Up to 36 registers were "owned" by each MCU. The MCU was responsible for looking after all of its Registers and deciding from the dialled information where the call was going to be routed. The local exchange numbering system would be available to the MCU via threadings in the cyclic store so the MCU would be able to read this information and in this way, all exchanges could be configured as required. If a call was identified by the MCU as internal to the exchange, usually by the first digit, then the MCU would tell the Register to come back when the complete number had been dialled. If the first dialled digit was a zero then this would normally be routed straight to the Group Switching Centre. However, if the exchange had an Alternative Available Route then the MCU would have to wait until enough routing digits had been received to make a routing decision. This AAR information was stored in the cyclic stores. Once the MCU had decided the routing, it sent a command to the Interrogator/Markers to set up the required path and also told the Register what dialled digits to forward on. Then the MCU would move on to the next task. Once the connection had been established, the Supervisory Processing Unit (SPU) took care of the path and all the call metering tasks. The MCUs had core memory to hold the dialled digits from all the Registers and also had other storage to manipulate call set-up information. There were three scan rates: 156ms for subscribers, 36ms for Registers, Outgoing Junctions and Routing Translations, and 12ms for Incoming Junctions. The last of these was the quickest scan, to ensure that no incoming digits were lost from the incoming junctions.