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(ECM) RAM using some kind of error detection andcorrection (EDAC) scheme. The two types of memory errors inRAM (especially DRAM) are "soft" errors due toradiation-induced bit switching, and "hard" errors due to theunexpected deterioration of a memory chip. Soft errors do notindicate lasting damage to the memory board, but they docorrupt programs or data. Hard errors demand physicalrepairs. Single bit memory failures are the most common. Ahard single bit failure, such as that caused by a completelydead chip can be corrected by EDAC if each chip supplies onlyone bit of each word. EDAC memory is the most common level ofprotection for minicomputers and mainframes whereas thecheaper parity protection is more common in microcomputers.[Clearpoint, "The Designer's Guide to Add-In Memory", ThirdAddition].