DDR and RDRAM

DDR RAM

These days DDR RAM is the standard.

DDR RAM DDR - double data rate - RAM is still synchronous DRAM (SDRAM), but runs at twice the system bus speed. This is because DDR RAM transfers data on both the rise and fall of each clock cycle. Thus, with a 133MHz system bus, the RAM effectively runs at 266MHz. This greatly relieves the memory access bottleneck which is becoming more and more problematic for today's high speed systems.

Of course, you must ensure that your RAM can match your systems bus speed. With a 133MHz system clock, you need RAM that will run at 266MHz (133MHz x 2), and such RAM is confusingly called PC2100-rated RAM. This nomenclature is based on the data transfer rate of the RAM:

DDR-RAM Bus width = 64 bits = 8 bytes
DDR-RAM Bus speed with 266MHz FSB = 266 666 000 cycles per second
DDR-RAM Bandwidth = 2100 megabytes per second (approx)

So this is where the the 2100 in PC2100 comes from.

Similary, PC2700 RAM runs at 333MHz and transfers approximately 2700 million bytes per second. PC3200 RAM runs at 400MHz and transfers 3200 million bytes per second. The image below is the RAM out of my current PC. It is GeIL PC3200: Overclocking RAM. The great thing about this stuff is that it's designed to dissipate heat rapidly and is therefore highly overclockable.

GeIL Oveclocking RAM


DDR Rating FSB Speed
(MHz)
RAM Clock Speed
(MHz)
Bandwidth
(MB/sec)
PC1600 100 200 1600
PC2100 133 266 2100
PC2700 166 333 2700
PC3200 200 400 3200

When using DDR RAM, you should ensure that the rating allows sufficient bandwidth for your system. This basically means that you should pick RAM that is matched to (or faster than) your FSB. So, if you have a 133MHz FSB, you should use PC2100 RAM or better. Since there is very little difference in price between the different DDR ratings, you might as well go for the best you can. So, if you have a 133MHz FSB, it would be wise to still purchase PC3200 RAM. This helps to future proof your system and also allows you more scope for overclocking by clocking up your FSB.

I was once asked if you can simply swap out PC133 SDRAM for PC2100 DDR RAM, since they are both types of SDRAM and since they both transfer 64 bits per cycle. The answer is no. Conventional (SDR) SDRAM DIMMs have 168 pins. DDR RAM has 184 pins.

RDRAM

A similar type of technology is Rambus DRAM (or RDRAM). RDRAM can be pumped to much higher clockspeeds than DDR RAM. However, we must also consider that DDR SDRAM is 64 bits wide, while RDRAM Rimm memory cards are only 16 bits wide. Let's calculate some bandwidths...

Bandwidth = Bus width x Bus speed

RDRAM Bus width = 16 bits = 2 bytes
RDRAM Bus speed with 400MHz FSB = 800 000 000 cycles per second ('double-pumped')
RDRAM Bandwidth = 1.5 gigabytes per second

DDR-RAM Bus width = 64 bits = 8 bytes
DDR-RAM Bus speed with 333MHz FSB = 333 300 000 cycles per second
DDR-RAM Bandwidth = 2.5 gigabytes per second

(For an explaination of bandwidths and buses, check out the Bus section.) Of course, the figures quoted above are only examples, based on a typical configuration you might find today (early 2003 at time of writing this).

Benchmarks of RDRAM indicate that sustained throughput appears to be highly dependent upon the full system configuration and may not be as high as we were initially led to believe. Although Intel strongly backed RDRAM, providing support only for RDRAM and not DDR RAM with their release of the Pentium 4, the much higher cost of RDRAM has made it unpopular. Despite significant drops in the price of RDRAM, even Intel have backed down by providing P4 chipsets that support DDR RAM instead. That said, the latest RDRAM offerings come in the form of a PC4200 Rimm; confused by the nomenclature yet? The PC4200 runs at 1066MHz, hence double pumped on a 533MHz Pentium 4 bus, yet is 32 bits wide. This results in a whopping bandwidth of 4.1Gb/sec. It should be noted that these 232-pin Rimms are not compatible with boards designed for the 16-bit cousins.

[12 months later...]

Just 12 months after I originally wrote this section it became clear that RDRAM was out and DDR was here to stay. And it gets better...

DDR2

DDR is slowly being replaced by DDR2. Both memory types use the same double-pumping principle. However, design improvements in DDR2 (such as changes to the electrical interface, and a doubling of the prefetch buffer width from 2 bits to 4 bits) allows for much higher clockspeeds.

Although it is supported by both Intel and AMD chipsets, it has been slow to catch on with current system bus speeds. This is partly because DDR2 has more latency than DDR. The result is that DDR2 RAM actually performs worse than its DDR counterpart on a FSB with the same clockspeed!

But with system buses getting faster and faster, DDR2 (and DDR3) will definitely be the way forward.

Note that DDR2 DIMMs have 240 bits. Thus, they will not run in a slot designed for DDR RAM.

What's next

Now go on to look at dual channel memory.