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

: Power and Latencies"> The latest memory for mainstream personal computers is DDR2. DDR2 solves some of the problems inherent with original DDR (now known as DDR1). For example, DDR2 has on-die termination, which improves signal integrity at higher clock rates. DDR2 also solves the power and heat problem in a clever way. The actual memory core clock is 1/2 the clock rate "seen" by the system. DDR1, by contrast, clocks the core at the same speed as the external I/O clock. For example, DDR2/533 clocks at 266MHz -- but the internal core clock is 133MHz. The I/O buffer clock is 266MHz, and thats the clock rate that the system understands. To get around this seeming contradiction, DDR2 batches up four bits per clock cycle.
Since the I/O buffers run twice as fast, it really only hands off two bits per I/O clock cycle. So internally, the core is presenting data to the I/O buffers at quad data rate, but externally, the system sees two data items per clock cycle. In other words, DDR2 prefetches four data items, while DDR1 only prefetches two data items, per I/O clock cycle.

Latencies are also different. DDR1 CAS latencies could be as low as 2 clock cycles, though typical modules in OEM systems are 2.5 or 3 clocks. The DDR write latency is one clock, but as the external frequency goes up, thats too little time. So DDR2 adopts a simple algorithmic approach, where write latency is always CAS latency - 1. So if CAS is set to 4 -- typical for current DDR2 modules, write latencies are 3 cycles. From the perspective of the system, the actual delay hasnt changed much. CAS2 for DDR400 is roughly 15ms, while CAS4 for DDR2/533 is about the same. Overall bandwidth goes up, because the relatively slow latency is for that first read of the memory row. After that, memory streams out to the system per the higher clock rate. Of course, if the system is running DDR2/400, you may see little or no gain in performance. Power issues are addressed by lowering the voltage from 2.5V to 1.8V. As memory capacities increase, the power required by the added memory also goes up. Dr. Michael Schuette of Lost Circuits estimates that 4GB of DDR1 memory consumes 35-40W of power. The reduced voltage means power requirements also goes down, to about 25-30W for a 4GB system. The lower voltage also helps enable higher clock frequencies.

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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