Intel has obviously recognized that the price of RDRAM was going to be high, thus they designed and released an add-on chip, the 82805 Memory Translator Hub (MTH), which, when implemented on the motherboard, allows the use of PC100 SDRAM. No, that isn't a typo! The i820 outfitted with the MTH will allow the use of PC100 SDRAM running the memory bus at 100 MHz while the FSB runs at the higher 133 MHz speed.

The MTH chip sits between the i820's Memory Controller Hub (MCH) and the RDRAM memory slots. (Please refer to our i820 Chipset Review for a view of the i820's chipset diagram.) The MTH is responsible for translating the data from the MCH to the SDRAM main memory. In order for the MTH outfitted i820 board to communicate with the SDRAM memory it has to go through the following scenario. The CPU has a 64bit wide parallel signal running at its FSB of 133 or at least 100 MHz. The information is then converted to the RDRAM's 16bit wide serial signal in the i820's MCH and then it gets passed through the MTH that changes the signal back to a 64bit wide parallel signal. Obviously this isn't an actually sufficient way to communicate with the SDRAM. Getting data from the CPU to the SDRAM and back would be like you communicating with other people through a good old telegraph instead of using a telephone. Since the i820's MCH was designed with just RDRAM in mind, this is the only way for i820 to communicate with SDRAM memory.

Because the MTH shares the same pathway as the RDRAM memory slots, it is possible for motherboard manufacturers to design i820 platforms that support both RDRAM and SDRAM. Supporting both memory types would require more real estate on the motherboard given the extra space required for the RDRAM RIMM slots, MTH chip and SDRAM DIMM slots. On motherboards that support both memory types the user will have to make a choice between RDRAM and SDRAM because mixing the memory types isn't an option.

Intel's CC820 Motherboard

The CC820 motherboard from Intel is their first platform equipped with the MTH. The MTH chip is located to the left of the IDE connectors and below the memory slots. By first glance this board can be easily mistaken for Intel's VC820. Basically the only difference between the two boards is the memory support, SDRAM vs. RDRAM.

Intel CC820	Intel VC820

CC820 Feature Table