Notice that wider bit paths can be supported by installing more than one memory module. This means your individual system sockets either supports SIMMs or DIMMs, not both. However, your computer can have both types of sockets. SIMMs and DIMMs Can Support Error Checking . It has to do with error checking.
Error checking on a memory module is like page verification on a fax machine.
A change in the odd or even status of the byte tells your computer that data may have been lost or otherwise corrupted. ECC is typically found in high-end PCs and file servers where data integrity is key. Because error checking requires an extra bit of data for every byte, memory manufacturers must adjust bit width when they configure a parity-enabled memory module. The easiest way to determine whether or not a module supports parity is to look at the bit-width in the product description. When most people think of memory, they think of megabytes.