The MCU contained a non-volatile data store, which used a core store. There were three types of data store, Main Ferrite Store (MFS), Special Ferrite Store (SFS) and Register Ferrite Store (RFS). The MFS was used by the MCU itself to hold data for various reasons and the SFS was used for manipulating data. An example of this was that SFS2 could take the data stored in positions 1-5 and swap them with data stored in positions 6-10. Each store held 10 decimal digits, represented in 2 out of 5 code. The RFS held data from each of the MCUs associated registers, e.g. dialled digits. There were 20 MFS, 4 SFS and up to 36 RFS.