1. A method for operating a computer system in an operating system test mode with respect to memory address pinning in virtual memory, the computer system comprising: a processor system comprising one or more processors; a physical memory system comprising one or more memories; and a secondary storage memory system comprising one or more computer readable hardware storage devices, wherein the processor system uses the physical memory system and secondary storage memory to provide the virtual memory, said method comprising, in response to a request to access a memory address in the virtual memory: said one or more processors determining that a memory page associated with the memory address is available in the physical memory system, a first flag is set to an indication that the processor system is being operated in the operating system test mode, the memory page associated with the memory address is not pinned in the virtual memory, and a second flag is set to an indication that paging is disabled, and in response, said one or more processors generating an interrupt, wherein the interrupt generated indicates that the interrupt was generated in accordance with the operating system test mode; and in response to said generating the interrupt, said one or more processors pinning the memory page in the virtual memory, which: prevents occurrence of a page fault in response to a subsequent attempt to access the memory page; and prevents disruption of further testing since an operating system in the computer system is configured to perform crash behaviour in response to the occurrence of the page fault.