DDR SDRAM - Double Data Rate Synchronous Dynamic Random Access Memory. The size of its name is fairly
relative to the increase in
speed of your standard SDRAM, in
fact its
doubled, as the name suggests also. Put
simply, this RAM works by using both sides of the
stick, the
falling and
rising edge. It works a little like this:
SDRAM:
#---------DATA-----------# Rising
# # Falling
#---------DATA-----------# Rising
# # Falling
#---------DATA-----------# Rising
# # Falling
#---------DATA-----------# Rising
# # Falling
DDR SDRAM:
#---------DATA-----------# Rising
#---------DATA-----------# Falling
#---------DATA-----------# Rising
#---------DATA-----------# Falling
#---------DATA-----------# Rising
#---------DATA-----------# Falling
#---------DATA-----------# Rising
#---------DATA-----------# Falling
So, basically, DDR SDRAM uses twice the data flow along its DIMMs (Dual Inline Memory Module) to produce twice the speed... theoretically. It works a bit like this:
100 MHz SDRAM -----> 200 MHz DDR SDRAM Required: 100 MHz FSB Motherboard
133 MHz SDRAM -----> 266 MHz DDR SDRAM Required: 133 MHz FSB Motherboard
That's not to say that 266 MHz DDR SDRAM won't work on a 100 MHz FSB Motherboard, so long as it is new enough, it just means that it will only run at 200 MHz, and not 266 MHz. Now there are two new types of DDR, but they are no longer based on SDRAM, but rather DRAM, and as such are named DDR DRAM. It is, however, a common misconception to call them DDR SDRAM, so I will still put them in this write up. The new DDR DRAM still works on precisely the same principals as DDR SDRAM, and come at these speeds:
333 MHz DDR DRAM Required: 333 MHz FSB Motherboard
433 MHz DDR DRAM Required: 533 MHz FSB Motherboard (no 433 MHz available)
In this case, these speeds of DDR are not backwards compatible with older motherboards. Even though I say here that a 333 MHz/533 MHz FSB Motherboard is required, most of these motherboards at the moment will not support this RAM, this is just a theoretical requirement. 333 MHz and 433 MHz DDR DRAM are both considerably new technologies, and as such most motherboards still only support DDR SDRAM PCI 2700 (266 MHz), so if you want this faster RAM you're going to have to do some serious spending. DDR has now won the battle against the faster RDRAM with lower latencies and, overall, better performance for price.