What is the X in X-mas?
(ECM) RAM using some kind of error detection and correction (EDAC) scheme. The two types of memory errors in RAM (especially DRAM) are "soft" errors due to radiation-induced bit switching, and "hard" errors due to the unexpected deterioration of a memory chip. Soft errors do not indicate lasting damage to the memory board, but they do corrupt programs or data. Hard errors demand physical repairs. Single bit memory failures are the most common. A hard single bit failure, such as that caused by a completely dead chip can be corrected by EDAC if each chip supplies only one bit of each word. EDAC memory is the most common level of protection for minicomputers and mainframes whereas the cheaper parity protection is more common in microcomputers.
[Clearpoint, "The Designer's Guide to Add-In Memory", Third Addition].