Today on the testbench we have a couple of boards based on VIA's Apollo Pro266 chipset. This chipset follows in a long line of chipsets from VIA and supports all modern socket370 CPU's including Intel Celerons, PentiumIII's and VIA's C3 processors. The chipset consists of VIA's VT8633 Northbridge and VT8233 Southbridge chips and provides 2 ATA100 IDE ports, 3 USB root hubs (for 6 ports), onboard audio and 10/100 ethernet as well as the usual features you expect from a modern Socket370 chipset.
The main point of interest for this chipset is that it allows the use of DDR SDRAM. In fact, it's the first Socket370 chipset to do so, presumably because Intel have been focussing their attentions on RAMBUS for Socket370, while VIA continue their commitment to DDR. DDR, or Double Data Rate, refers to SDRAM that, while still running at a base frequency of 100 or 133MHz, actually sends 2 signals per clock, so you get (in theory) the equivalent of 200 or 266MHz - we'll see how much actual difference it makes later in this article.
Numbering
One thing that DDR doesn't do is follow the existing naming convention for RAM. Normal non-DDR ram (or SDR, for Single Data Rate) is referred to by the frequency it's meant to run at. PC100 runs at 100MHz. PC133 runs at 133MHz. Pretty simple. Picking up a stick of DDR that says "PC2100" on the side, like this NANYA-chipped stick that was included with the Asus board:
..would be enough to cause heart palpitations if you didn't realise they'd let the marketing people have free reign over product naming again. No, it's not running at 2.1GHz. PC1600 is 100MHz DDR RAM. 100MHz, but 2 signals per clock. so (kinda) equivalent to 200MHz. PC2100 is 133MHz DDR, so (kinda) 266MHz. Where do these disproportionately large numbers come from? Glad you asked. 168-pin SDRAM DIMMs, as found in PC's, utilize 64-bit memory technology. 8 bits to a byte as we all know, so it's 8 bytes. 100MHz ram is capable of sending 8x100 = 800 bytes per second, then - but as it's DDR, and sends it twice per clock tick, that's 1600 bytes per second, and that's where the name PC1600 comes from. PC2100 is based on a 133MHz clock - if you do the math you get 2128, but I guess the marketing people thought PC2100 was easier to remember - I'm glad someone learned something from the whole PCMCIA thing.
So, the new naming system is a reference to the bandwidth provided by the module, not really a reference to the frequency it's running at. Why, we wonder, did they decide to change naming schemes? Surely PC200 and PC266 sounded speedy enough? Ahh, but not in a world where RAMBUS are marketing their products as PC600 and PC800. That's my theory, anyway.
So. PC1600 is 100MHz DDR or 200MHz. PC2100 is 133MHz DDR or 266MHz as it also says on the sticker:
And then there's this stuff:
..which Jim from Realtime Systems kindly sent over so we'd have something to test DDR boards with - it's what I'll be using on the boards in this article.
They're calling this PC2400, which by my calculations means they reckon it'll do 150MHz DDR, or 300MHz, at CAS2. DDR RAM seems to all be CAS2 or CAS2.5 - for a an explanation of how CAS works with DDR, Realtime have a page here with some info. Anyway, expect a full review of this RAM later, but for now let's focus on the boards themselves..
 HiSpeedPC Fan Expander |  Gainward GF3 Golden Sample |  Rheobus Kit |