storage (ECM) RAM using some kind of error detection andcorrection (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.