When FreeVMS server is booting, it uses sigma0. Sigma0 is the idempotent L4 memory pager and is replaced by PAGER.SYS. PAGER.SYS runs in the same address space than FreeVMS kernel. When memory subsystem is initialized, FreeVMS server starts a process named INIT.EXE. This new thread loads all initial modules, mounts SYS$ROOT device, initializes devices tree and logical names and runs scripts in SYS$ROOT:[SYSMGR].
SYS$INIT does not run in same address space than FreeVMS server. Unlike FreeVMS server, it does not use Sigma0 anymore. All default pages are caught by FreeVMS server.