Intel Pentium 4 1.4GHz & 1.5GHz

A new CPU requires a new chipset and a new bus

Once known to us only as “Tehama” we now know that the i850 chipset will be the platform the Pentium 4 will run on, and could well be the reason for its slow adoption, at least at first. 

The i850 chipset isn’t much different from the little used i840 chipset.  The chipset features AGP 4X support, it interfaces with Intel’s ICH2 chip that provides Ultra ATA/100 support and the only real difference between it and the i840 is that it doesn’t support multiple processors and it supports the Pentium 4’s AGTL+ bus. 

The difference between the Pentium 4’s AGTL+ bus and the AGTL+ bus used by the Pentium III is that the Pentium 4’s bus is a quad pumped 100MHz bus effectively running at 400MHz.  Since it is based off the same bus that we are used to from Intel, it is still a shared protocol meaning that when the multiprocessor version of the Pentium 4 hits (codename Foster) each CPU will share the 3.2GB/s of available bandwidth provided by the 400MHz FSB.  AMD on the other hand uses the EV6 bus which uses a point to point protocol that gives each CPU a dedicated 200 – 400MHz path to the North Bridge.  Intel’s shared protocol is cheaper for motherboard manufacturers to implement but of course there is a performance reduction that is paid for it.  For now this doesn’t matter since the Pentium 4 is strictly a uni-processor part, but it will matter when its SMP counterpart, Foster, is released.

Since the i850 has its roots in the i840 chipset, it also happens to share the same dual channel RDRAM memory controller as the i840.  Unfortunately this means that the Pentium 4 will have no SDRAM support at its launch.  Chances are that VIA will be the first out with a SDRAM and possibly a DDR SDRAM chipset for the Pentium 4, but at least we know that Intel is planning a chipset with support for both standards the only problem is that they aren’t scheduled to release this chipset until after Q3/2001. 

With two RDRAM channels the latency issues surrounding RDRAM are lessened and the amount of available bandwidth doubled, however it requires that you install RIMMs in pairs of two.  This means that i850 boards will have 4 RIMM slots, however since each RDRAM channel is only 16-bits wide it shouldn’t be too expensive for a motherboard manufacturer to implement.  This is still one of the benefits of RDRAM from a layout perspective. If you’ve noticed, most DDR boards are shipping with 3 or sometimes just 2 DIMM slots because of the number of traces required for a 64-bit data path to the North Bridge capable of handling such high transfer rates. 

With Intel involved in an agreement with Rambus they are in a difficult position.  They have to promote the Pentium 4, however doing so will require that they essentially promote Rambus.  There is no question about it, Intel will have to promote the Pentium 4 and in an effort to push sales of the processor and remove the price of RDRAM from the equation Intel will be bundling two 64MB PC800 RDRAM RIMMs with every boxed Pentium 4 processor.  This way, Intel can absorb most of the RDRAM price premium and take away one of the obstacles from owning a Pentium 4.  This does not apply to OEM Pentium 4 processors as they won’t come with any RDRAM, which leads us to wonder how big of a price difference will exist between retail (boxed) and OEM Pentium 4 CPUs.