One drawback of this method is that the size of the key determines how many jobs could be simultaneously active. Four bits means that there could be a maximum of 15 user jobs at any one time, since 0000 is reserved for the operating system. An advantage of this method is that the memory for a user program did not have to be allocated contiguously, i.e. in one long unbroken chunk. It could be spread around the memory. Later virtual memory systems have the same flexibility to break up memory as they need.