vmem_advance
vmem_advance(vmp, &walker, vsp);
vmem_advance(vmp, &walker, NULL);
vmem_advance(vmp, rotor, rotor->vs_anext);
vmem_advance(vmp, rotor, vsp);
vmem_advance(vmp, &walker, vsp);
vmem_advance(vmp, &walker, NULL);
vmem_advance(vmp, rotor, rotor->vs_anext);
vmem_advance(vmp, rotor, vsp);