Intel's 820 Chipset - Performance using SDRAM

There are some other interesting facts about the 82805AA MTH that must be taken into account when considering a motherboard outfitted with one. 

The MTH itself can only address up to 4 RAS lines, what this translates into is a realistic maximum of two DIMM slots per MTH present on the motherboard.  If a single MTH can address up to 4 RAS lines then why does this only result in support for 2 DIMM slots? 

You’ll first have to take note of the use of the word “realistic” in that statement.  The reason that 2 DIMM slots is a realistic maximum is because not all DIMMs take up a single RAS line, only single sided DIMMs do.  If you have a double sided DIMM, such as a 128MB DIMM, then it occupies 2 RAS lines and thus a single MTH would only be able to address 2 of those DIMMs.

ASUS has outfitted their P3C-2000 with 4 DIMM slots yet they only make use of a single MTH.  But if you read their documentation carefully, you can’t use 4 double sided DIMMs in the P3C-2000 at the same time. 

We alluded to the thought of using more than one MTH on a motherboard in order to support more DIMM slots, however this is only possible on an i840 motherboard.  The reason behind this is because on the i840 chipset you have two memory channels coming from the MCH, and thus can have one MTH per memory channel.  With the i820 chipset, you only have a single memory channel and you can’t split that single memory channel up into two separate channels for use with two MTHs.

While you can use ECC SDRAM on an i820 motherboard with a MTH, the MTH won’t take advantage of the ECC support provided by your SDRAM.  While this probably won’t be an issue for most i820 motherboard owners, if you are planning to use an i840 motherboard in a workstation or server environment, this may be an important loss.

Since your SDRAM obviously can’t operate at the incredible speeds that RDRAM does and since it would be too expensive for a motherboard manufacturer to include a separate clock generator just for the MTH, the MTH derives the SDRAM clock from the FSB frequency based on a multiplier (sound familiar?).

The MTH keeps the SDRAM running at either a 1/1 ratio with the FSB or a 3/4 ratio with the FSB.  The purpose of this is to allow for the use of the 133MHz FSB for 133MHz FSB CPUs while keeping the memory at the PC100 specification of 100MHz.  The reason for this is because Intel obviously does not support/acknowledge the VIA developed PC133 specification as a real memory specification and thus designed the MTH with PC100 SDRAM in mind.

PC133 SDRAM will work on an i820 motherboard, you just won’t be running the SDRAM at its 133MHz theoretical maximum. 

The ASUS P3C-2000 allows for the manual adjustment of the MTH clock divider, however if an FSB frequency of 133MHz or above is chosen, the divider defaults to 3/4 and won’t let you adjust it. 

CRIMMs

If your motherboard features both RIMM and DIMM slots, you naturally can't occupy both, but if you're using the DIMM slots then all of your empty RIMM slots must be filled with Continuity RIMM modules (CRIMMs). Motherboard manufacturers that feature both RIMM and DIMM slots generally ship their boards with enough CRIMMs to fill all of the RIMM slots.

Heat

The last point of concern when implementing a MTH on a motherboard is that during the translation process the MTH naturally generates quite a bit of heat. This has forced most motherboard manufacturers to place a heatsink on the MTH in their designs.