FSB & Memory Support
This brings us to the next feature of the 82815 GMCH, its Front Side Bus frequency support. Unlike the i820/820E, the i815 chipset officially supports 66, 100 and 133MHz FSB frequency settings. This makes the i815 Intel’s most flexible platform since it can run a 66MHz FSB Celeron, a 100MHz FSB Pentium III and even a 133MHz FSB Pentium III.
Because the chipset officially supports the 133MHz FSB, it also features a 1/2 AGP to FSB ratio, which allows the AGP bus to continue to run within spec at 66MHz even while the FSB is running at 133MHz. This is unfortunately one of the problems that keeps a decent number of BX motherboard owners away from the 133MHz FSB frequency. Using the 2/3 AGP to FSB ratio (the lowest supported by the BX chipset), a 133MHz FSB keeps the AGP bus on a BX motherboard running at 89MHz, which can give some graphics cards a pretty hard time.
Although the i815 chipset supports AGP 4X whereas the BX chipset only features AGP 2X support, as you can see by the below charts, the performance difference between AGP 2X and AGP 4X isn’t great enough to get too excited about.
The i815’s memory controller is a bit more interesting than the MCHs we’ve seen in the past. The controller itself supports two memory frequencies, 100MHz or 133MHz. So even if you are running a 66MHz FSB Celeron on an i815 motherboard, your memory bus will be running at 100MHz. This gives the chipset a slight advantage over the BX chipset, which is incapable of running its memory clock at a separate speed from its FSB clock.
Intel mentions that the 133MHz memory bus frequency can only be used alongside the 133MHz FSB, and while the motherboards we tested in lab only allowed us to use the 133MHz memory bus when the 133MHz FSB was selected, it is difficult to believe that a motherboard manufacturer wouldn’t be able to find a way around that limitation.
Another interesting chipset limitation is that although the memory controller supports 6 rows of SDRAM (3 banks) that is only when PC100 SDRAM is used. Intel only supports 4 rows of SDRAM (2 banks) when PC133 SDRAM is used. What this means is that you are only supposed to be able to use two double-sided DIMMs when running at 133MHz, and if you insert three double sided DIMMs, the motherboard is supposed to default back down to the 100MHz memory bus setting. None of the motherboards we’ve seen have done this with the exception of Intel’s 815 board, however we did notice a drop in stability when forcing the 133MHz memory bus with three double-sided DIMMs installed. Our test system would experience random lockups and would fail to complete some tests.