'MCH' And Its Buses
The 82820 chip is named MCH, which stands for 'Memory Controller HUB'. The MCH includes a couple of important buses.
The Memory Bus - 133 MHz, 128 Bit = 1.6 GB/s
The memory bus in combination with RDRAM is twice that of the BX chipset. However, let's not forget that the CPU can only take advantage of 33% more memory bandwidth, and this is only the case if the CPU runs at 133 MHz FSB. Also, because AGP has moved to 4X it requires increasing the memory bandwidth so it can keep up with the AGP bus. The new RDRAM or Rambus memory does have a much higher bandwidth, but it suffers from higher latencies vs. PC-100 and PC-133 SDRAMs.
The 'Interlink" between the MCH & ICH - 133 MHz, 2x, 8 bit = 266 MB/s
The MCH and ICH are able to directly communicate at 266 MB/s. This is much faster than the 440BX's (133 MB/s) communication between the North and South Bridge via the PCI-bus.
The AGP 4X bus - 133 MHz, 128 Bit = 1064 MB/s
The new AGP 4x bus provides much more bandwidth for the AGP graphics adaptor, twice as much bandwidth as the AGP 2X on the 440BX.
The 'ICH'
Right underneath the 'MCH' you can find 82801AA, the 'ICH' = 'I/O Controller Hub '. This part of the chipset talks to all the PCI-devices over the PCI bus, to the EIDE hard drives over the ATA66 interface, to external devices over the USB, to a low priced modem/audio-codec via AMR and finally to the 'FWH' = 'Firmware Hub'. The ICH is the place where all the outside communications run together, forwarded to main memory, the CPU or the AGP graphics controller over this new 266 MB/s bus. You could say that the 'ICH' replaces what used to be called 'South Bridge'.
The FWH - 82802
Behind the name 'FWH' = 'Firmware Hub ' you'll find a chip that's not much else than a 4 Mbit EEPROM plus a tiny bit of active silicon. The EEPROM contains the motherboard BIOS and the active silicon is a random number generator. This random number generator, otherwise known as the 'RNG', can supposedly be used by software to provide extra security during computer related transactions (e.g. ECommerce).
RDRAMs - Rambus Memory
Say goodbye to the old familiar SDRAM modules. The i820 chipset only supports RDRAM memory. This new memory technology provides twice the bandwidth of conventional SDRAMs. The only two drawbacks are the memory cost and slower latency. The RDRAM cost is nearly 5 times the cost of SDRAM PC-100 memory, but as SDRAM-prices rise, the difference may shrink. It still adds a whole lot of additional cost to an i820 based system though. Also, you will find that i820 based systems will always have every memory bank filled. If the memory slot doesn't have a RDRAM RIMM module installed it must have a continuity module called a CRIMM in the memory socket. The CRIMM(s) is required to avoid any reflections with the high speed Rambus-interface.
Future i82x chipsets will provide support for PC-133 SDRAM but all of the memory accesses will be forced to go through the MTH, otherwise called 'Memory Translator HUB'. I don't have all the details regarding this new HUB but I'm expecting some performance hits due to the translation process. The i820 chipset supports various RDRAM speeds. However, not all of these options are available to every FSB frequency. Here is a table that depicts supported frequencies.
| PCxxx | PC600 | PC700 | PC800 |
|---|---|---|---|
| Memory Bus Speed | 300 MHz | 356 MHz | 400 MHz |
| Processor Host Bus100 MHz | Supported | Supported | Supported |
| Processor Host Bus133 MHz | Not Supported | Supported | Supported |
RIMM Sizes 64 MB, 96 MB, 128 MB, 256 MB
Is There A New CPU That Supports 133 MHz Bus?
There is a different processor that supports the i820's 133 MHz FSB. Basically, it the same old Pentium III processor that has been qualified to run with the new higher bus speed. The main difference between the current Pentium III and the new one is the bus multiplier each CPU is clocked at. Today's Pentium III 600 runs with a FSB frequency of 100 MHz and a multiplier of 6x. A Pentium III that supports the higher 133 MHz FSB has a lower multiplier for a 600 MHz core. With a 133 MHz FSB the multiplier would be 4.5x. The new 133 MHz capable Pentium III's will be labeled 533B (for 533 MHz) and 600B (for 600 MHz). Other than the 'B' at the end of the CPU's frequency, nothing has changed about the new, or is it old, Pentium III processors. Even though most of the current shipping processors could probably handle the higher 133 MHz FSB, the problem would be the higher locked multiplier. It would be out of the question to expect a 600/100 MHz part with a locked multiplier of 6x to run with a 133 MHz FSB at 798 MHz, but it may be a nice chance to overclock e.g. a Pentium III 450 to 600 MHz.
