Frontside Bus, Memory Controllers

By Loyd Case  |  Posted 2004-07-19 Email Print this article Print

The current LGA775 Pentium 4 processors run their frontside bus speeds at 200MHz (800MHz effective). But DDR2/533 offers an effective memory clock of 266MHz. This results in reduced memory efficiency and throughput. In an ideal world, youd like to have the frontside bus clock be synchronous with the memory clock. Its likely that Intel will increase the frontside bus speed of the Pentium 4 line later this year.
AMD, on the other hand, is faced with a slightly different issue. The Athlon 64s memory controller is built onto the CPU die itself. The positive side of this is that memory controller latencies are radically reduced, since its running at the same speed as the processor. But it also means that AMD will need to re-spin the Athlon 64 to support DDR2. Whether theyll do that for the current CPU generation or wait until they make the move to 90nm is an open question. Given the excellent performance theyve been getting from vanilla DDR400, theyre certainly not hurting at present. Other Memories
While DDR and DDR2 gets the lions share of system sales, other memory types are vying for some OEM attention. These include QBM (quad-band memory) and Rambus new XDR memory.
QBM support was announced by VIA with some fanfare last year. Unlike DDR and DDR2, but similar to Rambus, QBM is the product of one company, Kentron. Kentron doesnt build the memory, but rather, takes off-the-shelf DDR and uses switching technology to double the throughput. Think of it conceptually as a kind of dual-channel memory on a single module. Kentrons idea is to take lower speed, lower cost memory and effectively double the performance while keeping the clock rate low. XDR is Rambus latest attempt to woo PC and memory manufacturers to the companys proprietary technology. Rambus past effort in this arena was RDRAM, which gained some sway due to Intels original acceptance of RDRAM in the Intel 820, 850, and 860 chipsets. XDR DRAM is really a forward looking technology. Rambus estimates that DDR2/667 may be end up being DDR2s maximum speed, though some other observers suggest the technology can progress as high as DDR2/800. Whatever the case, Rambus suggests that XDR DRAM could be the next step up in memory performance, sometime in late 2005 or early 2006. Of course, given Rambus litigious history, it remains to be seen who might actually work with them. XDR is based on Rambus Yellowstone signaling technology, which is clearly Rambus-owned IP. It certainly bears watching, but its at least 18 months before the technology will become a factor. Much can change in the technology world in that time.

Loyd Case came to computing by way of physical chemistry. He began modestly on a DEC PDP-11 by learning the intricacies of the TROFF text formatter while working on his master's thesis. After a brief, painful stint as an analytical chemist, he took over a laboratory network at Lockheed in the early 80's and never looked back. His first 'real' computer was an HP 1000 RTE-6/VM system.

In 1988, he figured out that building his own PC was vastly more interesting than buying off-the-shelf systems ad he ditched his aging Compaq portable. The Sony 3.5-inch floppy drive from his first homebrew rig is still running today. Since then, he's done some programming, been a systems engineer for Hewlett-Packard, worked in technical marketing in the workstation biz, and even dabbled in 3-D modeling and Web design during the Web's early years.

Loyd was also bitten by the writing bug at a very early age, and even has dim memories of reading his creative efforts to his third grade class. Later, he wrote for various user group magazines, culminating in a near-career ending incident at his employer when a humor-impaired senior manager took exception at one of his more flippant efforts. In 1994, Loyd took on the task of writing the first roundup of PC graphics cards for Computer Gaming World -- the first ever written specifically for computer gamers. A year later, Mike Weksler, then tech editor at Computer Gaming World, twisted his arm and forced him to start writing CGW's tech column. The gaming world -- and Loyd -- has never quite recovered despite repeated efforts to find a normal job. Now he's busy with the whole fatherhood thing, working hard to turn his two daughters into avid gamers. When he doesn't have his head buried inside a PC, he dabbles in downhill skiing, military history and home theater.

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