMC) is a new developed industry bus standard, dedicated specifically to the problem of transferring raw, uncompressed digital video and audio data through the PC, without incurring performance overheads. VMC operates at a clock speed of 33MHz, resulting in a maximum data transfer rate of 132M/sec. VMC supports 8-bit, 16-bit and 32-bit devices simultaneously, and handles up to 15 data streams at the time without affecting performance. The VESA Media Channel provides the option for a 68-pin multi-drop cable, allowing multiple devices to be combined in a modular fashion. Table 4.17 shows the pinouts of the 68-pin VESA Media Channel Connector.

Table 4.17  VESA Media Channel Connector

Pin Signal Name Pin Signal Name
1 SA 35 EVST(0)
2 EVST 36 Ground
3 BS(0) 37 BS(1)
4 Ground 38 SNRDY
5 CONTROL 39 Ground
6 RESET 40 Ground
7 CLOCK 41 Ground
8 Reserved 42 Ground
9 MASK 0 43 MASK 1
10 Ground 44 DAC Pixel Data 0
11 DAC Pixel Data 1 45 Ground
12 DAC Pixel Data 2 46 DAC Pixel Data 3
13 Ground 47 DAC Pixel Data 4
14 DAC Pixel Data 5 48 Ground
15 DAC Pixel Data 6 49 DAC Pixel Data 7
16 Ground 50 DAC Pixel Data 8
17 DAC Pixel Data 9 51 Ground
18 DAC Pixel Data 10 52 DAC Pixel Data 11
19 Ground 53 DAC Pixel Data 12
20 DAC Pixel Data 13 54 Ground
21 DAC Pixel Data 14 55 DAC Pixel Data 15
22 Ground 56 DAC Pixel Data 16
23 DAC Pixel Data 17 57 Ground
24 DAC Pixel Data 18 58 DAC Pixel Data 19
25 Ground 59 DAC Pixel Data 20
26 DAC Pixel Data 21 60 Ground
27 DAC Pixel Data 22 61 DAC Pixel Data 23
28 Ground 62 DAC Pixel Data 24
29 DAC Pixel Data 25 63 Ground
30 DAC Pixel Data 26 64 DAC Pixel Data 27
31 Ground 65 DAC Pixel Data 28
32 DAC Pixel Data 29 66 Ground
33 DAC Pixel Data 30 67 DAC Pixel Data 31
34 Ground 68 SB

USB Motherboard Connector

Newer motherboards often have embedded USB chipsets. When the USB connectors are not soldered onto the motherboard, a separate USB bracket is used. Table 4.18a, 18b and 18c show the pinouts of the different types of USB motherboard connectors for the USB bracket. Note that in some cases the two separate connectors are joined to one 8-pin or 10-pin connector.

Table 4.18a  Separate 4-pin USB Motherboard Connectors

Pin Signal Name Pin Signal Name
1 +5v 1 +5v
2 Data - 2 Data -
3 Data + 3 Data +
4 Ground 4 Ground

Table 4.18b  Separate 5-pin USB Motherboard Connectors Type 1

Pin Signal Name Pin Signal Name
1 +5v 1 +5v
2 Data - 2 Data -
3 Data + 3 Data +
4 Ground 4 Ground
5 Ground 5 Access Key

Table 4.18c  Separate 5-pin USB Motherboard Connectors Type 2

Pin Signal Name Pin Signal Name
1 +5v 1 +5v
2 Reserved / Access Key 2 Reserved / Access Key
3 Data - 3 Data -
4 Data + 4 Data +
5 Ground 5 Ground

NOTE: If you don't know which of these connectors is used, check the motherboard documentation. For the USB bracket, usually the wires are colored as follows: red = +5v, black = Ground, white = Data - and green = Data +.

Motherboard CMOS RAM Addresses

Table 4.19 shows the information maintained in the 64-byte standard CMOS RAM module. This information controls the configuration of the system and is read and written by the system Setup program.

In the original AT system, a Motorola 146818 chip was used. Newer systems incorporate the CMOS into the chipset, Super I/O chip, or use a special battery and NVRAM (Non-Volatile RAM) module from companies like Dallas or Benchmarq. The standard format of the information stored in the CMOS RAM is shown in Table 4.19.

Table 4.19  AT CMOS RAM Addresses

Offset Hex Offset Dec Field Size Function
00h 0 1 byte Current second in binary coded decimal (BCD)
01h 1 1 byte Alarm second in BCD
02h 2 1 byte Current minute in BCD
03h 3 1 byte Alarm minute in BCD
04h 4 1 byte Current hour in BCD
05h 5 1 byte Alarm hour in BCD
06h 6 1 byte Current day of week in BCD
07h 7 1 byte Current day in BCD
08h 8 1 byte Current month in BCD
09h 9 1 byte Current year in BCD
0Ah 10 1 byte Status register A
Bit 7 = Update in progress
0 = Date and time can be read
1 = Time update in progress
Bits 6-4 = Time frequency divider
010 = 32.768KHz
Bits 3-0 = Rate selection frequency
0110 = 1.024KHz square wave frequency
0Bh 11 1 byte Status register B
Bit 7 = Clock update cycle
0 = Update normally
1 = Abort update in progress
Bit 6 = Periodic interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 5 = Alarm interrupt
0 = Disable interrupt (default)
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 4 = Update-ended interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 3 = Status register A square wave frequency
0 = Disable square wave (default)
1 = Enable square wave
Bit 2 = Date format
0 = Calendar in BCD format (default)
1 = Calendar in binary format
Bit 1 = 24-hour clock
0 = 24-hour mode (default)
1 = 12-hour mode
Bit 0 = Daylight Savings Time
0 = Disable Daylight Savings (default)
1 = Enable Daylight Savings
0Ch 12 1 byte Status register C
Bit 7 = IRQF flag
Bit 6 = PF flag
Bit 5 = AF flag
Bit 4 = UF flag
Bits 3-0 = Reserved
0Dh 13 1 byte Status register D
Bit 7 = Valid CMOS RAM bit
0 = CMOS battery dead
1 = CMOS battery power good
Bits 6-0 = Reserved
0Eh 14 1 byte Diagnostic status
Bit 7 = Real-time clock power status
0 = CMOS has not lost power
1 = CMOS has lost power
Bit 6 = CMOS checksum status
0 = Checksum is good
1 = Checksum is bad
Bit 5 = POST configuration information status
0 = Configuration information is valid
1 = Configuration information is invalid
Bit 4 = Memory size compare during POST
0 = POST memory equals
1 = POST memory not equal to
Bit 3 = Fixed disk/adapter initialization
0 = Initialization good
1 = Initialization failed
Bit 2 = CMOS time status indicator
0 = Time is valid
1 = Time is Invalid
Bits 1-0 = Reserved
0Fh 15 1 byte Shutdown code
00h = Power on or soft reset
01h = Memory size pass
02h = Memory test pass
03h = Memory test fail
04h = POST end; boot system
05h = JMP double word pointer with EOI
06h = Protected mode tests pass
07h = Protected mode tests fail
07h = Protected mode tests fail
08h = Memory size fail
09h = Int 15h block move
0Ah = JMP double word pointer without
0Bh = used by 80386
10h 16 1 byte Floppy disk drive types
Bits 7-4 = Drive 0 type
Bits 3-0 = Drive 1 type
0000 = None
0001 = 360K
0010 = 1.2M
0011 = 720K
0100 = 1.44M
11h 17 1 byte Reserved
12h 18 1 byte Hard disk types
Bits 7-4 = Hard disk 0 type (0-15)
Bits 3-0 = Hard disk 1 type (0-15)
13h 19 1 byte Reserved
14h 20 1 byte Installed equipment
Bits 7-6 = Number of floppy disk drives
00 = 1 floppy disk drive
01 = 2 floppy disk drives
Bits 5-4 = Primary display
00 = Use display adapter BIOS
01 = CGA 40-column
10 = CGA 80-column
11 = Monochrome Display Adapter
Bits 3-2 = Reserved
Bit 1 = Math coprocessor present
Bit 0 = Floppy disk drive present
15h 21 1 byte Base memory low-order byte
16h 22 1 byte Base memory high-order byte
17h 23 1 byte Extended memory low-order byte
18h 24 1 byte Extended memory high-order byte
19h 25 1 byte Hard Disk 0 Extended Type (0-255)
1Ah 26 1 byte Hard Disk 1 Extended Type (0-255)
1Bh 27 9 bytes Reserved
2Eh 46 1 byte CMOS checksum high-order byte
2Fh 47 1 byte CMOS checksum low-order byte
30h 48 1 byte Actual extended memory low-order byte
31h 49 1 byte Actual extended memory high-order byte
32h 50 1 byte Date century in BCD
33h 51 1 byte POST information flag
Bit 7 = Top 128K base memory status
0 = Top 128K base memory not installed
1 = Top 128K base memory installed
Bit 6 = Setup program flag
0 = Normal (default)
1 = Put out first user message
Bits 5-0 = Reserved
34h 52 2 bytes Reserved

Table 4.20 shows the values that may be stored by your system BIOS in a special CMOS byte called the diagnostics status byte. By examining this location with a diagnostics program, you can determine whether your system has set trouble codes, which indicate that a problem has occurred previously.

Table 4.20  CMOS RAM (AT and PS/2) Diagnostic Status Byte Codes

Bit Number
7 6 5 4 3 2 1 0 Hex Function
1 . . . . . . . 80 Real-time clock (RTC) chip lost power
. 1 . . . . . . 40 CMOS RAM checksum is bad
. . 1 . . . . . 20 Invalid configuration information found at POST
. . . 1 . . . . 10 Memory size compare error at POST
. . . . 1 . . . 08 Fixed disk or adapter failed initialization
. . . . . 1 . . 04 Real-time clock (RTC) time found invalid
. . . . . . 1 . 02 Adapters do not match configuration
. . . . . . . 1 01 Time-out reading an adapter ID
. . . . . . . . 00 No errors found (Normal)

If the Diagnostic status byte is any value other than zero, you will normally get a CMOS configuration error on bootup. These types of errors can be cleared by re-running the Setup program.

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