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A long time ago - well September, 2004 to be pedantic - in a galaxy far away Samsung introduced some DDR2 memory. Now it has announced DDR3 memory just after making a big splash with XDR RAM. Confused? Don't worry. Here comes Techworld's RAM primer.

DDR
DDR memory is Double Data Rate memory. It is a form of SDRAM - Synchronous Dynamic Random Access Memory - and has a data rate twice that of ordinary SDRAM because its frontside bus is twice as fast. SDRAM has been in wide use since the later 90s. SDRAM chips are synchronised to a computer's CPU clock and the theory is that memory wait states don't thereby occur. There are two cell blocks in an SDRAM chip. When a bit is accessed from one block a bit in the second block is prepared for access and can be sent to the CPU faster than the first bit. It is a form of caching in a way.

The clock signal pulse has a rising phase and a falling phase. DDR transfers data on both the rising phase and the falling phase. This doubles the transfer rate over standard SDRAM. DDR chips are packaged in a 184-pin DIMM module compared to SDRAM's 168-pin DIMM. It's said Samsung produced the first DDR chip in 1998.

Now let's complicate matters. DDR chips run at one of four speeds. The highest is around 533 megabits per second and the chips can't run much faster because the signal quality in the chip fades.

DDR2
DDR2 doubles the speed of DDR (or DDR1 as it is sometimes referred to) because it has a 4-bit pre-fetch ability whereas DDR only has a 2-bit pre-fetch ability. This does not mean there is a faster clockspeed though. DDR2 chips also ratchet up signal quality by having terminating transistors, which eliminate signal noise, placed on the chip itself. The DDR2 DIMM has 240 pins. Apparently Samsung produced the first DDR1 memory in 2001.

Let's complicate matters some more.

The first DDR2 chips ran a lower internal clock speed than equivalent data rate DDR chips but the 4-bit pre-fetch meant the effective memory speed was the same. So DDR2-400 was as fast as DDR400. Other DDR2 speeds were 533, 667, 800 and 1200MHz. The data rate is four times the clock rate, since DDR2 has a 4-bit pre-fetch. So a DDR-2 400 module has a data rate of 4 x 400 = 1600Mb/sec or 1.6MB/sec or 0.8GB/sec.

OCZ has released a new DDR2 memory module, called the PC2-5400 Enhanced Bandwidth DDR2 Platinum Edition, with a clock rate of 667MHz.

DDR2 memory module capacities range from 256Mbit, through 512, 1Gbit and up to 4Gbit. They run at 1.8 volts instead of DDR's 2.5 volts.

Gartner thought, back in 2004, that DDR2 RAM would account for more than half of the memory market in 2006.

DDR3
Now Samsung has announced a 512Mbit DDR3 chip running at 1.5 volts, meaning less heat generation as well as lower power requirements. Samsung says its operational speed is twice that of DDR-2, meaning 1.6GB/sec.

It claims IDC market research shows that DDR3 will account for 65 percent of the memory market by 2009. Production should start early next year.

The picture is of roughly three year DRAM product cycles with SDRAM being mainstream in 1998-2000, DDR1 being mainstream in 2001-2003/4, DDR2 taking centre-stage in 2005-2006, and DDR3 then progressively taking over in the 2007-2009 perod.

XDR
We described XDR technology here. Its RAMBUS-developed technology moves data at 8 bits per clock cycle with a data rate of 8GB/sec. That is ten times faster than DD2-400's 0.8GB/sec.

A 12.8GB/sec XDR 512Mbit module is said to be coming. Rambus says that XDR can support from 3.2 up to 8GHz clock rates.

One reason XDR memory modules are faster than DDR modules is that the component modules in the chip use point-to-point connections to the memory controller rather than sharing a multi-drop wire as DDR modules do.

IDC reckons global XDR DRAM shipments will exceed 800 million 256Mbit-equivalent units by 2009. So perhaps XDR could be the generation after DDR3 DRAM. But XDR is a proprietary Rambus technology whereas the DDR module types are open standards.

The new IBM/Sony/Toshiba Cell processor uses Rambus XDR memory.

Test
Here are three questions designed to test your RAM knowledge:-

- How much faster than DDR1 is DDR2?
- How fast is DDR3?
- How fast is XDR?

If you answered 2x DDR1, 4X DDR1, and 20X DDR1 then consider yourself a RAM champion. If you didn't then go to the start of this article and read it again.