No memory test can catch all possible RAM faults with 100 percent accuracy. even though it may show that each cell can store and read a 0 or a 1. Some faulty RAMs can be pattern-sensitive. For instance, a RAM may be able to store and read 01010101 and 10101010, but it may fail when 11100011 is stored. Even for a small RAM system, it would take a prohibitively long time to try storing and reading every possible pattern in each location. For this reason, if a RAM system passes the checkerboard test, you can conclude that it is probably good; if it fails the test, then it definitely contains a fault.