Chipset

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Chipsets The system chipset and controllers are the logic circuits that are the intelligence of the motherboard. They are the “traffic cops” of the computer, controlling data transfers between the processor, cache, system buses, and peripherals—basically everything inside the computer. Since data flow is such a critical issue in the operation and performance of so many parts of the computer, the chipset is one of the few components that have a truly major impact on your PC’s quality, feature set, and speed. What exactly is a “chipset”? It sounds like something very complex but really is not, although many of the functions it performs are. A chipset is just a set of chips. At one time, multiple, smaller controller chips performed most of the functions of the chipset. There was a separate chip (often more than one) for each function: controlling the cache, performing direct memory access (DMA), handling interrupts, transferring data over the I/O bus, etc. Over time these chips were integrated to form a single set of chips, or chipset, that implements the various control features on the motherboard. This mirrors the evolution of the microprocessor itself: at one time many of the features on a Pentium for example were on separate chips. There are several advantages to integration, but the two primary ones are cost reduction and better compatibility (the more things that are done by a single chip or group of chips from one manufacturer, the simpler the design is, and the less chance of a problem). Sometimes the chipset chips are referred to as “ASICs” (application-specific integration circuits), although there are many other types of ASICs as well. Note: Intel also calls their chipsets “PCIsets” and “AGPsets”, referring to the system bus technologies the chipsets implement. The system chipset in most cases does not integrate all of the circuitry needed by the motherboard. Most motherboards have the following controllers on them: • •

The system chipset itself. The “Super I/O” chip, which handles input and output from the serial ports, parallel port, floppy disks, and in some cases, the IDE hard disks as well • Additional built-in controllers that are normally found in expansion cards: video, sound, network and SCSI controllers being the most common. Note: The term “chipset” is also used to refer to the main processing circuitry on many video cards. The name is used because the concept is similar: a highly integrated circuit used to perform a set of functions. However, this is a totally different type of chipset, and is not the same as a motherboard (system) chipset.

Chipset Functions and Feature This section describes the various functions performed by the system chipset, and the various PC features that the chipset plays a key role in supporting. In doing so, it touches upon the various system features that are enabled by the choice of chipset made by the motherboard designer. These capabilities are broken into several categories, as described.

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Chipset Processor Support One of the most important decisions made by anyone choosing or building a new PC is which processor is desired. The key to making the decision of what type, speed and even what number of processors to use is the motherboard, and in particular the chipset that controls it. •

Processor Class Support and Optimization

A chipset is designed to work with a specific set of processors in mind. In general, most chipsets only support one “class” or generation of processors: most chipsets are geared specifically for Pentium class systems, or Pentium III / Pentium 4 systems. The reason for this is simple: the design of the control circuitry must be different for each of these processor families due to the different ways they employ cache, access memory, etc. For example, the Pentium III & Pentium 4 have level 2 cache within the CPU package itself, so obviously these need different logic than the Pentium, which has level 2 cache on the motherboard. •

Processor Speed Support

Faster processors require chipset control circuitry capable of handling them. The specification of the processor speed is done using two parameters: the memory bus speed, and the processor multiplier. In short, the processor bus and memory bus connect the processor, chipset and memory together. The memory bus speed is the processor’s “external” speed, the speed it talks to the rest of the computer at (as opposed to its internal speed). Typical modern memory bus speeds are 66, 100 and 133 MHz. The multiplier represents the factor that the processor multiplies the memory bus speed in order to obtain its internal speed. Multipliers on modern PCs are normally 3x, 3.5x, 4x, 4.5x, 5x, 5.5x, 6x, 6.5x etc. 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. For example, a typical classic Pentium III chipset will support bus speeds of 66 to 133 MHz with a multiplier range of 3x to 12x. •

Multiple Processor Support

Some chipsets support the ability to create motherboards with two or four processors on them. 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 the CPUs for maximum efficiency. There is more to running multiple processors than the chipset supporting it, though that is one piece of the puzzle. In addition, the processors must be compatible, and your operating system must be able to take advantage of the extra CPUs for there to actually be a performance improvement.

Chipset Cache Support This section discusses chipset features related to the system or secondary cache. The cache buffers recent memory accesses by the processor, which improves performance drastically since it operates much faster than the system memory.

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Secondary Cache Size

The chipset determines how much level 2 (secondary) cache is supported. Most modern chipsets support cache of either 256KB or 512KB. Note that this is not relevant for modern Pentium III or Pentium 4 machines, which have level 2 cache in the same package as the processor. •

Secondary Cache Type

There are three major types of cache currently in use; in increasing order of performance, they are: asynchronous, synchronous burst, and pipeline burst. Each requires different control circuitry, and therefore must be explicitly supported by the chipset. •

Secondary Cache Write Policy

There are two different “policies” for controlling the way writes to memory are done in a cached system: a “write through” cache means that memory writes are sent to memory as soon as the processor sends out the information; a “write back” cache means the processor writes the information only to the cache, which later writes the information to memory at the appropriate time. Write back is in most cases superior to write-through •

System Memory Cacheability

Chipset characteristics control the maximum amount of memory the system can cache. This is a very different question than the amount of memory the system can hold. Many chipsets (unfortunately) can hold more memory than they can support with the level 2 cache. The amount of cacheable memory depends on the chipset control circuitry and the amount of tag RAM on the board. It does not depend on how much memory you currently have in the system

Chipset Memory Support There are many different styles and sizes of system memory (RAM), and many different technologies in use today. The chipset dictates many of the allowable characteristics of the memory used on your motherboard •

Maximum Memory Support

The chipset dictates the maximum amount of RAM you can have on the motherboard. For modern systems, this can be as low as 64 MB, or as high as 4 GB. This number can be higher (in some cases much higher) than the maximum amount of memory that can be cached, and this is something to be aware of. •

DRAM Technology

The chipset controls whether your motherboard can use FPM, EDO, or SDRAM, DDR SDRAM or RDRAM memory. Changing the memory type impacts the way that memory is read and written to, which is controlled by the chipset. In addition, some chipsets are better than others with certain types of memory; a chipset can be optimized to provide

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faster access to a certain kind of memory, while being less efficient at using a different kind. Memory for desktop computers comes in few basic types of packages: the SIMM (single in-line memory module) DIMM (dual in-line memory module), RIMM (Rambus in-line memory module) & DDR (Double Data Rate) DIMM. SIMMs come in two sizes: 30 pins and 72 pins. DIMMs are 168 pins. These packages differ not only in physical size but also in the “width” of the memory they can output at once. 30-pin SIMMs produces 8 bits of data at a time, 72-pin SIMMs 32, and DIMMs 64 bits. The width of memory needed by the motherboard depends on how the chipset accesses memory, and on the data bus width of the processor. In general, 486 class machines require 32-bit wide memory and Pentium, Pentium Pro and Pentium III machines require 64-bit wide memory, but the chipset design has an influence on this. Some Pentium motherboards will handle 32-bit wide memory (slowly) by changing the way memory is accessed. The design of the chipset is also a limiting factor on how many SIMM, DIMM or RIMM slots the motherboard can have. Better chipsets support more slots, giving you more flexibility in setting up and upgrading your PC. •

Parity and Error Correction Support

Error correction logic is provided as part of the memory control circuits of the chipset. On modern PCs, both parity and ECC functions are provided by the use of parity memory; some chipsets support ECC only, using ECC memory. A chipset without at least parity support has no ability to detect or correct memory read errors.

Chipset Timing and Flow Control One of the chipset’s most important functions is controlling memory reads and writes, and transfers to the local bus (usually PCI and/or AGP) by the processor. The design quality of the chipset, the type of cache, the speed of your system memory, and the type of processor used all play a part in determining how efficiently information can be transferred to and from the processor, which will dictate, in part, the overall performance of your machine. •

Address Decoding

The chipset performs the function of translating the processor’s requests for instructions and data into addresses that represent the locations in memory where this information can be found. •

Cache and Memory Data Transfer

When the processor requests information from the memory, the cache is first checked to see if it contains the data (since it is much faster than the memory). If it does, the data is read from there. Otherwise, it is read from the memory and given both to the processor for computation, and to the cache to store it in case the processor needs it again soon. The chipset controls the timing of these transfers. Better chipsets can do this work more efficiently than others.

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Bus Buffering and Data Flow

The chipset controls and manages the flow of information from the local bus (PCI on a Pentium or later system) to memory, as well as from PCI directly to the processor. On AGP systems, the chipset also coordinates transfers between the CPU and AGP video card. For higher efficiency, there can be several transactions pending at the same time. Since the processor, memory, and PCI bus are all operating at different speeds, buffers (temporary data storage areas) are used to hold data during transfers. Chipsets vary in the amount of these buffers that they provide; more buffers mean more ability to support concurrent operation of the processor, PCI bus, and memory. •

Memory System Timing

Since memory is in most cases much slower than the processor it serves, the processor often must wait for the memory to provide it with information it needs. A “wait state” is a clock cycle (or “tick”) where the processor is idle because it is waiting for the system memory. The chipset’s goal is to reduce this waiting as much as possible; it inserts these wait cycles where necessary to make sure the processor doesn’t get ahead of the system cache or memory. The faster your system memory and cache, the fewer of these wait states need to be performed, which increases performance (very few wait states are needed for the cache, compared to the system memory, which is kind of the point of cache. All of this is also a function of the chipset memory access circuitry. Cache on a modern system is stored in 32-byte “lines”, meaning information is read from and written to the cache 32 bytes at a time. Since the memory is normally read 8 bytes (64 bits) at a time, this means it takes 4 memory reads (or writes) to fill an “entry” in the cache. On the first of these reads or writes, the address information must be provided to the memory, to tell it which location must be used. After this, the next three reads or writes are from consecutive locations, so the speed is much higher, because there is no need to send the address for the last three accesses (since they are consecutive with the first one). This, of course, greatly improves performance. The delay in accessing the first memory location is referred to as “latency”. Reflecting this technology, cache and memory access timing is often specified using terminology like this: F-S-S-S, where “F” is the number of cycles for the first access, and “S” is the number for each subsequent consecutive access. An example of this speed specification would be “5-2-2-2” which means the first access takes 5 clock cycles, and three following it take 2 each. You may also see a speed parameter in your BIOS settings like “x-2-2-2”, or “5-x-x-x”, because the timing for the first access, and for the subsequent accesses, can be set and controlled independently on many systems. Remember that the first number doesn’t represent just “wait states”, part of the reason it takes longer is specifying the address to read from, as stated above. •

Memory Auto detection

Most modern chipsets can automatically detect the type of memory and its speed, and adjust its wait states accordingly to ensure maximum performance without data loss. If this isn’t supported, the user must enter the BIOS setup program and set the timing values there.

Chipset Peripheral and I/O Bus Control

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Most computers use two buses: the ISA (industry standard architecture) bus for slower peripherals, and for compatibility with older components, and PCI (peripheral component interconnect), a high-speed “local bus” for hard disks, video cards and other high-speed devices. The newest PCs also use the new AGP port for video. The chipset controls these buses, and transfers information to and from them and the processor and memory. The chipset’s capabilities determine what kinds of buses the system can support, what speed they can run at, and what additional features they may have. •

Bus Types

The chipset dictates what sort of buses the system can support; in fact, Intel calls its chipsets “PCIsets” and “AGPsets”. Most modern PCs support the ISA and PCI buses, but older chipsets (on 486 class machines) support the VESA Local Bus (VLB) instead of PCI. There are even some 486 PCs that support all three buses on the same motherboard. The newest PCs also add AGP for video. •

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 an analogous way, the chipset must employ bus bridges to connect together the different system 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 integrated into them support for four IDE (ATA) hard disks, 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 hence reduces cost). There are several features related to the IDE interface and its use of the PCI bus that are controlled by the chipset. 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 chipset feature. Without this, both devices must run at the slower of the two devices’ speed. •

DMA Controller and DMA Mode Support

Direct memory access (DMA) provides a way for devices to transfer information directly to and from memory, without the processor’s intervention. This was originally conceived as a more efficient means of transferring information to and from memory than programmed I/O, as it allows the processor to do other work (when using an operating system that supports it). 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

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chipset’s DMA controller, and the better (and 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 (relatively slow) DMA controller talk to the device doing the transfer, further improving performance. Bus mastering support is also provided by the chipset. •

Interrupt Controller

The interrupt controller provides the means by which I/O devices request attention from the processor to deal with data transfers. This work is performed by a pair of Intel 8259 interrupt controllers (now integrated into the chipset). •

USB Support

USB (Universal Serial Bus) is a new technology intended to replace the current dedicated ports used for keyboards and mice. Support for USB is implemented as part of the chipset. •

AGP Support

AGP (Accelerated Graphics Port) is a new “bus” specifically designed by Intel to connect processors to high-speed graphics cards, especially ones performing 3D operations. It isn’t really a bus because it only supports two devices. Plug and Play Plug and Play (PnP) is a specification that uses technology enhancements in hardware, BIOSes and operating systems, to enable supported devices to have their system resource usage (IRQs, I/O port addresses, DMA channels) set automatically. Intended to help eliminate some of the problems with getting peripheral devices to work without stepping on each other’s toes, Plug and Play requires support from the chipset as well.

Chipset Power Management Support Most recent chipsets support a group of features that work together to reduce the amount of power used by the PC during idle periods. These initiatives came as a result of efforts in two main areas: those concerned about the power consumption of PCs which are left running for as much as 90% of the time even though idle, and laptop PC owners trying to get more life from a battery charge. Power management works through a number of BIOS settings that dictate when to shut down various parts of the computer when it becomes idle. In general, the settings are progressive, so that you have the option to shut down more parts of the PC as the idle time increases. There are a number of different protocols that work together to make power management work. •

Energy Star

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Energy Star is the program started by the EPA to “certify” PCs that are considered energy efficient and incorporate power management or power use reduction features. Most PCs these days are Energy Star compliant, and display its distinctive logo on the screen when the BIOS boots up. •

Advanced Power Management (APM)

Advanced Power Management or APM is the name given to the component in some operating systems (such as Windows 98) that works with the BIOS to control the power management features of the PC. For example, APM allows you to set parameters in the operating system to control when various power management features will be activated. •

Display Power-Management Signaling

This standard, developed by VESA, specifies a set of signals that can be sent by compliant video cards to compliant monitors to instruct them to go into power-conserving modes •

System Management Mode

System Management Mode or SMM is a power-reduction standard for processors, that allows them to automatically and greatly reduce power consumption. It also incorporates features such as suspend/resume. •

Hard Disk Spindown

Both IDE/ATA and SCSI hard disk accept a command to “spin down” when instructed, their contribution to the power conservation effort. The savings here of course is quite minimal, because modern hard disks use little power.

Examples of Chipsets Major Chipset vendors are:

1. Intel 2. VIA 3. SiS 4. AMD 5. NVIDIA Examples of Few Chipsets are: 1. 2. 3. 4. 5. 6.

Intel 810 series chipsets Intel 815 series chipsets Intel 845 chipset Intel 850 chipset VIA PLE133 chipset VIA 694X chipset

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7. SiS 630ET chipset 8. SiS 650 chipset 9. Server works LE, HE etc.

Example of a chipset Layout

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