Unbuffered, registered, buffered and fully buffered RAM

In this video from ITFreeTraining, I will look at unbuffered, registered, buffered and fully buffered RAM. Each different type of RAM has different advantages and disadvantages. By the end of this video you will understand the differences between these RAM types and where you might find them.

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In this video
0:17 In this video, I will first look at unbuffered RAM. This is the most commonly sold RAM on the market. Next, there is registered and buffered RAM. Both refer to the same type of RAM and the names can be used interchangeably. The last RAM type I will look at is fully buffered. This RAM was only used for a short period of time, but you never know it may return one day.

Unbuffered
0:40 First I will look at unbuffered RAM. Unbuffered is the cheapest and also the most common form of RAM on the market. Consider that you have an external memory controller or a memory controller inside the CPU. In this example I will use DDR2 memory, but the same process applies to other memory modules.

In DDR2, there are 240 pins on each memory module and 64bits are transferred at once. Different memory modules may transfer different amounts, but 64bit is pretty common now days. DDR also uses the same pins to read as it does to write. So, considering that you have 240 pins, the math’s does not seem to work out.

What is missing is that there are multiple clock lines as well as control lines. These all work in unison so the 16 chips on the memory module work together to transmit data. Having so many lines all working together at the same time is an amazing feat of engineering, but also means more chance of something going wrong. So, using a system like this, there is an increased chance of data corruption.

However, the chance of corruption is quite low so it comes down to a trade-off between cost and reliability. For this reason, unbuffered memory is often used in low-end computing. With low-end computing, a rare memory corruption is an acceptable price to pay for the cheaper cost. Although there is a difference of opinion on how often these errors will occur, it is estimated that in a correctly set up computer system, a memory error will occur maybe once or twice a year. The chance of it occurring in something critical is quite small so you probably will not even notice it when it happens.

Registered/Buffered
2:22 The next type of memory that I will look at is registered and buffered memory. This memory type is used in high-end workstations and servers. The basic principal behind the memory module works much the same as for unbuffered. The terms registered and buffered memory are used interchangeably.

The difference is that the memory controller connects to what is referred to as registers or a buffer. You can generally tell if a memory module has this feature as it has an extra chip in the middle of the memory module. This chip will generally be a different size to the other chips on the memory module. Later in the course I will look at error correcting memory modules. If a memory module has this feature, there will also be additional chips on the memory module. However, in a basic registered or buffered memory module, there will be generally be only the one extra chip in the middle of it.

The memory controller is connected directly to this register or buffer. This means this extra chip is responsible for the transfer of data between the memory controller and the memory chips on the memory module.

What this essentially does is reduce the load on the DRAM chips on the memory module. Since the memory controller only has to deal with the one chip rather than multiple chips, this makes it more stable when there are a lot of memory modules in the computer. The memory module, after accessing the DRAMs on the chip, may delay transferring data to the memory controller for one cycle. This gives the DRAM chips time to become more electrically stable. In servers with a lot of memory, this becomes important.

Description too long for youtube, please see the following link for the rest of the description http://itfreetraining.com/ap/3a15

References
“CompTIA A+ Certification Exam Guide Ninth Edition” page 164
“DDR2 SDRAM” https://en.wikipedia.org/wiki/DDR2_SDRAM
“Registered memory” https://en.wikipedia.org/wiki/Registe...
“buffered & unbuffered RAM”    • buffered & unbuffered RAM  
“Fully Buffered DIMM” https://en.wikipedia.org/wiki/Fully_B...
“DRAM Errors in the Wild: A Large-Scale Field Study” http://static.googleusercontent.com/m...

Credits
Trainer: Austin Mason http://ITFreeTraining.com
Voice Talent: HP Lewis http://hplewis.com
Quality Assurance: Brett Batson http://www.pbb-proofreading.uk