the fork$()$ system call in xv$6$ copies all of the parent process's user$-$space memory into the child. if the parent is large, copying can take a long time. worse, the work is often largely wasted: fork$()$ is commonly followed by exec$()$ in the child, which discards the copied memory, usually without using most of it$.$ on the other hand, if both parent and child use a copied page, and one or both writes it$,$ the copy is truly needed.