A method for in-service RAM testing in computer systems and networks having a virtual memory. The method involves identifying a number n of physical memory units pi, i=1 . . . n, in the RAM and a number n−1 of virtual memory units vj, j=1 . . . n−1, in the virtual memory, performing a one-to-one mapping to map the physical memory units pi to virtual memory units vj such that a physical memory unit px is left unmapped, and then testing data in the unmapped physical memory unit px by destructive or non-destructive tests. After the test data from another physical memory unit py is copied to physical memory unit px and a one-to-one re-mapping of physical memory units pi to virtual memory units vj is performed such that physical memory unit py is left unmapped. The data in physical memory unit py is then tested and the steps of copying, one-to-one re-mapping and testing can be repeated until all physical memory units pi are tested. The testing can be performed with the aid of an MMU and it can involve more than one physical memory page at a time.