iXBT Labs - Computer Hardware in Detail






VIA chipsets:
myths and reality

March 17, 2001

An article on the practical features of assembling and adjusting x86-systems on the VIA Technologies chipsets is necessary for a long time already, this issue has become of utmost importance the last year when their number has increased much. And today we are introducing this article in the form of an overall practical "guide to action" which at the same time contains the required technical information.

In order the principles on which VIA chipsets are based to be comprehensive we divided the technical part into two sections devoted to the chips known as north and south bridges. First it may seem to be not very convenient but in the end you will understand that for the VIA Technologies chipsets such approach is the most suitable.

South bridges

Currently, they use three south bridges on the boards based on the VIA chipsets: VT82C596B (Mobile South Bridge), VT82C686A (Super South) and VT82C686B.

The 596B is the oldest south bridge. One USB controller (2 ports, correspondingly) has neither hardware monitoring nor AC'97/MC'97-interface. An integrated IDE-controller corresponds to the Ultra ATA/66 specification. The name Mobile South was given due to the fact that VIA positioned it as a solution for portable computers, that's why they developed the chip with low power consumption in mind.

686A/B. The VIA VT82C686A was, not so much time ago, the most multifuctional south bridge: a dual-channel USB-controller (4 USB ports), Ultra ATA/66 IDE, AC'97/MC'97-interface, system monitoring. Its modification VT82C686B differs only in the built-in IDE-controller (Ultra ATA/100).

If the 596B had the system monitoring it would have suit the vast majority of ordinary users. AC'97 Audio and a AMR slot is unnecessary for the most users. Besides, one hardly uses more than two USB-devices on one computer, plus, the Ultra ATA/100 support is not a meaning advantage. That's why the old bridge can satisfy the needs of 90% users.

For the rest 10% the speciality lies in the fact that all chipsets from VIA Technologies are defined only by the north bridge, since a south bridge fits any. Nobody disturbs to make a board on the latest VIA KT133A but with the aging 596B - and everything will work perfectly! Within the 596B frames it will perform without failures and errors.

North bridges

North bridges of even very different VIA chipsets are very similar to each other. As a rule, the most advanced version differs from the earlier one in 2-3 functions supported, and, therefore, in the same slight improvement in the chip design. As an example I will take a curious "phenomenon" connected with the meaning of Vendor ID/Device ID codes of the whole series of the VIA north bridges.

First, I want you get a clear idea about these codes. Vendor ID is a manufacturer code, and all chips produced by the definite manufacturer must send this code to the interested program on the special request of the latter (in the beginning it's BIOS). Afterwards, a manufacturer assigns the Device ID for each their chip which allows to define which of the chips from this vendor answered. We can watch the process of identification of chips based on Vendor ID/Device ID in the course of installation of new devices in the Windows OS. First, there appears a window which informs that a new device is found, then Windows either recognizes it itself or asks for a diskette or a CD with drivers and INF files on failing to find any data in its internal base. The OS defines the type of the device and its name according to the Vendor ID/Device ID which are written in "textual" form in INF-files.

The surprise lies in the fact that all north bridges of the VIA Apollo Pro, Pro Plus, Pro133 and Pro133A chipsets have these both codes equal. It means that all they are formally one chip. But there is one more code - Revision, and exactly it allows to tell, for example, VIA Apollo Pro from VIA Apollo Pro133A. But the fact that these chips differ only its version says that it's one chip which just undertook some improvings with time.

Now we can built some virtual raw of the VIA north bridges having concentrated not on the "external" chipsets' and bridges' names but on the "internal" qualities, i.e. functions. There we have to play with assumptions since there is no official information on their validity.

Apollo Pro133

The first VIA chipset which supports FSB 133 MHz for the Intel platform (Slot 1/Socket 370). Like all other VIA chipsets it is asynchronous. It supports Intel Pentium II/III and Celeron CPUs, AGP (2X) slot, up to 6 PCI Master devices (including those which are integrated in the south bridge).

Apollo Pro133A, Apollo KX133/KT133/KT133A

The Apollo Pro133A differs from the Apollo Pro133 in the AGP 4X support. Probably, this core served a base for the Apollo KX133 (for the AMD Athlon, Slot A), and after that for the Apollo KT133 (AMD Athlon/Duron, Socket A). The KT133 doesn't support the FSB 133 MHz, it's connected with the fact that when the chipset was developing there were no one AMD processor with such bus. After the release of new versions of Athlon they produced Apollo KT133A which was completely identical to the Apollo Pro133A in characteristics (except the processor bus: AGTL+ for Pro133A and EV-6 133 (266) MHz for KT133A).

Apollo PLE133 (PM601) and ProSavage PM133/KM133

In fact, it a north bridge from Apollo Pro133A (PLE133, PM133) or KT133 (KM133) with an integrated graphics core. In case of the PM133/KM133 they use a graphics core from S3 (a combination of the 3D-component of Savage4 and 2D from Savage2000), in case of the PLE133 the graphics core is taken from Trident Blade3D. Besides, the PLE133 doesn't support an interface for the external AGP-video card what simplified (and made cheaper) the chipset and boards on its base.


What chipset drivers are and what they are for

The drivers are necessary for the OS for correct operation with any chipset. The common mistake is that Intel chipsets do not need drivers. The drivers for them are simply included into the set of driver database of the Windows 98, and the OS itself defines the corresponding functional elements of Intel chips and sets support drivers for them in the course of installation. By the way, the Windows Me (Millennium) contains the information on the most VIA chipsets as well as the corresponding drivers - as a result of an official agreement between VIA Technologies and Microsoft.

However, the users of the Windows 98/98 SE/2000 still have to install VIA 4-in-1 Drivers Pack or each drivers that is included in this pack. That's why I want to pay special attention to these drivers, what they are for and why their installation is necessary for the normal operation of the system.

VIA INF Driver

It's a set of INF-files with the description of devices which are included in the VIA south bridges (ACPI-controller, in particular). These INF-files are copied into the device database of the Windows in the course of installation of the driver (usually to C:\WINDOWS\INF), and after rebooting the OS can define (according to the Vendor ID/Device ID) the presence of the corresponding devices as well as to get the "instruction on handling" from the INF-files (I/O port addresses etc.).

IRQ Routing Miniport Driver

The IRQ routing, i.e. distribution of requests interrupt between devices is a very important function which is typical for any OS that supports Plug-and-Play specification. If the routing is incorrect many devices won't be able to implement their functions correctly and can even rouse errors. The standard router installed with the Windows 98/98 SE is intended for the IRQ distribution mechanism supported by Intel chipsets, for VIA chipsets it differs. The VIA IRQ Routing Driver is used for the system to distribute accurately interrupt vectors to all devices installed in the computer.

Note: before installation of the IRQ Routing Driver VIA you should enable the USB controller in BIOS (the both channels) and IRQ for the USB controller. After installation and rebooting put everything in the initial state.

IDE BusMaster Driver

This driver is in fact divided into two independent parts, the both parts are interchangeable. The VIA 4-in-1 Drivers features only one variant - the VSD (Vendor Support Driver), in case of a separate packet - VIA IDE BusMaster Driver use can choose between the aforementioned VSD and a full value Miniport Driver (VXD). The VSD is a driver which works together with a universal Windows driver for all IDE BusMaster controllers, while the Miniport Driver completely replaces a standard Windows driver.

VIA looks at it this way: VSD-drivers provides the best compatibility with all IDE/EIDE/ATAPI devices while the Miniport Driver is a bit faster but can appear to be incompatible with some definite storage devices. The 4-in-1 Drivers Pack includes only VSD variant as a solution for everybody, and if you are an advanced user I recommend you to set the Miniport Driver from a separate distributive disc. On the whole the both drivers serve for correct usage of "speed" modes of data transfer which are supported by the IDE controller integrated in the south bridge (Ultra ATA/66/100).

VIA GART (Graphics Address Relocation Table) Driver (VIA AGP Driver)

Note that the majority of AGP video cards can operate without this driver. The GART Driver allows the AGP video card to use a part of the system RAM as an extension of its own memory organizing there a buffer for texture storage (DiME mode - Dynamic in Memory Execution).

Those video chips that can work in the DiME mode do it via the GART-driver. But the fact that the card is inserted in the AGP slot doesn't imply that it uses DiME. For example, all 3dfx based video cards don't support DiME (it's connected with architecture of the chips), that's why the GART-driver gives them nothing. If a video card tries to enable the GART and the latter is not installed, the system hangs with an attempt to start an application using 3D-accelerator.


Some problems can't be solved with the standard means. You should know these problems and you should be able to use non-standard means for their solving.

VIA chipset based boards and NVIDIA chip based video cards

In combinations of some motherboards on the VIA Apollo Pro133/133A and video cards on the NVidia of TNT2 and GeForce series the incompatibility is on the hardware level when the programs use API Direct3D. It becomes difficult to define what sort of hardware is guilty (motherboard, chipset, video card, video chip). That's why there is only one thing I can recommend you - you should either avoid the specified combination or test properly the machine on some Direct3D-application when assembling the computer.

IDE-controller of the VIA south bridge and Ultra ATA/33 CD-ROM (CD-RW, DVD)

Some ATAPI CD-ROM (CD-RW, DVD) drives which support Ultra ATA/33 mode work incorrectly when connected to the VIA BusMaster IDE-controller included in the VIA south bridge via a standard 40-thread cable. The problem lies in breaking of the integrity of data while reading discs or in a wrong recognition of the format when first applying to the disc. There are two ways to solve it. The simplest one is to prohibit usage of Ultra ATA for such storage device in the BIOS or in the diagnostic utility from the VIA IDE BusMaster Drivers' set. The second way can be used in those cases if you don't want to restrict drive's performance, then you have to connect it with a 80-thread Ultra ATA/66-cable (despite the fact that the device will work in Ultra ATA/33 mode!).

Disabling of the secondary IDE channel with BIOS

In case the secondary channel of the IDE-controller was disabled by BIOS the system manager in Windows 98 can lose the both IDE channels (showing them as disabled), as well as define the USB-controller as a disabled device with "Code 12" status. The Windows 98 SE has a bit different reaction on disabling the secondary channel with BIOS: the IDE controller is recognized as an incorrectly operating device and rouses conflict of resources when making a request for interrupt. The users of the Windows 98 SE can solve this problem having loaded the patch from Microsoft which can be found at www.entry.kiev.ua/Support/245682US8.exe. The Windows 98' users should either renew the OS up to Windows 98 SE (and then use the aforementioned method) or use the Windows system manager functions (not BIOS) in order to disable the secondary IDE channel.

Sound subsystem of the VIA 686A/B south bridge. Game port

Some boards with the VIA VT82C686A(B) south bridges may feature the following problem: after the integrated sound card (AC'97 Audio) was deleted from the system manager the joystick for a game port left in the list of existing devices. The reason is that the driver of the game port (vjoyd.vxd) can not in fact define whether the device is included in the system, that's why once installed it starts up every time with the system' booting. In order to solve the problem you should delete the device manually.

Speciality of the system monitoring of 686A/B south bridges

There are two temperature sensors on the boards (the first is for the CPU, the second is for system block). The VIA Hardware Monitoring System program doesn't know from which sensor the signal comes. So, for elimination the error you should update this program up to the version 2.02, and edit the HMHARD.DAT file. The file must contain the following lines:



For inverse operation of the sensors:



Wake on USB-keyboard

Some systems based on the motherboards using VIA south bridges can refuse to wake up from the USB-keyboard even if the USB Keyboard wake-up function is included into the specification. It's connected with the fact that not all USB-keyboards support ACPI. Note that wake-up service for USB devices appeared in the chipsets starting only from the VIA VT82C686A CF.

Driver support for VIA chipsets under Windows 2000

In case VIA chipset based systems run under the Windows 2000 there can arise some failures connected with incorrect operation of AGP when the system comes out from the Suspend mode. It caused by a wrong order of installation of chipset support drivers, in particular in case of installation of the GART-driver for VIA chipsets earlier than VIA INF Driver. This can be provoked also by installation of earlier versions of VIA 4-in-1 Drivers Pack, earlier than 4.03. In this case you should undertake the complete reinstallation of the system with the correct setting of the drivers.

Concluding remarks

The main aim of this article is to explain how to solve problems which arouse with handling VIA chipsets. VIA Technologies for the shortest time became a leading manufacturer of chipset for motherboards. That's why today many will have to deal with their products.

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