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UMD ENEE 759H - Types of Memory Errors

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CGS 3269 – Memory - 52 Types of Memory Errors Unfortunately, no memory system is perfect. Although misrepresentation of a single bit from millions of bits might not seem like much of a concern, it can lead to system crashes or alter the answer to an important calculation. It is important at this point to make the distinction between a memory error and a transmission error. When sending data over communication lines (note that this could be a local bus and thus the transmission is entirely inside your system) an error in the transmission of the data may occur which can be detected and/or corrected or simply ignored. The choice of which approach to use is in many ways application dependent. For example, if you are listening to music on your system’s CD player with a 48x sampling rate – the odd bit error will simply be ignored. On the other hand, if your application is to make sure that the space shuttle comes out of orbit so that it hits the center of the runway at the Cape, you would like to be sure that your data is accurate or you may be fishing it out of the adjacent swamp while you contemplate your job options! Memory errors fall into two broad categories, soft errors and hard errors, we’ll examine each type separately. Soft Errors Soft errors are unexpected or unwanted changes in the value of a bit (or bits) somewhere in the memory. One bit may suddenly, randomly change state, or noise (electronic interference) may get stored as if it were valid data. In either case, one or more bits become something other than what they are supposed to be, possibly changing an instruction in a program or a data value used by a program. Soft errors result in changes in your data rather than changes in the hardware. Through replacement or restoring the erroneous data value (or program code) the system will once again operate exactly as it should. Typically, a system reset (reboot - a cold boot) will effect this restore. Soft errors are why you apply the old rule of thumb - "save often". Most soft errors result from problems within the memory chips themselves or in the overallCGS 3269 – Memory - 53 circuitry of the system. The mechanism behind these two different types of soft errors is completely different. Chip-Level Errors: The errors which occur inside the memory chips themselves are almost always a result of radioactive decay. The culprit is the epoxy of the plastic chip package, which like most materials contains a few radioactive atoms. One of these minutely radioactive atoms will spontaneously decay and produce an alpha particle. Practically every material will contain a few radioactive atoms, not enough to make the material radioactive (the material is well below background levels), but they are there. By definition, a radioactive particle will spontaneously decay at some time. An alpha particle consists of a helium nucleus, two protons and two neutrons, having a small positive charge and a lot of kinetic energy. If such a charged particle "hits" a memory cell in the chip, the charge and the energy of the particle will typically cause the cell to change state in a microscopic nuclear explosion. The energy level of this nuclear explosion is too small to damage the silicon structure of the chip itself. Whether a given memory cell will suffer this type of soft error is unpredictable, just as predicting if or when a given radioactive atom will decay is impossible. However, when you deal with enough atoms this unpredictability becomes a probability, and chip designers can predict that one of the memory cells in a chip will suffer such an error. They just can't predict which one of the cells will be affected. In the early days of PCs, radioactive decay was the most likely cause of soft errors in the computer. Improvements in design and technology have made memory chips more reliable. For example, any given bit in a 16 KB chip might suffer a decay-based soft error every billion or so hours of operation. The likelihood that a given bit in a modern 16 MB chip will suffer an error is on the order of once in two trillion hours of operation. This makes modern chips about 5000 times more reliable than those in the first generation PCs, and the contents of each cell in the memory is about five million times more reliable when you consider that the capacities have increased about 1000 times. Although conditions of use will obviously influence the occurrence of soft errors, the error rate of modernCGS 3269 – Memory - 54 memory chips is such that the typical PC with 128 MB of RAM would suffer a decay-based soft error once in 10 to 30 years. This probability is so small that most manufacturers simply ignore this factor. System-Level Errors: Sometimes data traveling through the circuits of the computer gets hit by a noise glitch. If a noise pulse is strong enough and occurs at an especially inopportune instant, it can be misinterpreted by the PC as a data bit. Such a system-level error will have the same effect as a soft error in memory. In fact, many such errors are reported as memory errors (glitch occurs between the memory and the memory controller, for example). The most likely place for system-level errors to occur is on the buses. A glitch on a data line will cause the PC to try to use or execute a bad bit of data or program code, resulting in an error. An error on the address bus will cause the PC to find the wrong bit or byte of data and the unexpected value might have the same results as if it were a data bus error. The probability of a system-level soft error occurring depends almost entirely on the design of the PC. Poor design can leave the system especially vulnerable to system-level error and may in fact assist in the generation of such errors. Overclocking a system is a common cause of system-level soft errors. Hard Errors When some part of a memory chip actually fails, the result is a hard error. One common cause of a hard error is a jolt of static electricity introduced into the system by someone carrying a static charge. Initially, the hard error may appear to be a soft error (i.e., a memory glitch) however, rebooting the system does not alleviate the symptom and possibly the system will not reboot if it cannot pass the memory system self-test. Hard errors require attention. Commonly the chip or module in which the error has occurred will require replacement. [Note: operating memory chips beyond their speed


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