Computer Basics Page 2

Motherboards

The motherboard is the most important part of your computer.  It is also one of the most compared, critiqued, and reviewed pieces of hardware. Often, on the internet,  you'll find reviews and debates over which board is best or which chipset is best.  Sometimes the average reader gets left in the dust. We will try to explain what's going on and what it all means.

The chipset controls the system and its capabilities. It is the hub of all data transfer. It is a series of chips on the motherboard, easily identified as the largest chips on the board with the exception of the CPU. Chip sets are integrated, meaning they are soldered onto the board and are not upgradable without buying a whole new motherboard.

All data must go through the chipset. All components talk to the CPU through the chipset. To make order out of all this data, the chipset makes use of the DMA controller and the bus controller.

Since chipsets are so important and have to know how to communicate with all components, they must be designed for your configuration and CPU. The chipset maker needs to keep up with BIOS and memory makers, since all of these parts work together and the chipset is the hub of it all.

A chipset is designed by the manufacturer to work with a specific set of processors. Most chipsets only support one generation of processors: most chipsets are geared specifically for 486 type systems, Pentium class systems, or Pentium Pro / Pentium II systems. Why make it complicated like that? Well, the reason is simple. The design of the control circuitry must be different for each processor generation due to the different ways they employ cache, access memory, etc. For example, the Pentium Pro and Pentium II have level 2 cache within the CPU itself, so obviously they would need a different circuitry design than the Pentium, which has level 2 cache on the motherboard.

Most motherboards that support Intel Pentium processors also support their equivalents from AMD and Cyrix. In fact, usually, these chips install just as an Intel chip, other than the fact that you may need to set different jumper settings for bus speed or voltages. I must note here, though, that the different voltages of the CPU's and whether the board will support it is not a function of the chipset, but of the voltage regulator. But, since Intel is the largest manufacturer of Pentium-class and higher chipsets, AMD and Cyrix are at a disadvantage. AMD has evened out the field a tad with their AMD-640 chipset, aimed at optimizing the performance of AMD's K6. But also, companies such as Via and ALI are producing Super 7 chipsets aimed at non-Intel processors.

Processor Speed Support

Faster processors require chipsets capable of handling them. The specification of the processor speed is done using two parameters: the memory bus speed and the processor multiplier. The memory bus speed is the processor's "external" speed, the speed it talks to the rest of the computer at. The memory bus speed also (normally) dictates the speed of the PCI local bus, which in most motherboards runs at half the memory bus speed. Typical modern bus speeds are 50, 60, 66 and 75 MHz. Faster systems use 83MHz or even 100MHz bus speeds. The multiplier represents the number by which the memory bus speed must be multiplied to obtain the processor speed. Multipliers on modern PCs are normally 1.5x, 2x, 2.5x, 3x, 3.5x, or 4x, though faster processors will eventually increase this.

The chipset runs at the speed of the motherboard bus, usually 66MHz in most systems. With chipsets such as Intel's 440BX, Via's MVP3, and ALI's Aladdin V, many newer PC's are pushing 100MHz bus speeds. This particularly helps the performance of Super 7 systems because the L2 cache runs at the speed of the motherboard. This doubles L2 cache speeds. With Pentium II's, the L2 cache is already running at 1/2 the speed of the processor, so increasing the bus to 100MHz won't help out as much.

The range of the processor speeds supported by the chipset is indicated, generally, by looking at the range of supported memory bus speeds and multipliers. A typical Pentium chipset will support bus speeds of 50 to 66 MHz with a multiplier range of 1.5x to 3.0x. This yields speeds of 75, 90, 100, 120, 133, 150, 166 and 200 MHz.

Multiple Processor Support

Some chipsets support the ability to make motherboards that support two or four processors. The chipset circuitry coordinates the activities of the processors so that they don't interfere with each other, and works with the operating system software to share the load between them. The standard for multiprocessing in Pentium and Pentium Pro PCs is Intel's SMP (symmetric multiprocessing). It only works with Intel processors. Of course, I should make note that, in order to successfully have a multi-processor system, that much more than a supporting chipset is needed. You must have compatible CPU's and a supporting OS.

Most modern computers use three bus types: the ISA bus for slower, older peripherals, the PCI bus, and the AGP Bus.

The chipset controls these buses. It transfers information to and from them and the processor and memory. The chipset's capabilities in this area determine what kinds of buses the system supports and how fast they can get. For this reason, Intel calls its chipsets "PCIsets". Most modern PCs support the ISA and PCI buses, but older chipsets support the VESA Local Bus instead of PCI.

Bus Bridges

A "bridge" is a networking term that refers to a piece of hardware that connects two dissimilar networks and passes information from the computers on one network to those on the other, and vice-versa. In this way, the chipset must use bus bridges to connect together the different bus types it controls. The most common of these is the PCI-ISA bridge, which is used to connect together devices on these two different buses.

IDE/ATA Hard Disk Controller

Almost all motherboards now have support for four IDE (ATA) hard disks integrated into them, two on each of two channels. Integrating this support makes sense for a number of reasons, among them the fact that these drives are on the PCI bus, so this saves an expansion slot and reduces cost. The data transfer rate of IDE drives is based on their using programmed I/O (PIO) modes, and use of the fastest of these modes depends on support from the PCI bus and chipset. The ability to set a different PIO mode for each of the two devices on a single IDE channel, called independent device timing, is also a function of the chipset. Without this feature, both devices must run at the speed of the slowest drive.

More recently, ATA-33 drives have become the thing to have. These enhanced IDE drives are appealing mainly because of their attractive price. Earlier chipsets only supported PIO modes, which required CPU involvement in every hard drive access. This isn't good when trying to multi-task. ATA-33 drives use DMA to work without CPU intervention. This allows speeds of up to 33MBps. The concept of DMA is described below.

DMA Mode Support and Bus Mastering

Direct memory access (DMA) provides a way for devices to transfer information directly to and from memory, without the processor's intervention. It is still used by many devices, although newer transfer modes are now used for high-performance devices like hard disks. DMA is controlled by the chipset's DMA controller, and the newer the controller, the more DMA modes its supports.

Bus mastering is an enhancement of DMA whereby the remote device not only can send data to the memory directly, it actually takes control of the bus, and performs the transfer itself instead of using the DMA controller. This cuts down on the overhead of having the slow DMA controller talk to the device doing the transfer, further improving performance. Bus mastering support is provided by the chipset.

USB & AGP Support

USB (Universal Serial Bus) is a new technology intended to replace the current ports used for keyboards and mice. It is still unclear as to whether this standard will catch on and become popular. USB has been around for a while now, although it is still rather rare to see in action. Despite this, most modern chipsets support USB.

AGP is another high-speed bus used for graphics cards. This bus must be supported by the chipset. The Intel 440LX used to be the only chipset the supported it, but since then, many more have emerged, including many not made by Intel.

Plug and Play

Plug and Play (PnP) is a specification that uses technology enhancements in hardware, BIOS and operating systems, to enable supported devices to have their system resource usage set automatically. Intended to help make installation easier by eliminating some of the problems with getting peripheral devices to work together, PnP requires support from the chipset as well.

Chipsets offer support for power management on the computer. Most recent chipsets support a group of features that reduce the amount of power used by the PC during idle periods. These types of features are deemed important for a few reasons. First, many get concerned over the amount of power consumed by PC's when they are left on for long periods of time. Secondly, with the use of laptops, many are concerned about the life of their battery.

Power management works through a number of BIOS settings that tell the computer when to shut down various pieces of hardware when it becomes idle. While, in theory, this is a good idea, it does sometimes get in the way. One example is that all-too-common wait time when returning to the computer to wait for the hard drive to power up. Sometimes, the hard drive will power down too soon, and when you come back, you have to wait a few seconds for the drive to power up again.

There are a number of terms commonly heard in relation to these power management features. Energy Star is a program started by the EPA to brand PCs that are considered energy efficient and incorporate power management. Most modern PCs are Energy Star compliant, and display its logo on the top of the screen when the BIOS boots up. Advanced Power Management or APM is the name given to the component in some operating systems (such as Windows 95) that works with the BIOS to control the power management features of the PC. APM allows you to set parameters in the operating system to control when various power management features will be activated. System Management Mode or SMM is a power-reduction standard for processors. This allows them to automatically and greatly reduce power consumption.

One of the biggest issues with chipsets is what types of memory they will support as well as how much.

When purchasing a chipset, make sure you get one with support for SDRAM. With this, 66MHz is fine for most applications, but with the prices for 100MHz chipsets coming down so much, opt for a chipset supporting the 100MHz frontside bus. You'll see the difference in many aspects of the computer's use, especially the more involved AGP-enabled applications.

One needs to pay attention to how the memory is supported. A chipset can support a certain amount of memory as well as is able to cache a certain portion of it. This means that a certain amount of the main system memory will be cached by the L2 cache, increasing performance. One of the more famous horror stories is the 430TX chipset by Intel. Although it could support up to 256MB of SDRAM, it could only cache the first 64MB of it. This meant that with memory amounts over 64MB, you were probably degrading the system's performance by quite a bit. Because Windows 95 loads itself into the higher memory areas, leaving the lower areas free for DOS compatibility, this meant that the OS and all system-critical applications were being hampered by the crappy cache support.

When purchasing a chipset, make sure it can address 1MB or 2MB of L2 cache. Some come with 512K, which is adequate, but don't consider 256K or lower. The higher the L2 cache, the more memory the chipset is likely to be able to cache.

The chipset market will have to evolve along with memory enhancements. Minor tweaks to SDRAM, such as Double Date Rate (DDR) SDRAM will extend the life of SDRAM, but will include some tweaks to the chipset support. Intel's eventual move to Rambus DRAM, or RDRAM, will change everything. While SDRAM delivers data steadily at 66MHz of 100MHz, RDRAM will use an 8-bit interface and fire data off at 800MHz. Because of the close integration of the chipset to the memory subsystem, the move to RDRAM will require drastic changes to chipset design.

 

Computer Basics Page 3