VIA Apollo Pro 133A & VCSDRAM - Part 1: The Technology

Although most BX motherboards already support FSB frequencies of 133MHz and above, the problem with overclocking the FSB past 100MHz on current BX and Apollo Pro Plus motherboards is the frequency it places the rest of your system at.  The problem with the North/South bridge architecture of most of our chipsets is that most of the operating frequencies are dependent upon one another.  For example, if you overclock your FSB to 133MHz (on a 100MHz BX board) your PCI bus that was once running at 100MHz/3 is now running at 133MHz/3.  Granted that most newer boards now support the 1/4 PCI clock divider which would put your PCI bus at 133MHz/4, but what about your AGP card? 

The current AGP 2X/4X specification calls for a 66MHz clock frequency of all AGP cards (AGP 2X runs at 66MHz but transfers at twice that speed, AGP 4X transfers at 4X that speed). Unfortunately the AGP frequency is determined by multiplying your FSB by a certain fraction.  In the case of the BX and Apollo Pro Plus chipsets, the only fractions available are 1/1 and 2/3.  With the 66MHz FSB, the 1/1 ratio is used because this keeps the AGP frequency at 66MHz or at a 1-to-1 ratio with the FSB.  With the 100MHz FSB, the 2/3 ratio is used as this keeps the AGP frequency at 66MHz, or at 2/3 of the FSB.  But what happens if you overclock to 133MHz?  If you multiply 133MHz by 2/3, you get approximately 89MHz, or a clock frequency 33% over the AGP 2X/4X specification. 

The truth of the matter is that there isn't a single 3D AGP accelerator on the market today that can work reliably at 89MHz in mass quantities.  There have been some reports of AGP cards working at 89MHz, but the general consensus is that they don't. Even the most reliable cards have some difficulty working at the 83MHz frequency they are forced to work at if your FSB is running at 124MHz.  This puts overclocking your current BX platform to 133MHz an almost impossible task to accomplish if you still plan on using your system for more than showing off your overclocked creation.

The Apollo Pro 133A fixes the "problems" that kept most of us from overclocking to the 133MHz FSB on our BX and Apollo Pro Plus motherboards.  By adding support for the 1/2 AGP clock divider, the chipset now allows for much greater FSB frequencies such as 133MHz and even higher overclocked frequencies as your AGP clock will be running at 1/2 the FSB.  In the case of the 133MHz FSB, this leaves your AGP clock running at a comfortable 66MHz. 

At a 133MHz FSB, all your PCI devices operate at 1/4 the FSB, or 33MHz, which is exactly what the PCI 2.2 specification calls for.  This includes your on-board EIDE controller as it is tied into the PCI bus, so you don't have to worry about your hard drives crapping out when you use the officially supported 133MHz FSB frequency. 

The 1/2 AGP clock divider opens up a word of new possibilities for overclocking.  The 133MHz+ FSB settings are now much more viable options, even at 150MHz, the AGP bus is still running at 75MHz which is around 14% over the specification, and is manageable for most AGP cards (it's almost the equivalent of running the 112MHz FSB on a BX board).  At 150MHz, your PCI bus will be running at 37.5MHz, or 1/4 of 150MHz, which is also very manageable for most PCI cards;it is the equivalent of running your FSB at 75MHz on a motherboard that only supports the 1/2 PCI clock divider.