Old Motherboard - Intel 440BX


New Motherboard - ASUS P5AD2-E


The Motherboard  .  .  .  the foundation  .  .  .  the core  .  .  .  the engine  .  .  .  the PC's brain, skeletal and nervous system, arteries and veins arteries combined.  The Motherboard is a large circuit board that supports virtually every part of your PC.  

This is a long section, due to the many components of the motherboard.  Read it thoroughly, because if you understand the motherboard - you understand the PC.

The circuit board itself contains the ROM and CMOS chips discussed earlier, and the CPU which is the central brain.  It also has a number of chips that work together to supply supporting functionality to the CPU and device drivers - which is called the "chipset".  In addition, the motherboard has sockets and ports for just about everything else.  The motherbaord pictured, is a Supermicro P6SBA (based on the Intel 440/BX chipset, ATX form factor, Slot 1 CPU architecture) - one of the biggest selling Intel versions of all time : 

The primary components are shown above - and the chipset is comprised of all the gray chips you see scattered about.  The  CPU slot is a newer version called "Slot 1" as opposed to the older, white square with hundreds of holes for pins, called "Socket 7".   The L2 cache is a small amount of extremely high-speed memory - above and beyond the main RAM - that manufacturers have found to boost performance considerably.  Older PC's had little or L2 cache.  There are adapters available that plug into the Slot1 style CPU slot, and have a Socket 7 on their side - these are called "slocketts", and are extremely popular.  They allow users with the slower L2 cache (1/2 the processor speed) - to upgrade to faster PIII processors with L2 cache's that run at the processor speed.  I have done this to my Dell 450 PII with L2 cache at 1/2 that at 225 MHz  -  upgraded it to a 650 MHz PIII with L2 at full CPU speed, which is 650 MHz.  What a difference that made !!!

Most modern PC motherboards come with the following :

Reading the CPU Markings

This is only easy on a Pentium - AMD's use a more cryptic approach and are not covered here.  The following shows the markings on a Pentium II and a Pentium III :

PII Markings

Layman's Description (if that's even possible)

If you are reading these sections for an overview then read this, and briefly skim the rest, because to do the motherboard even a small amount of justice, I had to go into detail in the later sections.

The motherboard has a connection to all of the devices you see on your PC, and acts as a central director.  It has a path between the brain (the CPU chip) and the video card, so that it can display the programs you use on your monitor.  It has plugs on the side that connect to cables from your keyboard and your your mouse.  That way, when  you move the mouse, the motherboard picks up on it, and shows you the movement on the screen.

If you double-click on an icon, such as Microsoft Word, the motherboard realizes that, and using it's connection to your hard drive, it load in files from the Microsoft Word directory and then displays an empty new document on the screen for you.  When you type, the letters are place into the empty Word document by the motherboard (via the video card which in turn feeds into your monitor).  Once you are done typing, and click "Print", the motherboard realizes that you want to print the document, and sends the characters out the parallel port (printer port) to your printer. 

When you run programs that add, subtract, and perform various mathematical operations, the motherboard sends the data into the CPU, and the operations are completed there, within the CPU, and the results are fed back out and either stored on your hard drive or maybe temporarily stored in memory (RAM) and can also be displayed on the screen.  The motherboard runs off a system clock, so that it runs at a high speed of digital "ticks" and does one thing at a time - but very very quickly, so that it seems to us like it is doing many things at once.

Real-Time and System Clocks

The computer has two clocks . . . a "real-time clock" which is used to keep the time of day, and runs off the same battery that the CMOS uses, and a system clock, which is mounted on the motherboard, and runs off the power supply voltage, accurately generating high-speed "ticks" or "cycles" which are used to synchronize the CPU cycles.

The system clock is synchronized, and must be extremely accurate, so that the computer does not generate errors.  The system clock stops running when you turn the PC off.   The clock speed is given in MHz (Megahertz = millions of cycles per second).  The system clock runs at the same speed as the Front Side Bus, and in fact the bus speed is controlled by the system clock !!

It is an interesting fact the the processor does not run at the same speed of the CPU clock.  Instead, the CPU uses a multiplier circuit to actually boost the speed of the system clock.  For example, a PII 450 MHz PC, will have a bus speed (system clock speed) of 100 MHz.  It uses a multiplier of  x4.5 to create the 450 MHz CPU speed.

NOTE:  all motherboards have an adjustable multiplier, either by using jumpers, or the CMOS setup program (which can be accessed during boot-up).  Therefore, if you purchased a 350 MHz PII, and want to upgrade - you can buy a 450 MHz PII CPU, and insert it  -  just make sure to set the jumpers so that the multiplier is now 4.5 instead of 3.5


This is the practice of increasing the multiplier beyond what the recommend limits are.  It was very popular a few years ago.  Amazingly, when you buy a 350 MHz Intel PII CPU, it is exactly the same thing as a 450 MHz CPU - only the multiplication factor of the system clock changes.

Unfortunately, modern CPU's starting in the fall of 1999 have a "lock" on the speed they will run at, which foils most overclocking schemes.  Some creative people have found ways around it, but it is risky in that it may burn up your CPU - I would advise you to stay away from it.

NOTE:  video cards can be overclocked in a similar fashion, but they also have a propensity for burning up !!

CPU (Central Processing Unit)

There is a separate section on the CPU, which plays the most critical role for the motherboard. It is used in virtually every operation that your computer is involved in.  The CPU can only do one thing at a time.  THE CPU CAN ONLY DO ONE THING AT A TIME.  Therefore, it must run at a very high speed . . . so high in fact, that to us, it appears that many things are occurring simultaneously.  There is even a term for it  .  .  . "multitasking".

In reality, there is no multitasking within the CPU, unless your PC has two or more CPU's.  Multitasking actually involves a "round-robin" servicing of different processes by the CPU.  For example, it may send a number of bytes of a print job to the printer, then add some figures in an operation that you are running in a spreadsheet, then move some data between the keyboard and the screen, then go back and send some more data to the printer, and so on.  It blazes around, servicing different tasks,  at such a speed that it appears that they are all being performed simultaneously.  Hence the term "multitasking".

Heat Sinks

The CPU has thousands of tiny transistors, all packed tightly together, and all have electrical current running through them, which generates a significant amount of heat.  The heat is very intense and would destroy the CPU if allowed to go unchecked.  

Therefore "heat sinks" are used to dissipate the heat.  A heat sink is made of metal, and functions using the same concept as a car radiator. In order to increase the surface area - so that more heat can be dissipated - the outer portion of the heat sink is fashioned into a number of "fins", as shown below in the Socket 7 CPU heat sink, which also fits on Socket 370 (same size exactly):


Metal is great conductor of heat (and cold for that matter).  The heat sink is attached to the back of the CPU with a clamp, so that the two surfaces are flush against one another.  The heat flows into the heat sink, which radiates it out into the air.  This is why a cooling fan is so important - to keep a flow of cool air moving across the fins of the heat sink.  

NOTE:  newer video video cards also use heat sinks

Expansion Slots 

15 interrupts used to be more than enough.  Then came the scanner, the digital camera, the video capture card, the CD Recorders, etc.  Soon, power users found that they had no interrrupts left !!!  Assume for this example the device is a card, inserted into one of the motherboard's expansion slots (see the pic above).  If the device is in the older type of slot called ISA (Industry Standard Architecture) slot - actually it is the bus type underneath the slots which is ISA  -  then it must have one interrupt, all to itself.  If it is the newer and faster PCI slot (Peripheral Component Interconnect) then it can share an interrupt with another PCI device.  PCI was a godsend to the computer world, since many users were in a bind, and had to sacrifice - disconnecting expensive equipment they had bought, due to lack of interrupts.

Another type of slot called the AGP (Accelerated Graphics Port) is typically used exclusively for a video card, and PC's only have one AGP slot.

Other devices, which are neither ISA or PCI (such as your disk drive controller, system timer, numeric data processor, etc,) also receive an interrupt - and these generally cannot be shared.  Only PCI devices can share interrupts.


You can think of a bus as a highway on which data travels within a computer. When used in reference to personal computers, the term bus usually refers to internal bus. This is a bus that connects all the internal computer components to the CPU and main memory. There's also an expansion bus that enables expansion boards to access the CPU and memory. Please see the Buses section for more details, and a description of the primary 5 types of buses.

All buses consist of two parts -- an address bus and a data bus. The data bus transfers actual data whereas the
address bus transfers information about where the data should go.  The size of a bus, known as its width, is important because it determines how much data can be transmitted at one time. For example, a 16-bit bus can transmit 16 bits of data, whereas a 32-bit bus can transmit 32 bits of data.   

Every bus has a clock speed measured in MHz. A fast bus allows data to be transferred faster, which makes
applications run faster. On PCs, the old ISA bus is being replaced by faster buses such as PCI.  Nearly all PCs made today include a local bus for data that requires especially fast transfer speeds, such as video data. The local bus is a high-speed pathway that connects directly to the processor, bypassing RAM. 

Bus Speed - typically, when someone mentions bus speed, they are talking about the FSB (Front Side Bus).  An odd name - it is called this because the area we are referring to is at the front of the CPU, facing the memory and I/O buses.    Anyway, it was long ago found that one of the primary bottlenecks in a PC is the bus speed.  The faster the bus speed, the quicker the cards (sound card, video card, etc.) can communicate with the CPU.

Older computers used a 33 MHz FSB bus, which was then increased to 66 MHz.  More recently the 100 and 133 MHz FSB buses were introduced.  The FSB runs at the same speed as the system clock.

Cache (pronounced "cash")

Cache memory is small and is very high-speed.  Therefore they use expensive static RAM (SRAM) for cache memory.  It is still affordable, since the amount of memory is so small.  Cache memory rarely exceeds 1 MB, and 256k and 512k are the most common values used.  The idea behind cache memory is that you can place a small amount of memory directly next to the CPU, and it can be used to perform the most common instruction which require small amounts of memory.  A large number of operations are done repetitively, and if there is cache memory to perform them - the farther away and slower RAM does not need to be accessed.

In order to use a cache effectively, the CPU needs to determine which data is most frequently accessed.  The methods of figuring this out is known as "smart caching".

L1 Cache (Level 1 Cache) - Primary, or Internal Cache - you don't hear much about this type of cache, because it has been largely done away with.  It is cache that is integrated within the CPU.  Older PC's used it . . . the Intel 486 CPU contained an 8 kB L1 Cache, and the Pentium had a 16 kB L1 Cache.

L2 Cache (Level 2 Cache)- External Cache, that later became an Internal Cache - the L2 cache is what has enabled modern PC's to become the speed demons that they are.  Initially the L2 cache was a separate chip on the Motherboard, between the CPU and Memory.  Later it was integrated into the CPU (see explanation below), same as the L1 cache.

The FSB and BSB -
the circuit board traces (leads) that connect the L2 cache to Memory, sit in front of the cache and therefore are called the FSB (FrontSide Bus).  The leads that connect the L2 cache to the CPU, are tucked away behind the L2 cache, and therefore are called the "BSB - Back Side Bus".

L3 Cache (Level 3 Cache)- External Cache - like the initial L2 cache, the L3 cache is a separate chip on the Motherboard, between the CPU and Memory.  What was once L2 cache on motherboards, has now become the L3 cache - since the L2 cache is now contained within the CPU.  Few motherboards today (as of early 2005) have L3 cache.

Why the L2 Cache was Integrated into the CPU Core - 

Intel's first attempt to integrate the L2 cache directly into the processor failed, because the sheer mass of processor rejects drove the manufacturing costs for the Pentium Pro through the roof. While its successor, the Pentium II, also had an integrated L2 cache, the same difficulties prevented it from being integrated directly into the processor core. Instead, Intel integrated the processor onto a small circuit board, added memory components for the L2 cache, wrapped it all up in a plastic box, and dubbed it the "Slot 1." The processor had morphed into a clunky plug-in board that was more expensive than its socket-based rivals. 

Once 0.25 µm manufacturing methods were introduced, though, Intel was able to integrate the L2 cache into the core, stepping up performance considerably. The first processor to benefit from this then-innovative technique was the Celeron Mendocino (128 KB L2 cache). It was not until many months later that the second one, the Pentium III with a Coppermine core (256 KB), came out.

Motherboard Classes - Many people identify the type of motherboard with the type of CPU that it takes.  For example, a Motherboard that accepts PIII CPU's (Pentium 3) is often called a Pentium III Motherboard.  Others use the chipset to classify the board (such as the 440BX for example).  However, there are several important factors such as chipset that is permanently soldered onto them, the form factor, type of CPU, number of expansion slots, and types of  memory.  There is no way to cover all mother boards here, so we will instead go over the more popular versions.

NOTE:  there are two main makes/models of CPU's - Intel Pentium and AMD Athlon (Cyrix was a third brand that closed their business).  We will do not discuss the AMB CPU's here - however they are excellent !!

Pentium - designed to work with 3.3 volt Pentium CPU's, 75-to-200 MHz, including Intel Pentiums, clones, AMD K5's, and Cyrix 6x86.  Most use a Socket 5 or Socket 7 CPU, and the Intel versions have the old 430HX, 430VX, and 430FX chipsets.  The single-volt architecture does not allow the MMX instruction set to be used.  RAM is limited to either EDO DRAM or SDRAM.  No DMA, USB ports, and the L2 cache was poorly designed.  The bus speed is typically 66 MHz.

Pentium MMX - uses two voltages (split-voltage) at the same Pentium 3.3 volts for the motherboard compnents, except the CPU which runs at 2.8 volts.  The lower voltage in the CPU allows higher speeds.  Only Socket 7 is used for the CPU mount,  MMX is actually a set of 57 multimedia instructions built into the microprocessor. MMX-enabled microprocessors can handle many common multimedia operations, such as digital signal processing (DSP), that are normally handled by a separate sound or video card. However, only software especially written to call MMX instructions -- so-called MMX-enabled software -- can take advantage of the MMX instruction set. 

Pentium II - includes the MMX instruction set - the first PC's to use Intel's Slot 1 architecture, which is a slot that the CPU plugs into, instead of a flat  socket.  The L2 cache runs at 50% the speed of the CPU.

Pentium III - the biggest improvements here was the 133 MHz bus, and the L2 cache runs at the same speed as the CPU.  The PIII features 70 new instructions--Internet Streaming SIMD extensions- that dramatically enhance the performance of advanced imaging, 3-D, streaming audio, video and speech recognition applications. It was designed to significantly enhance Internet experiences, allowing users to do such things as browse through realistic online museums and stores and download high-quality video. The processor incorporates 9.5 million transistors, and was introduced using 0.25-micron technology - the CPU is remarkably smaller than the .35 micron PII, and therefore it runs cooler.. Again, similar to MMX, only software especially written to call SIMD instructions - can take advantage of the instruction set. 

Pentium 4 - (see ) - the latest Intel CPU, with extremely high speeds of up to 4 GHz.  Includes a 32-bit microprocessor, hyper-pipelined technology, a rapid execution engine and a 400 to 800 MHz system bus.  There is also a 1066 MHz bus available with the very expensive CPU family known as the "Extreme Edition".  Designed to enhance online gaming, digital video and photography, speech recognition and MP3 encoding. Current speeds run at 2.4 to 3.8 GHz. The Pentium 4 processor also features a new Level 1 cache technology - Execution Trace Cache, NetBurst micro-architecture doubles the pipelength depth to 20 stages, and increases the frequency capability.  Streaming SIMD extension 2 (SSE2) has 144 new instructions, a 128-bit SIMD integer arithmetic and 128-bit SIMD double precision floating point instructions.  Here are the two recent P4 packages and CPU families for sale by Intel (March 2005):


775 -Land package

Intel® Pentium® 4 Processor
Intel® Pentium® 4 Processor
Supporting Hyper-Threading
Processor NumberΔ 660, 650, 640, 630 570J°, 560J, 560, 550J, 550, 540J, 540, 530J, 530, 520J, 520 NA
Architecture 90 nm process technology
L2 Cache 2 MB 1 MB 1 MB
Clock Speed 3 to
3.60 GHz
2.80 to 3.80 GHz 3.20F, 3.40F, and 3.60F
Front Side Bus 800 MHz
Chipset 800 MHz system bus Intel® 925XE Express**, 925X Express, 915G Express, 915GV Express, 915GL Express, 915P Express, and 915PL Express Chipsets Intel® E7221, and 925X Express chipsets
Socket LGA775
Intel® Desktop Boards Download matrix for desktop boards compatible with the Intel Pentium 4 processor [PDF 91KB] Intel® Entry Server Board SE7221BK1-E
Hyper - Threading Technology Yes
Intel® Extended Memory 64 TechnologyΦ Yes NA Yes
Execute Disable Bit° Yes 570J, 560J, 550J, 540J, 530J, 520J °NA

** This chipset also supports 1066 MHz system bus if you use an "Extreme Edition CPU".



478-pin package

Intel® Pentium® 4 Processor
Intel® Pentium® 4 Processor supporting Hyper-Threading Technology
Intel® Pentium® 4 Processor
Intel® Pentium® 4 Processor

Architecture 90 nm, 130 nm process technology 90 nm, 130 nm, 180 nm process technology
L2 Cache 512 KB, 1 MB 256 KB, 512 KB, 1 MB
Clock Speed 2.40 to 3.40 GHz 1.30 to 2.80 GHz
Front Side Bus 800 MHz 400, 533 MHz
Chipset 800 MHz system bus Intel® E7221, 875P, 865PE, 865G, 865GV, and 848P chipsets NA
Chipset 533 MHz system bus Intel® 865P, 850 Chipset Family, 850E, 845PE, 845GE, 845GV, 845E and 845G Intel® 865P, 850 Chipset Family, 850E, 845PE, 845GE, 845GV, 845E and 845G chipsets
Chipset 400 MHz system bus NA Intel® 845GL and 845 chipsets
Socket mPGA478
Intel® Desktop Boards Download matrix for desktop boards compatible with the Intel Pentium 4 processor [PDF 91KB]


Form Factors

An odd term - but all it means is the size of a motherboard, and has been extended to mean the type of case as well.  Why they didn't call it size, I'll never know.  As mentioned in the overview, there are two main "form factors" for motherboards  .  .  .  the AT or Baby AT  .  .  . and the ATX.  ATX is not an abbreviation - it is simply an Intel copyrighted name.  There are a few others worth mentioning, such as LPX and NLX.  Here is a comparison :




Where Found

Match to Case and Power Supply

Full AT



Very Old PCs

Full AT, Full Tower

Baby AT



Older PCs

All but Slimline, ATX




Newer PCs


Mini ATX



Newer PCs





Older Retail PCs


Mini LPX



Older Retail PCs





Newer Retail PCs



Baby AT

A Baby AT motherboard is 8.5" wide and nominally 13" long, but some odd ones are 10" or 11" instead or 13".  Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard
connector soldered onto the board. The serial and parallel port connectors are almost always attached using cables that go between the physical connectors mounted on the case, and pin "headers" located on the motherboard.

The AT and Baby AT form factors put the processor socket(s)/slot(s) and memory sockets at the front of the motherboard, and long expansion cards were designed to extend over them. When this form factor was designed, over ten years ago, this worked fine: processors and memory chips were small and put directly onto the motherboard, and clearance wasn't an issue. However, now we have memory in SIMM/DIMM sockets, not directly inserted onto the motherboard, and we have larger processors that need big heat sinks and fans mounted on them.  That is why the ATX form factor was created.


The first significant change in case and motherboard design in many years, the ATX form factor was invented by Intel in 1995, and has taken over the old Baby AT's.  

Characteristics of ATX form factor motherboards:

Some ATX advantages over Baby AT :