vm_startcopy
memset(&ovm->vm_startcopy, 0,
(caddr_t) (ovm + 1) - (caddr_t) &ovm->vm_startcopy);
memcpy(&vm2->vm_startcopy, &vm1->vm_startcopy,
(caddr_t) (vm1 + 1) - (caddr_t) &vm1->vm_startcopy);