Double Data Rate SDRAM can theoretically improve RAM speed to at least 200 MHz. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output.

Unfortunately, DDR SDRAM (Also known simply as DDR RAM) has not been designed to be idiot-proof, and if there's one thing people should have learnt by now, there are a lot of idiots out there.

Last night, I was very tired and...(enough of the excuses) OK, I put my RAM in the wrong way round, resulting in a terrifying smell of burning coming from the inside of my computer.
You see, this horribly embarrassing blunder never used to be possible with normal 168-pin single data rate DIMMs. They were designed with two "keys" meaning the RAM could only be inserted one way into the socket. Something like this:

 -----------------------------------------------------------------
| ---- ---- ---- ---- ---- ---- ---- ---- |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| |____| |____| |____| |____| : : |____| |____| |____| |____| |
|_________________________________________________________________|
############ ################### ################################

DDR RAM, on the other hand, has only one key and it's only slightly wonky, like so:

 -----------------------------------------------------------------
| ---- ---- ---- ---- ---- ---- ---- ---- |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| | | | | | | | | : : | | | | | | | | |
| |____| |____| |____| |____| : : |____| |____| |____| |____| |
|_________________________________________________________________|
#################################### ############################

Frankly, this is shit -- if you'll excuse my bluntness -- because it doesn't serve its intended purpose. The key is very close to the middle of the stick, and it is quite easy (well, if you force it hard enough it is,) to insert the wrong way round. I understand the need to have a different pattern of keys to the old SDR, so it is impossible for morons to insert the wrong sort of RAM, but this way leaves the door wide open for morons to... well... it's all too painful to recall. ;(

Update: The next day:
I inserted the burnt RAM (still smelling of burnt... whatever it is) into the burnt slot. What's more, the burnt RAM has one of the gold contacts missing. It is sitting right in front of me now on the desk. Guess what? My computer works. I'm in Linux right now, with broken RAM in a broken RAM slot. Evidently two brokens cancel to a working.
I shit you not.
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.

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