<storage> (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].
