FreeVMS
Virtual Memory System

Virtual memory

When system is booting, vmskernel.sys is lauched as L4 roottask. 
Its pager is sigma0 default pager. Sigma0 is an idempotent pager and roottask mainly handle physical memory. In some circumstances, roottask can map virtual memory when a thread attemps to access to nonexistent memory. 
Sigma0 only maps memory for roottask and only once. Thus, roottask, after complete initialization, enters in infinite loop that can dispatch pagefault protocol (sys$pagefault()).