System Buses

System Buses

The components inside your computer talk to each other in various different ways. Most of the internal system components, including the processor, cache, memory, expansion cards and storage devices, talk to each other over one or more "buses".

A bus, in computer terms, is simply a channel over which information flows between two or more devices (technically, a bus with only two devices on it is considered by some a "port" instead of a bus). A bus normally has access points, or places into which a device can tap to become part of the bus, and devices on the bus can send to, and receive information from, other devices. The bus concept is rather common, both inside the PC and outside in the real world as well. In fact, your home telephone wiring is a bus: information flows through the wiring that goes through your house, and you can tap into the "bus" by installing a phone jack, plugging in the phone and picking it up. All the phones can share the "information" (voice) on the bus.

This whole section focuses specifically on the system I/O (input/output) buses, also called expansion buses. First the buses and their characteristics are discussed, and then the most common types of I/O buses found on the PC are described with details on their features.

Bus Hierarchy

The PC has a hierarchy, in a way, of different buses. Most modern PCs have at least four buses. I consider them a hierarchy because each bus is to some extent further removed from the processor; each one connects to the level above it, integrating the various parts of the PC together. Each one is also generally slower than the one above it (for the pretty obvious reason that the processor is the fastest device in a modern PC):

The Processor Bus: This is the highest-level bus that the chipset uses to send information to and from the processor.
The Cache Bus: Higher-level architectures, such as those used by the Pentium Pro and Pentium II, employ a dedicated bus for accessing the system cache. This is sometimes called a backside bus. Conventional processors using fifth-generation motherboards and chipsets have the cache connected to the standard memory bus.
The Memory Bus: This is a second-level system bus that connects the memory subsystem to the chipset and the processor. In some systems the processor and memory buses are basically the same thing.
The Local I/O Bus: This is a high-speed input/output bus used for connecting performance-critical peripherals to the memory, chipset, and processor. For example, video cards, disk storage devices, high-speed networks interfaces generally use a bus of this sort. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect Bus (PCI).
The Standard I/O Bus: Connecting to the above three buses is the "good old" standard I/O bus, used for slower peripherals (mice, modems, regular sound cards, low-speed networking) and also for compatibility with older devices. On almost all modern PCs this is the Industry Standard Architecture (ISA) bus.

The system chipset is the conductor that controls this orchestra of communication, and makes sure that every device in the system is talking properly to every other one.

Some newer PCs actually use an additional "bus" that is specifically designed for graphics communications only. The word "bus" is in quotes because it isn't actually a bus, it's a port: the Accelerated Graphics Port (AGP). The distinction between a bus and port is that a bus is generally designed for multiple devices to share the medium, while a port is only for two devices.

Data and Address Buses

Every bus is composed of two distinct parts: the data bus and the address bus. The data bus is what most people refer to when talking about a bus; these are the lines that actually carry the data being transferred. The address bus is the set of lines that carry information about where in memory the data is to be transferred to or from.

In addition, there are a number of control lines that, well, control how the bus functions, and allow users of the bus to signal when data is available. These are sometimes refered to as the control bus, though often they are simply not mentioned.

Bus Width

A bus is a channel over which information flows. The wider the bus, the more information can flow over the channel, much as a wider highway can carry more cars than a narrow one. The original ISA bus on the IBM PC was 8 bits wide; the universal ISA bus used now is 16 bits. The other I/O buses (including VLB and PCI) are 32 bits wide. The memory and processor buses on Pentium and higher PCs are 64 bits wide.

The address bus width can be specified independently of the data bus width. The width of the address bus dictates how many different memory locations that bus can transfer information to or from.

Bus Speed

The speed of the bus reflects how many bits of information can be sent across each wire each second. This would be analogous to how fast the cars are driving on our analogical highway. :^) Most buses transmit one bit of data per line, per clock cycle, although newer high-performance buses like AGP may actually move two bits of data per clock cycle, doubling performance. Similarly, older buses like the ISA bus may take two clock cycles to move one bit, halving performance.

Bus Bandwidth

Bandwidth, also called throughput, refers to the total amount of data that can theoretically be transferred on the bus in a given unit of time. Using the highway analogy, if the bus width is the number of lanes, and the bus speed is how fast the cars are driving, then the bandwidth is the product of these two and reflects the amount of traffic that the channel can convey per second.

The table below shows the theoretical bandwidth of most of the common I/O buses on PCs today. Note the italics on the word "theoretical"; most buses can't actually transmit anywhere near these maximum numbers because of command overhead and other factors. This is especially true of older buses. For example, the theoretical bandwidth of the 8-bit ISA bus might be about MBytes/sec, but in reality there are wait states inserted during I/O that drop this figure down dramatically.

Most of these buses can run at many different speeds; the speed listed is the one most commonly used for the bus type. See here for a similar table showing processor and memory bus bandwidth for various processors.

Bus	Width (bits)	Bus Speed (MHz)	Bus Bandwidth (MBytes/sec)
8-bit ISA	8	8.3	7.9
16-bit ISA	16	8.3	15.9
EISA	32	8.3	31.8
VLB	32	33	127.2
PCI	32	33	127.2
64-bit PCI 2.1	64	66	508.6
AGP	32	66	254.3
AGP (x2 mode)	32	66x2	508.6
AGP (x4 mode)	32	66x4	1,017.3

Note: You may be somewhat confused by the bandwidth numbers I have listed in the table above. For example, shouldn't the bandwidth of standard PCI be 32/8*33.3=133.3 MB/sec? This is how most people and even companies write it, but this is not technically correct, because of the old problem of different definitions of what "M" stands for. The "M" in "MHz" is 1,000,000 (10^6), but the "M" in "MBytes/second" is 1,048,576 (2^20). So the bandwidth of the PCI bus is more properly stated as 32/8*33.3*1,000,000/1,048,576=127.2 MBytes/second.

A few words on the last four entries. In theory, the PCI bus can be extended to 64 bits in width, and 66 MHz in speed. However (here it comes again) for compatibility reasons almost all PCI buses and the devices they run in, are rated for only 33 MHz at 32 bits. AGP is based upon this theoretical standard and does run at 66 MHz, but remains only 32 bits wide. AGP has additional modes, dubbed x2 and x4, that allow the port to perform data transfers two or four times per clock cycle, respectively, leading to an effective bus speed of 133 or 266 MHz.

Bus Interfacing

On a system that has multiple buses, circuitry must be provided by the chipset to connect the buses and allow devices on one to talk to devices on the other. This device is called a "bridge", the same name used to refer to a piece of networking hardware that connects two dissimilar networks. By far the most commonly found bridge is the PCI-ISA bridge, which is part of the system chipset on a Pentium or Pentium Pro PC. The PCI bus also has a bridge to the processor bus; you can see these devices under "System devices" in the Device Manager in Windows 95.

Bus Mastering

On the higher-bandwidth buses, a great deal of information is flowing through the channel every second. Normally, the processor is required to control the transfer of this information. In essence, the processor is a "middleman", and as with many similar cases in the real world, it is far more efficient to "cut out" the middleman and perform the transfer directly. This is done by having capable devices take control of the bus and do the work themselves; devices that can do this are called bus masters. In theory, the processor can do other work simultaneously; in practice there are several complicating factors. In order to do bus mastering properly, a facility to arbitrate between requests to "take over the bus" must exist; this is provided by the chipset. Bus mastering is also called "first party" DMA since the work is controlled by the device doing the transfer.

Currently most bus mastering in the PC world is done on the PCI bus; in addition, support has been added for IDE/ATA hard disk drives to do bus mastering on PCI under certain conditions. IDE hard disk bus mastering on PCI is discussed in more detail here, as well as in the section on PCI. DMA and bus mastering are discussed more generally in the section on DMA.

The Local Bus Concept

The switch from character-based applications to graphics-based ones began in earnest at the start of the 90s, with the rapid growth in popularity of the Windows operating system. The increase in the amount of information that must be moved between the processor, memory, video and hard disks when using a graphical operating system compared to a text-based one is tremendous. A complete, standard screen of monochrome text is just 4,000 bytes of information (2,000 bytes for the characters, and 2,000 bytes for screen attributes). However, a standard 256-color Windows screen requires over 300,000 bytes, an increase of about 15,000%! (Out of interest, the highest-end resolution and color depth generally used today, 1600x1200 at 16 million colors, requires 5.8 million bytes of information per screen!).

The transformation of the software world from text to graphics also meant much larger programs and more storage requirements. From an I/O standpoint, much more I/O bandwidth was needed to handle the additional data going to and from the video card and the increasingly larger and faster hard disks. By this time, Intel had also moved on to the 80486 processor that provided many times the performance of earlier CPUs. The ISA bus, still running at the same speed and bus width that it did on the IBM AT, was finally and totally outmatched by these increasing demands and became a major bottleneck to improving system performance. Increasing the speed of the processor accomplished little if it was always waiting for the slow system bus to transmit data.

The solution was to create a new, faster bus, that would augment the ISA bus and be used especially for high-bandwidth devices such as video cards. This new bus would be put on (or near) the processor's much faster memory bus, to let it run at or near the external speed of the processor, and to allow data to flow between these devices and the processor without having to go through the much slower ISA bus. By placing these devices "local" to the processor, the local bus was born.

The first local bus was the VESA local bus. The current local bus of choice on modern computers is the Peripheral Component Interconnect or PCI bus.

System Bus Types

This section looks at the various input/output buses used on PCs, focusing most of the attention on the ones most commonly found on a modern machine.

Older Bus Types

While most of the attention today is given to the current local bus standard, PCI, and the new AGP port that is likely to become the next standard interface for video, these have evolved from a series of older buses that you will still find in service today on older PCs. The oldest one, ISA, is in fact still used even on the newest PCs! This section takes a look at these older bus types in some detail.

Industry Standard Architecture (ISA) Bus

The most common bus in the PC world, ISA stands for Industry Standard Architecture, and unlike many uses of the word "standard", in this case it actually fits. The ISA bus is still a mainstay in even the newest computers, despite the fact that it is largely unchanged since it was expanded to 16 bits in 1984! The ISA bus eventually became a bottleneck to performance and was augmented with additional high-speed buses, but ISA persists because of the truly enormous base of existing peripherals using the standard. Also, there are still many devices for which the ISA's speed is more than sufficient, and will be for some time to come (standard modems being an example).

(As a side note, after 17 years it appears that ISA may finally be going the way of the dodo. Market leaders Intel and Microsoft want to move the industry away from the use of the ISA bus in new machines. My personal prediction is that they will succeed in this effort, but that it will take at least five years to do it fully. There are few standards in the PC world as pervasive as ISA, and the hundreds of millions of existing ISA cards will ensure that ISA sticks around for some time.)

The choices made in defining the main characteristics of the ISA bus--its width and speed--can be seen by looking at the processors with which it was paired on early machines. The original ISA bus on the IBM PC was 8 bits wide, reflecting the 8 bit data width of the Intel 8088 processor's system bus, and ran at 4.77 MHz, again, the speed of the first 8088s. In 1984 the IBM AT was introduced using the Intel 80286; at this time the bus was doubled to 16 bits (the 80286's data bus width) and increased to 8 MHz (the maximum speed of the original AT, which came in 6 MHz and 8 MHz versions).

Later, the AT processors of course got faster, and eventually data buses got wider, but by this time the desire for compatibility with existing devices led manufacturers to resist change to the standard, and it has remained pretty much identical since that time. The ISA bus provides reasonable throughput for low-bandwidth devices and virtually assures compatibility with almost every PC on the market.

Many expansion cards, even modern ones, are still only 8-bit cards (you can tell by looking at the edge connector on the card; 8-bit cards use only the first part of the ISA slot, while 16-bit cards use both parts). Generally, these are cards for which the lower performance of the ISA bus is not a concern. However, access to IRQs 9 through 15 is provided through wires in the 16-bit portion of the bus slots. This is why most modems, for example, cannot be set to the higher-number IRQs. IRQs cannot be shared among ISA devices.

Micro Channel Architecture (MCA) Bus

The MCA bus (also called the Micro Channel bus; MCA stands for "Micro Channel Architecture") was IBM's attempt to replace the ISA bus with something "bigger and better". When the 80386DX was introduced in the mid-80s with its 32-bit data bus, IBM decided (much like it did with the AT) to create a bus to match this width. MCA is 32 bits wide, and offers several significant improvements over ISA. (One of MCA's disadvantages was rather poor DMA controller circuitry.)

The MCA bus has some pretty impressive features considering that it was introduced in 1987, a full seven years before the PCI bus made similar features common on the PC. In some ways it was ahead of its time, because back then the ISA bus really wasn't a major performance limiting factor:

32 Bit Bus Width: The MCA bus features a full 32 bit bus width, the same width as the VESA and PCI local buses. It had far superior throughput to the ISA bus.
Bus Mastering: The MCA bus supported bus mastering adapters for greater efficiency, including proper bus arbitration.
Plug and Play: MCA automatically configured adapter cards, so there was no need to fiddle with jumpers. This was eight years before Windows 95 brought PnP into the mainstream!

MCA had a great deal of potential. Unfortunately, IBM made two decisions that would doom MCA to utter failure in the marketplace. First, they made MCA incompatible with ISA; this means ISA cards will not work at all in an MCA system, one of the few categories of PCs for which this is true. The PC market is very sensitive to backwards-compatibility issues, as evidenced by the number of older standards that persist to this day (such as ISA!) Second, IBM decided to make the MCA bus proprietary. It in fact did this with ISA as well; however in 1981 IBM could afford to flex its muscles in this manner, while by this time the clone makers were starting to come into their own and weren't interested in bending to IBM's wishes.

These two factors, combined with the increased cost of MCA systems, led to the demise of the MCA bus. With the PS/2 now discontinued, MCA is dead on the PC platform, though it is still used by IBM on some of its RISC 6000 UNIX servers. It is one of the classical examples in the field of computing of how non-technical issues often dominate over technical ones.

Extended Industry Standard Architecture (EISA) Bus

EISA stands for Extended Industry Standard Architecture. Unlike ISA, here the name is not indicative of reality, for the EISA bus never became widely used and cannot by any stretch of the imagination be considered an industry standard. EISA began as Compaq's answer to IBM's MCA bus, and followed a similar path of development--with very similar results.

Compaq avoided the two key mistakes that IBM made when they developed EISA. First, they made it compatible with the ISA bus. Second, they opened the design to all manufacturers instead of keeping it proprietary, by forming the non-profit EISA committee to manage the design of the standard. EISA was similar to MCA both in terms of technology and market acceptance: it had significant technical advantages over ISA, and it never caught on with the PC-buying public.

Some of the key features of the EISA bus:

ISA Compatibility: ISA cards will work in EISA slots.
32 Bit Bus Width: Like MCA, the bus was expanded to 32 bits.
Bus Mastering: The EISA bus supports bus mastering adapters for greater efficiency, including proper bus arbitration.
Plug and Play: EISA automatically configures adapter cards, similar to the Plug and Play standards of modern systems.

EISA-based systems have today been mostly relegated to a specialty role; they are sometimes found in network fileservers. The EISA bus is virtually non-existent on desktop systems for several reasons. First, EISA-based systems tend to be much more expensive than other types of systems. Second, there are few EISA-based cards available. Finally, the performance of this bus is quite low compared to the popular local buses like the VESA Local Bus and PCI. EISA is not totally dead as a platform the way MCA is, but it is pretty close.

VESA Local Bus (VLB)

The first local bus to gain popularity, the VESA local bus (also called VL-Bus or VLB for short) was introduced in 1992. VESA stands for the Video Electronics Standards Association, a standards group that was formed in the late eighties to address video-related issues in personal computers. Indeed, the major reason for the development of VLB was to improve video performance in PCs.

The VLB is a 32-bit bus which is in a way a direct extension of the 486 processor/memory bus. A VLB slot is a 16-bit ISA slot with third and fourth slot connectors added on the end. The VLB normally runs at 33 MHz, although higher speeds are possible on some systems. Since it is an extension of the ISA bus, an ISA card can be used in a VLB slot, although it makes sense to use the regular ISA slots first and leave the (small number of) VLB slots open for VLB cards, which won't work in an ISA slot of course. Use of a VLB video card and I/O controller greatly increases system performance over an ISA-only system.

While VLB was extremely popular during the reign of the 486, with the introduction of the Pentium and its PCI local bus in 1994, wholesale abandonment of the VLB began in earnest. While Intel pushing PCI was one reason why this happened, there were also several key problems with the VLB implementation. First, the design was strongly based on the 486 processor, and adapting it to the Pentium caused a host of compatibility and other problems. Second, the bus itself was tricky electrically; for example, the number of cards that could be used on the bus was low (often only two or even one), and occasionally there could be timing problems on the bus when more than one card was used. Finally, the bus did not support bus mastering properly since there was no good arbitration scheme, and did not support Plug and Play.

Today VLB is obsolete for new systems; even the latest 486 motherboards use PCI, and all Pentiums and higher use PCI. However, these systems do still offer reasonable performance, and are now plentiful and very inexpensive--if you can still find them.

Peripheral Component Interconnect (PCI) Local Bus

Currently by far the most popular local I/O bus, the Peripheral Component Interconnect (PCI) bus was developed by Intel and introduced in 1993. It is geared specifically to fifth- and sixth-generation systems, although the latest generation 486 motherboards use PCI as well.

Like the VESA Local Bus, PCI is a 32-bit bus that normally runs at a maximum of 33 MHz. The key to PCI's advantages over its predecessor, the VESA local bus, lies in the chipset that controls it. The PCI bus is controlled by special circuitry in the chipset that is designed to handle it, where the VLB was basically just an extension of the 486 processor bus. PCI is not married to the 486 in this manner, and its chipset provides proper bus arbitration and control facilities, to enable PCI to do much more than VLB ever could. PCI is also used outside the PC platform, providing a degree of universality and allowing manufacturers to save on design costs.

PCI Bus Performance

The PCI bus provides superior performance to the VESA local bus; in fact, PCI is the highest performance general I/O bus currently used on PCs. This is due to several factors:

Burst Mode: The PCI bus can transfer information in a burst mode, where after an initial address is provided multiple sets of data can be transmitted in a row. This works in a way similar to how cache bursting works.
Bus Mastering: PCI supports full bus mastering, which leads to improved performance.
High Bandwidth Options: The PCI bus specification version 2.1 calls for expandability to 64 bits and 66 MHz speed; if implemented this would quadruple bandwidth over the current design. In practice the 64-bit PCI bus has yet to be implemented on the PC (it does exist in non-PC platforms such as Digital Equipment's Alpha,) and the speed is currently limited to 33 MHz in most PC designs, most likely for compatibility reasons. For mainstream PCI, we may be limited to 32 bits and 33 MHz for some time to come. However, it appears that the higher-performance PCI options are going to live on, albeit in modified form, through the new Accelerated Graphics Port.

The speed of the PCI bus can be set synchronously or asynchronously, depending on the chipset and motherboard. In a synchronized setup (used by most PCs), the PCI bus runs at half the memory bus speed; since the memory bus is usually 50, 60 or 66 MHz, the PCI bus would run at 25, 30 or 33 MHz respectively. In an asynchronous setup the speed of the PCI bus can be set independently of the memory bus speed. This is normally controlled through jumpers on the motherboard, or BIOS settings. Overclocking the system bus on a PC that uses synchronous PCI will cause PCI peripherals to be overclocked as well, often leading to system stability problems.

PCI Expansion Slots

The PCI bus offers more expansion slots than most VLB implementations, without the electrical problems that plagued the VESA bus. Most PCI systems support 3 or 4 PCI slots, with some going significantly higher than this.

Note: In some systems, not all of the slots are capable of bus mastering. This is now far less common than it was, but with early systems the motherboard manual should be checked to see if this is the case.

The PCI bus offers a great variety of expansion cards compared to VLB. The most commonly found cards are video cards (of course), SCSI host adapters, and high-speed networking cards. (Hard disk drives are also on the PCI bus but are normally connected directly to the motherboard on a PCI system). However, it should be noted that certain functions cannot be provided on the PCI bus. For example, serial and parallel ports must remain on the ISA bus. Fortunately, the ISA bus still has more than enough speed to handle these devices, even today.

PCI Internal Interrupts

The PCI bus uses its own internal interrupt system for dealing with requests from the cards on the bus. These interrupts are often called "#A", "#B", "#C" and "#D" to avoid confusion with the normal numbered system IRQs, though they are sometimes called "#1" through "#4" as well. These interrupt levels are not generally seen by the user except in the BIOS setup screen for PCI, where they can be used to control how PCI cards operate.

These interrupts, if needed by cards in the slots, are mapped to regular interrupts, normally IRQ9 through IRQ12. The PCI slots in most systems can be mapped to at most four regular IRQs. In systems that have more than four PCI slots, or that have four slots and a USB controller (which uses PCI), two or more of the PCI devices share an IRQ.

If you are using Windows 95 OEM SR2, you may see additional entries for your PCI devices under the Device Manager. Each device may have an additional entry entitled "IRQ Holder for PCI Steering". PCI steering is in fact a feature that is part of the Plug and Play portions of the system, and enables the IRQ used for PCI devices to be controlled by the operating system to avoid resource problems. Having this listed in addition to another device under the IRQ list does not mean you have a resource conflict.

PCI Bus Mastering

As discussed in the section on system bus functions and features, bus mastering is the capability of devices on the PCI bus (other than the system chipset, of course) to take control of the bus and perform transfers directly. The PCI bus is the first bus to popularize bus mastering; probably in part because for the first time there are operating systems and software that are really capable of taking advantage of it.

PCI supports full device bus mastering, and provides bus arbitration facilities through the system chipset. PCI's design allows bus mastering of multiple devices on the bus simultaneously, with the arbitration circuitry working to ensure that no device on the bus (including the processor!) locks out any other device. At the same time though, it allows any given device to use the full bus throughput if no other device needs to transfer anything. In a way, the PCI bus acts like a tiny "local area network" inside the computer, in which multiple devices can each talk to each other, sharing a communication channel that is managed by the chipset.

PCI IDE Bus Mastering

The PCI bus also allows you to set up compatible IDE/ATA hard disk drives to be bus masters. Under the correct conditions this can increase performance over the use of PIO modes, which are the default way that IDE/ATA hard disks transfer data to and from the system. When PCI bus mastering is used, IDE/ATA devices use DMA modes to transfer data instead of PIO; IDE/ATA DMA modes are described in detail here.

Since this capability was made available to newer machines, it has been one of the most talked about (and most misunderstood) functions of the modern PC. There is a lot of confusion amongst PC users about what PCI IDE bus mastering does and how it works. In particular, there are a lot of misconceptions about its performance advantages. In addition, there have been a lot of problems with compatibility in getting this new technology to work.

IDE bus mastering requires all of the following in order to function at all:

Bus Mastering Capable System Hardware: This includes the motherboard, chipset, bus and BIOS. Most newer motherboards using the Intel 430 Pentium chipset family (FX, HX, VX, TX) or the Intel 440FX Pentium Pro chipset, will support bus mastering IDE.
Bus Mastering Hard Disk: Normally this means that the drive must be capable of at least multiword DMA mode 2 transfers. All Ultra ATA hard disks also support bus mastering.
32-Bit Multitasking Operating System: This means usually Windows NT, Windows 95, Linux, or similar (but see below for caveats.)
Bus Mastering Drivers: A special driver must be provided to the operating system to enable bus mastering to work.

Getting this all set up can be a great deal of work. In particular, the following are common problems encountered when trying to set up bus mastering:

Driver bugs and incompatibility issues, especially with older drivers that are "generic" and not tuned to a particular motherboard.
Older hard disk drives not working properly.
Problems with dissimilar drives connected to the same IDE channel as master and slave.
Problems when using a CD-ROM drive alone on an IDE channel without a hard drive.
Bus mastering drivers that don't work on certain motherboards; also, some motherboards or systems just will not work with bus mastering at all.

Assuming that you get bus mastering IDE to work, you will see improvement if you are using a true multi-tasking operating system, and you are running multiple applications that are disk-access-intensive. This would not generally include most regular Windows 95 users, for example. Bus mastering IDE will not help at all in the following situations:

It will not make that 100 MB transfer from C: to D: that you are sitting and watching go much faster at all.
It will not speed up DOS games.
It will not make applications load more quickly (unless you somehow are loading more than one at a time).
It will not speed up single applications.

Especially: IDE bus mastering will not really speed up Windows 95 in general. Windows 95 does not do "true" multitasking and in many cases the processor will be held up waiting for the transfer to complete even if bus mastering is employed. So even though the processor in theory is freed up to do other things, it doesn't really do other things. Also, most people multitask by switching between applications that are open, but rarely have anything running in two or more simultaneously.

For most people, IDE bus mastering is not worth the effort and problems, and I now do not bother with it on new installs of Windows 95. This may be somewhat controversial, but in my opinion it is very overrated as a potential system improvement, given how much effort it requires. You're better off working overtime for a few hours and buying another 16 MB of RAM. :^) If you feel like trying it, contacting the company that made your motherboard for a driver set is a good place to start. You can also try Intel for a generic driver that may work on your Intel-chipset system. I'd recommend that you back up your hard disk first before trying any of these... refer to this section of the Troubleshooting Expert for more help resolving problems with these drivers if you have difficulties with them.

I am hopeful that in time, bus mastering over the IDE/ATA interface will be improved and these problems will be just a distant memory. With the creation of Ultra ATA and the DMA-33 high-speed transfer mode, it appears that the future lies in the use of PCI bus mastering with the IDE/ATA interface. There is just some work to do until this support is both universal and well-implemented.

PCI Plug and Play

The PCI bus is part of the Plug and Play standard developed by Intel, with cooperation from Microsoft and many other companies. PCI systems were the first to popularize the use of Plug and Play. The PCI chipset circuitry handles the identification of cards and works with the operating system and BIOS to automatically set resource allocations for compatible peripheral cards.

Accelerated Graphics Port (AGP)

The need for increased bandwidth between the main processor and the video subsystem originally lead to the development of the local I/O bus on the PCs, starting with the VESA local bus and eventually leading to the popular PCI bus. This trend continues, with the need for video bandwidth now starting to push up against the limits of even the PCI bus.

Much as was the case with the ISA bus before it, traffic on the PCI bus is starting to become heavy on high-end PCs, with video, hard disk and peripheral data all competing for the same I/O bandwidth. To combat the eventual saturation of the PCI bus with video information, a new interface has been pioneered by Intel, designed specifically for the video subsystem. It is called the Accelerated Graphics Port or AGP.

AGP was developed in response to the trend towards greater and greater performance requirements for video. As software evolves and computer use continues into previously unexplored areas such as 3D acceleration and full-motion video playback, both the processor and the video chipset need to process more and more information. The PCI bus is reaching its performance limits in these applications, especially with hard disks and other peripherals also in there fighting for the same bandwidth.

Another issue has been the increasing demands for video memory. As 3D computing becomes more mainstream, much larger amounts of memory become required, not just for the screen image but also for doing the 3D calculations. This traditionally has meant putting more memory on the video card for doing this work. There are two problems with this:

Cost: Video card memory is very expensive compared to regular system RAM.
Limited Size: The amount of memory on the video card is limited: if you decide to put 6 MB on the card and you need 4 MB for the frame buffer, you have 2 MB left over for processing work and that's it (unless you do a hardware upgrade). It's not easy to expand this memory, and you can't use it for anything else if you don't need it for video processing.

AGP gets around these problems by allowing the video processor to access the main system memory for doing its calculations. This is more efficient because this memory can be shared dynamically between the system processor and the video processor, depending on the needs of the system.

The idea behind AGP is simple: create a faster, dedicated interface between the video chipset and the system processor. The interface is only between these two devices; this has three major advantages: it makes it easier to implement the port, makes it easier to increase AGP in speed, and makes it possible to put enhancements into the design that are specific to video.

AGP is considered a port, and not a bus, because it only involves two devices (the processor and video card) and is not expandable. One of the great advantages of AGP is that it isolates the video subsystem from the rest of the PC so there isn't nearly as much contention over I/O bandwidth as there is with PCI. With the video card removed from the PCI bus, other PCI devices will also benefit from improved bandwidth.

AGP is a new technology and was just introduced to the market in the third quarter of 1997. The first support for this new technology will be from Intel's 440LX Pentium II chipset. More information on AGP will be forthcoming as it becomes more mainstream and is seen more in the general computing market. Interestingly, one of Intel's goals with AGP was supposed to be to make high-end video more affordable without requiring sophisticated 3D video cards. If this is the case, it really makes me wonder why they are only making AGP available for their high-end, very expensive Pentium II processor line. :^) Originally, AGP was rumored to be a feature on the 430TX Pentium socket 7 chipset, but it did not materialize. Via and other companies are carrying the flag for future socket 7 chipset development now that Intel has dropped it, and several non-Intel AGP-capable chipsets will be entering the market in 1998.

AGP Interface

The AGP interface is in many ways still quite similar to PCI. The slot itself is similar physically in shape and size, but is offset further from the edge of the motherboard than PCI slots are. The AGP specification is in fact based on the PCI 2.1 specification, which includes a high-bandwidth 66 MHz speed that was never implemented on the PC. AGP motherboards have a single expansion card slot for the AGP video card, and usually one less PCI slot, and are otherwise quite similar to PCI motherboards.

AGP Bus Width, Speed and Bandwidth

The AGP bus is 32 bits wide, just the same as PCI is, but instead of running at half of the system (memory) bus speed the way PCI does, it runs at full bus speed. This means that on a standard Pentium II motherboard AGP runs at 66 MHz instead of the PCI bus's 33 MHz. This of course immediately doubles the bandwidth of the port; instead of the limit of 127.2 MB/s as with PCI, AGP in its lowest speed mode has a bandwidth of 254.3 MB/s. Plus of course the benefits of not having to share bandwidth with other PCI devices.

In addition to doubling the speed of the bus, AGP has defined a 2X mode, which uses special signaling to allow twice as much data to be sent over the port at the same clock speed. What the hardware does is to send information on both the rising and falling edges of the clock signal. Each cycle, the clock signal transitions from "0", to "1" ("rising edge"), and back to "0" ("falling edge"). While PCI for example only transfers data on one of these transitions each cycle, AGP transfers data on both. The result is that the performance doubles again, to 508.6 MB/s theoretical bandwidth. There is also a plan to implement a 4X mode, which will perform four transfers per clock cycle: a whopping 1,017 MB/s of bandwidth!

This is certainly very exciting, but we must temper this excitement somewhat (and not just because AGP is new and we don't have much that is practical to evaluate yet). It's great fun to talk about 1 GB/s bandwidth for the video card, but there's only one problem: this is more than the bandwidth of the entire system bus of a modern PC! If you recall, the data bus of a Pentium class or later PC is 64 bits wide and runs at 66 MHz. This gives a total of 508.6 MB/s bandwidth, so the 1 GB/s maximum isn't going to do much good until we get the data bus running much faster than 66 MHz. Future motherboard chipsets will take the system bus to 100 MHz, which will increase total memory bandwidth to 763 MB/s, a definite step in the right direction, but still not enough to make 4X transfers feasible.

Also worth remembering is that the CPU also needs to have access to the system memory, not just the video subsystem. If all 508.6 MB/s of system bandwidth is taken up by video over AGP, what is the processor going to do? Again here, going to 100 MHz system speed will help immensely. In practical terms, the jury is still out on AGP and will be for a while, though there can be no denying its tremendous promise.

AGP Video Pipelining

One performance enhancing benefit of AGP is its ability to pipeline requests for data. Pipelining was first used by modern processors as a way to improve performance by letting the sequential parts of tasks overlap; see here for a full description of how it works. With AGP, the video chipset can use a similar technique when requesting information from memory, which improves performance.

AGP Video System Memory Access

One of the key features of AGP is the ability to share the main system memory with the video chipset. The reason that this has been incorporated into the design is to allow the video subsystem to have access to larger amounts of memory for 3D and other processing, without requiring that large quantities of special video memory be put on the video card for this purpose. Right now the memory on the video card is shared between the frame buffer and the other uses that the video card has for memory. Since the frame buffer requires high-performance memory technologies, most cards use this better technology (such as VRAM) for all of the memory on the video card, which is usually overkill for the parts other than the frame buffer.

Note that AGP is not the same as the ill-fated unified memory architecture (UMA). Under UMA, all of the video card's memory, including the frame buffer, is taken from main system memory. Under AGP, the frame buffer remains on the video card, where it belongs. Tthe frame buffer is the most important part of the video memory and it requires the highest performance, so it makes sense to leave it on the video card so special video-specific technologies like VRAM can be used.

What AGP does is to allow the video processor to access the system memory for other tasks that require memory, such as texturing and other 3D operations. The theory is that this memory isn't as crucial as the frame buffer, and doing this design means that higher-end video cards can be made more inexpensively by saving on the cost of dedicated video card memory that is only being used for these operations.

AGP Requirements

There are a number of different requirements in order to allow a system to take advantage of AGP:

AGP Video Card: This is pretty obvious.
AGP Chipset Motherboard: The chipset on the motherboard must be able to support AGP of course.
Operating System Support: The operating system must be able to support the new interface through its internal drivers and routines.
Driver Support: Of course, the video card needs special drivers in order to support AGP and also to exploit special features such as "2X" mode.

Operating system support is going to be especially problematic. I am not really sure how this will end up being handled in the early days of AGP, but I suspect that it may not be until Microsoft provides AGP support in the upcoming Windows 98 that we will see AGP's popularity really start to pick up.

System BIOS

BIOS stands for Basic Input/Output System, although the full term is used very infrequently. The system BIOS is the lowest-level software in the computer; it acts as an interface between the hardware (especially the chipset and processor) and the operating system. The BIOS provides access to the system hardware and enables the creation of the higher-level operating systems (DOS, Windows 95, etc.) that you use to run your applications. The BIOS is also responsible for allowing you to control your computer's hardware settings, for booting up the machine when you turn on the power or hit the reset button, and various other system functions.

System BIOS Functions and Operation

This section takes a look at the various functions that the BIOS performs, and discusses some of its characteristics. In addition to what is discussed in the sections below, the BIOS's other main responsibilities include booting the system, and providing the BIOS setup program that allows you to change BIOS parameters.

The BIOS Program

In order for any computer to function, it must have software to run on it. All that a processor--or any hardware for that matter--knows how to do is to follow instructions. The software is that collection of instructions, as described in this part of the Introduction. All regular programs that you run on your PC are stored permanently on your hard disk, and are loaded into your system memory (RAM) when you need to use them. From the memory, the processor can access the instructions coded into the program and run them, which lets you do your work.

When you first turn on your PC, the processor is "raring to go", but it needs some instructions to execute. However, since you just turned on the machine, your system memory is empty; there are no programs to run. To make sure that the BIOS program is always available to the processor, even when it is first turned on, it is "hard-wired" into a read-only-memory (ROM) chip that is placed on your motherboard.

A uniform standard was created between the makers of processors and the makers of BIOS programs, so that the processor would always look in the same place in memory to find the start of the BIOS program. The processor gets its first instructions from this location, and the BIOS program begins executing. The BIOS program then begins the system boot sequence which calls other programs, gets your operating system loaded, and your PC up and running.

The BIOS program is always located in a special reserved memory area, the upper 64K of the first megabyte of system memory (addresses F000h to FFFFh). Some BIOSes use more than this 64K area.

Other PC BIOSes

Although most people don't realize it, there are in fact several different BIOSes in your PC. When people say "the BIOS" they are of course referring to the main system BIOS. However there are also BIOSes to control peripherals that you put into your machine. Your video card has its own BIOS (in most cases) that contains hardware-driving instructions for displaying video information. Hard disks and other peripherals can contain their own BIOS instructions as well. Many SCSI host adapters for example have their own BIOS.

The BIOS and the Software Layer Model

As mentioned in many other places, one of the key factors in the success of the PC platform is the combination of a huge choice of hardware and software options, and at the same time compatibility between many different types of hardware and software. Backwards compatibility is particularly key; nobody wants to have to throw out their old software when hardware changes, and companies that have tried to ignore this have often suffered the consequences (e.g., the MCA bus).

Have you ever marveled at how most of the applications that ran on your AT-class machine in 1985 will still run on your Pentium today? This, despite radically different processors, system buses, memory, in fact, all the hardware is different, and the operating system is as well. I personally have many applications that I run still today that are 12 years old or more. How is this possible?

The key to this universality is the use of multiple software layers. Let's consider the use of a program like Microsoft Word 6.0, running on Windows 3.x. In a simplified view, when you run this application, you are actually employing four main software layers: Word 6.0 is the application; it runs on Windows; Windows runs on top of DOS and DOS runs on top of the system BIOS. The BIOS interfaces with the hardware. This table shows the various layers, from lowest to highest. The operating system and application can (and often are) composed themselves of multiple layers:

Layer #	Layer
0	Hardware
1	System BIOS
2	Operating System
3	Application

Each of these layers contributes to compatibility in an essential way: it talks to the level below it using a standard interface. In order for Word 6.0 to work in Windows, it must follow certain rules set forth by the designers of Windows. Windows must follow rules set forth by the creators of DOS. And DOS must use a standardized way of talking to the system BIOS.

Each layer provides an abstraction model to the software that runs on it, by providing to the layer above it a set of services and functions that the layer above it can use. Word 6.0 doesn't worry about the hardware or DOS much at all; it simply calls Windows functions and lets Windows worry about DOS. Windows talks to DOS using DOS functions, etc. (This is somewhat simplified because in some cases the layers aren't this cleanly separated).

By using these standards, it makes it possible to mix and match various layers, as long as the programmers follow the rules. If you want to update your DOS version, Windows will still work as long as the new DOS version provides the same standard interface that the old one did; it can provide new functions, but not take away any of the old ones. Similarly, Word 6.0 will work on Windows 95 because Windows 95 provides the same facilities to Word that Windows 3.x did. This is how compatibility is maintained across changes to operating systems.

What does all this have to do with the BIOS? :^) The BIOS is actually the pillar that supports all of this, because it provides the standard interface that DOS uses (or whatever the operating system is). The system hardware itself is the "mess at the bottom of the pile" that we are trying to have to avoid dealing with. In some ways, the most amazing part of all of this is that DOS itself will run on so many different machines. The BIOS is what makes this possible. Instead of DOS having to talk directly to the hardware, of which there are many, many possibilities. It talks to the BIOS, which is what is customized to the hardware. The BIOS hides the hardware from the operating system so it doesn't have to worry about it, by providing standardized services to the operating system.

As alluded to above, programs do not always have to follow this model exactly. It is possible for a DOS program to bypass DOS and the BIOS functions, and interface directly with the hardware. This is done normally for performance reasons; games do it most often. The problem is that this breaks the compatibility model; now the software does have to figure out what hardware it is using, and this makes these types of programs much less portable and compatible than ones that "play by the rules".

BIOS Services and Software Interrupts

The BIOS provides various software routines (subprograms) that can be called by higher-level software such as DOS, Windows, or their applications, to perform different tasks. Virtually every task that involves accessing the system hardware has traditionally been controlled using one or more of the BIOS programs (although many newer operating systems now bypass the BIOS for improved performance). This includes actions like reading and writing from the hard disk, processing information received from devices, etc.

BIOS services are accessed using software interrupts, which are similar to the hardware interrupts except that they are generated inside the processor by programs instead of being generated outside the processor by hardware devices. One thing that this use of interrupts does is to allow access to the BIOS without knowing where in memory each routine is located.

Normally, to call a software routine you need to know its address. With interrupts, a table called an interrupt vector table is used that bypasses this problem. When the system is started up, the BIOS puts addresses into this table that represent where its routines are located for each interrupt it responds to. Then, when DOS or an application wants to use a BIOS routine, it generates a software interrupt. The system processes the interrupt, looks up the value in the table, and jumps to the BIOS routine automatically. DOS itself and application programs can also use this interrupt vector table.

Hard disk access and control is, in particular, an important part of the BIOS's role in the PC.

BIOS Feature "Magic Dates"

There are several important BIOS features or capabilities that have been introduced over the last few years. The only reliable way to ensure that your BIOS supports these is to check your BIOS or motherboard manual, or contact the manufacturer or dealer. However, there are some rules of thumb relating the BIOS date that you can use to give you an idea of whether or not a given machine is likely to support a given feature.

Here are the most common BIOS features and the approximate date that they were introduced. Obviously these are just rules of thumb; don't take all of this as definitive. It is fairly safe to say that if the BIOS date is more than a year earlier than a magic date, the feature almost certainly is not supported, and if it is more than a year after the date, it almost certainly is supported:

Geometry Translation for IDE/ATA Hard Drives Over 504 MB: The magic date for this feature is around July 1994.
Plug and Play Specification: Support for PnP was introduced around mid-year in 1995. (I'm not 100% sure on this one.)
Year 2000 Compliance: For AMI BIOSes, the magic date is July 15, 1995. For Award BIOSes the situation seems more complex. Check out the Year 2000 web site.
Large IDE/ATA Hard Drives Over 2 GB: Support for very large hard drives seems to have begun with BIOSes dated in January 1996.

BIOS System Boot Operations

One of the most important functions that the BIOS plays is to boot up the system. When the PC is first turned on, its main system memory is empty, and it needs to find instructions immediately to tell it what to run to start up the PC. These it finds within the BIOS program, because the BIOS is in read-only permanent memory and so is always available for use, even when the rest of system memory is empty.

This section takes a look at what is involved in booting the PC, including a discussion of the steps in the system boot process, and a look at the power-on self-test (POST) that is conducted whenever the system starts up.

System Boot Sequence

The system BIOS is what starts the computer running when you turn it on. The following are the steps that a typical boot sequence involves. Of course this will vary by the manufacturer of your hardware, BIOS, etc., and especially by what peripherals you have in the PC. Here is what generally happens when you turn on your system power:

The internal power supply turns on and initializes. The power supply takes some time until it can generate reliable power for the rest of the computer, and having it turn on prematurely could potentially lead to damage. Therefore, the chipset will generate a reset signal to the processor (the same as if you held the reset button down for a while on your case) until it receives the Power Good signal from the power supply.
When the reset button is released, the processor will be ready to start executing. When the processor first starts up, it is suffering from amnesia; there is nothing at all in the memory to execute. Of course processor makers know this will happen, so they pre-program the processor to always look at the same place in the system BIOS ROM for the start of the BIOS boot program. This is normally location FFFF0h, right at the end of the system memory. They put it there so that the size of the ROM can be changed without creating compatibility problems. Since there are only 16 bytes left from there to the end of conventional memory, this location just contains a "jump" instruction telling the processor where to go to find the real BIOS startup program.
The BIOS performs the power-on self test (POST). If there are any fatal errors, the boot process stops. POST beep codes can be found in this area of the Troubleshooting Expert.
The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. This BIOS is normally found at location C000h in memory. The system BIOS executes the video card BIOS, which initializes the video card. Most modern cards will display information on the screen about the video card. (This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself).
The BIOS then looks for other devices' ROMs to see if any of them have BIOSes. Normally, the IDE/ATA hard disk BIOS will be found at C8000h and executed. If any other device BIOSes are found, they are executed as well.
The BIOS displays its startup screen.
The BIOS does more tests on the system, including the memory count-up test which you see on the screen. The BIOS will generally display a text error message on the screen if it encounters an error at this point; these error messages and their explanations can be found in this part of the Troubleshooting Expert.
The BIOS performs a "system inventory" of sorts, doing more tests to determine what sort of hardware is in the system. Modern BIOSes have many automatic settings and will determine memory timing (for example) based on what kind of memory it finds. Many BIOSes can also dynamically set hard drive parameters and access modes, and will determine these at roughly this time. Some will display a message on the screen for each drive they detect and configure this way. The BIOS will also now search for and label logical devices (COM and LPT ports).
If the BIOS supports the Plug and Play standard, it will detect and configure Plug and Play devices at this time and display a message on the screen for each one it finds. See here for more details on how PnP detects devices and assigns resources.
The BIOS will display a summary screen about your system's configuration. Checking this page of data can be helpful in diagnosing setup problems, although it can be hard to see because sometimes it flashes on the screen very quickly before scrolling off the top.
The BIOS begins the search for a drive to boot from. Most modern BIOSes contain a setting that controls if the system should first try to boot from the floppy disk (A:) or first try the hard disk (C:). Some BIOSes will even let you boot from your CD-ROM drive or other devices, depending on the boot sequence BIOS setting.
Having identified its target boot drive, the BIOS looks for boot information to start the operating system boot process. If it is searching a hard disk, it looks for a master boot record at cylinder 0, head 0, sector 1 (the first sector on the disk); if it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector.
If it finds what it is looking for, the BIOS starts the process of booting the operating system, using the information in the boot sector. At this point, the code in the boot sector takes over from the BIOS. The DOS boot process is described in detail here. If the first device that the system tries (floppy, hard disk, etc.) is not found, the BIOS will then try the next device in the boot sequence, and continue until it finds a bootable device.
If no boot device at all can be found, the system will normally display an error message and then freeze up the system. What the error message is depends entirely on the BIOS, and can be anything from the rather clear "No boot device available" to the very cryptic "NO ROM BASIC - SYSTEM HALTED". This will also happen if you have a bootable hard disk partition but forget to set it active.

This process is called a "cold boot" (since the machine was off, or cold, when it started). A "warm boot" is the same thing except it occurs when the machine is rebooted using {Ctrl}+{Alt}+{Delete} or similar. In this case the POST is skipped and the boot process continues roughly at step 8 above.

BIOS Power-On Self Test (POST)

The first thing that the BIOS does when it boots the PC is to perform what is called the Power-On Self-Test, or POST for short. The POST is a built-in diagnostic program that checks your hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot. It later continues with additional tests (such as the memory test that you see printed on the screen) as the boot process is proceeding.

The POST runs very quickly, and you will normally not even noticed that it is happening--unless it finds a problem (amazing how many things are like that, isn't it?) You may have encountered a PC that, when turned on, made beeping sounds and then stopped without booting up. That is the POST telling you something is wrong with the machine. The speaker is used because this test happens so early on, that the video isn't even activated yet! These beep patterns can be used to diagnose many hardware problems with your PC. The exact patterns depend on the maker of the BIOS; the most common are Award and AMI BIOSes. This part of the Troubleshooting Expert will help you figure out what the POST beep codes mean and what to do about them, if you are having this problem.

Note: Some POST errors are considered "fatal" while others are not. A fatal error means that it will halt the boot process immediately (an example would be if no system memory at all is found). In fact, most POST boot errors are fatal, since the POST is testing vital system components.

Many people don't realize that the POST also uses extended troubleshooting codes that you can use to get much more detail on what problem a troublesome PC is having. You can purchase a special debugging card that goes into an ISA slot and accepts the debugging codes that the BIOS sends to a special I/O address, usually 80h. The card displays these codes and this lets you see where the POST stops, if it finds a problem. These cards are obviously only for the serious PC repairperson or someone who does a lot of work on systems.

BIOS Startup Screen

When the system BIOS starts up, you will see its familiar screen display, normally after the video adapter displays its information. These are the contents of a typical BIOS start up screen:

The BIOS Manufacturer and Version Number.
The BIOS Date: The date of the BIOS can be important in helping you determine its capabilities, since the "magic dates" of some features are fairly well known.
Setup Program Key: The key or keys to press to enter the BIOS setup program. (This is usually {Del}, sometimes {F2}, and sometimes another key combination.
System Logo: The logo of the BIOS company, or in some cases the PC maker or motherboard manufacturer.
The "Energy Star" Logo: This distinctive logo is displayed if the BIOS supports the Energy Star standard, which almost all newer ones do.
The BIOS Serial Number: This is normally located at the bottom of the screen. Since BIOSes are highly customized to the particular motherboard, this serial number can be used in many cases to determine the specific motherboard and BIOS version you are using. Check out Wim Bervoets' BIOS site for a huge list of these numbers.

System Configuration Summary

Just before the BIOS begins booting the operating system from disk, it will display an ASCII-graphics box on the screen containing summary information about your system's configuration. What is in this box depends on your BIOS and system, of course, but typically you will find the following:

Processor (CPU) Type: This is usually the general family of processor, like "Pentium" or "Pentium Pro" etc. Newer BIOSes will recognize the Intel-compatibles directly; older ones may display "Pentium" when the chip really isn't one. You may see this displayed as "Pentium-S" on a system with Award BIOS; this just means that the processor incorporates SMM, enhanced power management features that were introduced in the 486SL and incorporated into almost all current processor.

Coprocessor: This used to be used to indicate if the machine had a separate coprocessor or floating point unit (FPU) in it. Modern processors have this built-in so this will almost always say "Installed" or "Integrated".

Clock Speed: The speed of the processor in MHz. Some BIOSes put the speed of the processor together with the processor type display (the first line).

Floppy Drive A: The size and capacity of the first detected floppy disk, A:

Floppy Drive B: The size and capacity of the second detected floppy disk, B:

IDE/ATA Drives: Most BIOSes will show an entry for each of the four devices on an IDE system: the primary slave and master, and the secondary slave and master. Older machines only support two IDE drives and will often just call these "Drive C" and "Drive D", which is misleading (since each drive can actually contain more than one drive letter). For each drive installed in the system, the screen will show its size and what sort of modes it is using for access (PIO modes, whether or not it is using LBA, etc.). Some BIOSes will also display the drive's manufacturer. Most modern BIOSes will also detect and show IDE (ATAPI) CD-ROMs.

Base Memory Size: This will (almost?) always be 640K. This is also called conventional memory.

Extended Memory Size: How much extended memory your system has. If you add this to the base memory you will usually find that 128K, 256K or 384K of memory is "missing" from the number that it should be (see the "powers of 2" table for usual byte totals for 16MB, 32MB etc.) The BIOS usually will not report the upper memory area that is reserved for the BIOS ROM and other hardware adapters.

Cache Size: The amount of cache memory in the system.

Memory Type and Configuration: Most newer systems will display how many banks of memory have been found, and what memory technology is in use; for example, you may see "EDO DRAM at Bank 1" or "FP: 0" (FPM DRAM) or something similar.

Display Type: This will normally say "VGA/EGA". It doesn't really tell you anything at all about the video adapter since this describes generically every video adapter produced for the last ten or so years (why do they even bother?)

Serial Port(s): The I/O port addresses of your serial or COM ports. Normally there are two, with addresses 3F8h and 2F8h, but there can be three or four as well.

Parallel Port(s): The I/O port addresses of your parallel or LPT ports. Normally there is one, with the address 378h, but you may see 278h or 3BCh here.

Plug and Play Devices: Some BIOSes will report the descriptions of any Plug and Play expansion cards they identify during the boot process.

BIOS Components and Features

There are several components and features that are commonly found these days as part of a modern BIOS. This section discusses the various physical components that make up the BIOS feature of a typical machine. It also discusses flashing, or updating, the system BIOS.

The BIOS ROM

The main hardware component of the system BIOS is the system BIOS ROM itself. This is normally located in an electrically-erasable read-only memory (EEPROM) chip, which allows it to be updated through software control. This is commonly called a flash BIOS.

The BIOS ROM is located in a socket on the motherboard and is relatively easy to locate because it is usually labelled with the name of the BIOS manufacturer. Most times this is Award, American Megatrends (AMI) or Phoenix. There is also often a version number on the chip, although the actual BIOS version within the chip may be different from what is labelled, because of the ability to flash the BIOS mentioned above.

Under normal circumstances, the BIOS ROM is permanent and there is normally no reason to need to deal with it in any way. If for some reason the BIOS ROM were to become corrupted due to an aborted flash update, for example, you might find your PC left in a state where it could not be booted. In this situation, you might have to physically replace the BIOS ROM, but this is a very rare happening.

BIOS CMOS Memory

The BIOS settings that you use to control how your PC works must be saved in non-volatile memory so that they are preserved even when the machine is off. This is as opposed to regular system memory, which is cleared each time you turn off the PC. A special type of memory is used to store this information, called CMOS memory, and a very small battery is used to trickle a small charge to it to make sure that the data it holds is always preserved. These memories are very small, typically 64 bytes, and the batteries that they use typically last for years. This non-volatile memory is sometimes called NVRAM.

CMOS stands for "Complementary Metal Oxide Semiconductor". This is one type of technology used to make semiconductors (integrated circuits) such as processors, chipset chips, DRAM, etc. CMOS has the advantage of requiring very little power, compared to some other semiconductor technologies. This is why it was chosen for this use, so that the amount of power required from the battery would be minimal, and the battery would be able to last a long time. This memory came to be called just "CMOS" since in the early days most parts of the computer did not use CMOS. Ironically, with today's processors having to do more and more and needing to do it with lower power consumption, they themselves are typically made entirely with CMOS technology. However, "CMOS" by itself usually still refers to the BIOS settings memory. Old habits die hard in the computer world.

The system uses something called a CMOS checksum as an error-detecting code. Each time you change the BIOS settings, the checksum is generated by adding together all the bytes in the CMOS memory and then storing the lowest byte of the sum. Then, each time the system booted, the system recomputes the checksum and compares it to the stored value. If they are different, then the system knows that the CMOS has been corrupted somehow and will warn you with an error, typically something like "CMOS Checksum Error".

There are many different types of batteries used to power the CMOS; mostly, these have changed over time as technology has evolved. These batteries are discussed here. You will not normally have to deal with the CMOS memory directly; it holds the settings you enter in the BIOS setup program. Over time, it is possible you will have CMOS problems; for example, you may find that the machine may begin to forget its settings when it is booted. These are usually signs of trouble with the motherboard battery.

In addition to the standard CMOS memory used to hold system settings, Plug and Play BIOSes use an additional non-volatile memory to hold extended system configuration data (ESCD). This is used to record the resource configurations of system devices when Plug and Play is used.

BIOS Updates and The Flash BIOS

As you can see by looking at the rest of the information on the BIOS, it is a very important program indeed. Its quality and modernness determine the features and capabilities of your machine to a large degree. The most famous example of this is support for IDE/ATA hard disks over 504 MB; the only change necessary to support these larger drives in most cases is a BIOS capable of doing the geometry translation. There are other examples as well; many motherboard manufacturers are able to expand the capabilities of their boards, or fix problems with them, by making changes to the BIOS and either giving free downloads or selling upgrades, just as software application houses like Microsoft, Lotus, etc. do.

The BIOS program in your PC is programmed into a read-only memory (ROM). ROMs are, of course, not rewriteable the way RAM is; that is why they are called "read-only". This presents a problem when you want to update your BIOS. In "the old days" when you wanted to update your BIOS, the manufacturer sent you a new BIOS chip; you opened the box, pulled out the old chip, and put the new one in. Needless to say, this is a pain. Fortunately, technology came to the rescue through the invention of the flash BIOS. Some machines still require the physical upgrade but by using the flash BIOS, most newer machines can upgrade using special software without having to open the case at all.

Machines with flash BIOS capability use a special type of BIOS ROM called an EEPROM; this stands for "Electrically Erasable Programmable Read-Only Memory". As you can probably tell by the name, this is a ROM that can be erased and re-written using a special program. This procedure is called flashing the BIOS and a BIOS that can do this is called a flash BIOS. The advantages of this capability are obvious; no need to open the case to pull the chip, and much lower cost.

Many motherboards have a special "safety feature" to prevent accidental (or malicious) changes to the flash BIOS--a jumper that must be changed before performing a flash BIOS upgrade. While this is a security feature, it also obviates one of the great advantages of the flash BIOS, namely not having to open the case! Your motherboard manual will tell you whether you have a jumper or not. With the increasing commonality of viruses that can change flash BIOS code, this may soon be a feature on every motherboard.

There is one big disadvantage to using the flash process to upgrade your BIOS. While the BIOS is actually being flashed, it is in a very vulnerable state. If you are really unlucky and something very bad happens in the middle of the upgrade (for example, a power outage), it is possible to end up with a corrupted BIOS chip. You can also end up with a corrupted system if you boot the wrong flash BIOS image into the chip--motherboard manufacturers usually use an unintelligent, universal flash program that will happily program the entirely wrong BIOS image into your system if you tell it to! Fortunately, some manufacturers are now making these programs smarter. For example, Award's flash program checks the name of the image file against the model of motherboard and will complain if there is a mismatch. (Good idea, guys!)

If your system becomes corrupted to the point where it will not boot, you are in a catch-22. Your BIOS is corrupted, so you can't boot the PC, and since you can't boot the PC there's no conventional way to re-flash the BIOS. See this section in the troubleshooter for ideas on recovering from this problem.

Fortunately, BIOS chip corruption is quite rare; the BIOS is a small program and flashing it usually takes only a few seconds. Still, it's not something to try in the middle of a thunderstorm, and I try to use a UPS when doing this type of work, if possible. And be sure you are using the right BIOS image.

The Boot Block

The preceding section discusses updating the program code in the system BIOS by flashing it (replacing it under software control). If the flash procedure is done improperly, the BIOS code can become corrupted, which will cause the system to go into an unbootable state. Many newer systems come with a feature where a 4 KB "boot block" program is included as part of the BIOS. This is a tiny piece of code whose job it is to recover from a situation where the BIOS code is incorrect or corrupted.

If your motherboard supports this feature, when the PC tries to boot and finds the BIOS code corrupted, the boot block will try to recover the BIOS code, usually by reading it from a specially-prepared floppy disk. You may have to change a jumper on the motherboard to enable this capability, and you may need to make use of a "plain vanilla" ISA video card. The boot block will load the BIOS code and then when you next reboot, the regular BIOS code should be in place and the problem resolved.

You will need to follow the special recovery procedure outlined by your motherboard manufacturer if you want to take advantage of this feature.

Plug and Play

The BIOS is one of the four major parts of the system whose cooperation is required in order to implement Plug and Play features on a PC. In short, Plug and Play (also called "PnP" for short, I won't get into its sarcastic nicknames here :^) ) is a system whereby your PC hardware, BIOS and operating system work together to identify and configure resource usage by hardware devices automatically. The intention is to reduce the number of resource conflicts, jumper settings, and manual driver setups required to get the system to work.

The BIOS is a key part of this; in fact it does much of the work of identifying and configuring expansion cards, and passing configuration information on to the operating system.

BIOS Setup Program

Despite its many functions and the important role it plays in running your PC, the system BIOS is most "famous" for the BIOS setup program, the little built-in utility that lets you set the many functions that control how your computer works. In fact, some people even call this program "the BIOS" or "the CMOS" which of course is inaccurate ("CMOS" refers to the technology used to create the tiny memory where the BIOS settings are stored).

Setup Program Manual

Your motherboard manual should come with instructions on how to use the setup program; the BIOS program and its settings typically take up the second half of the entire manual. Unfortunately, most of these manuals are pitifully inadequate. They will usually provide a list of each setting with one or two sentences to describe them, and that's about it. Better motherboard companies will do better than this.

Warning: Be sure to check the manual closely and compare it against the settings you actually see on the screen. If you see mismatches, I would recommend you try to get an updated manual from the various sources on the Internet. It is, unfortunately, all too common to find incorrect or outdated BIOS manuals shipped with motherboards. This is usually worse with cheaper motherboards, but even good ones tend to come with bizarrely cryptic or inaccurate documentation.

Setup Program Interfaces

There is no standard interface for the BIOS setup program; it varies depending on the manufacturer and even within BIOSes by the same manufacturer. You would think that with only two companies (Award and American Megatrends, better known as AMI) dominating the field that they could coordinate and come up with some sort of a standard, but this has not happened. In fact, just the opposite, since AMI's newer BIOSes are actually Windows-like, complete with cursor and mouse control! Most traditional setup programs are text-based and use menus.

As PCs have grown more complex, they have started to have more and more settings that can be controlled. Generally speaking, this is a good thing, because you have much more control over how your system works than you ever have had before. However, it can also mean some confusion. Early PCs had a very simple setup program, sometimes even just a list of 10 or 12 questions displayed on the screen. Newer ones use sections to organize the various settings. Most PCs of the same general caliber will have similar settings, even though they may be organized differently.

Entering the BIOS Setup Program

The BIOS setup programs can normally be entered only during the boot process, either a cold boot or a warm boot (after hitting {Ctrl-Alt-Del}). Some setup programs will let you go into setup program using a key combination at any time.

At least one thing is finally becoming somewhat standard: the use of the {Del} key to enter the setup program during boot. This is true of AMI and Award BIOSes, and some others as well. Older BIOSes can use any of a myriad of strange key combinations, including {Esc}, {F1}, {F2}, {Ctrl-Esc}, {Alt-Esc}, {Ctrl-Alt-Esc}, {Ctrl-Alt-Enter}, {Ins} or others.