Not All Memory is Created Equal Parity, ECC, Registered and Conclusions

Parity and ECC

Both parity and ECC (Error Correcting Code) are forms of error detection for memory modules. Parity is a simple form of error detection that adds an extra bit for every 8 bits on a memory module. This extra bit records whether there is an even or odd number of 1's registered in the 8 bits. If they don't match, then an error has been detected within the memory. ECC is a more advanced form of error detection that goes beyond the single parity bit and can actually handle error correction.

Most consumer level computers do not specifically use parity or ECC modules. They are typically reserved for more expensive servers dealing with more processing and memore intensive applications. The important things to know is that if the motherboard supports parity or ECC modules and some are in use, additional memory must be of the same type otherwise the system will not properly function. ECC and parity memory will generally not work properly in systems that do not support these functions.
Registered Memory

Registered memory is a specific form of memory designed to support a larger number of chips on a given memory module. To allow for all the memory chips on the module to properly function, it is necessary for the module to add a delay to all command requests to the memory. The delay is actually generated by an additional clock generator that resides on the module instead of relying on the motherboard. This will decrease the overall performance of a given memory module. The memory type is typically used in large server environments where large amounts of RAM are required. In these environments, the benefits of the increase in RAM outweigh the performance impacts caused by memory swapping. Memory swapping, also referred to as virtual memory, is the function of a computer using the hard disk to store system memory when there is insufficient physical memory.

Most consumer desktop computer systems will not use this form of memory at all. Memory that does not use the registered clocking is typically referred to as unbuffered memory.
So What Does it All Mean?

Hopefully this guide has helped sort out some of the mysteries behind computer memory. With the knowledge of all the terms and acronyms and how the pieces work together, it should be possible to determine how the different types of memory can influence the performance and functionality of PC computer systems. Memory isn't just something that it placed into a system to make it run. It must be properly matched with the other components in the computer to achieve the proper performance.

Memory technologies change just like at computer components do. Synchronous DRAM and Rambus have become outdated and DDR is heading in that direction. It wouldn't be prudent to select a new computer that uses any of these types of memory. The performance of the system is less than that of one with DDR2 or DDR3 and the future upgradability is in question. As memory technologies are phased out, the supply and availability of chips decreases. This can result in older memory technologies ending up costing more for less memory than a more current technology. Then again, DDR3 is also quite new and will be priced more than DDR2 memory for the same capacity due to lower volumes.