a redundancy of one. A problem was discovered very late in the development of the TXE4 in that if an equipment number was threaded with the wrong directory number on the cyclic stores, it could clash with the directory number of another equipment number leading to multiple directory numbers. This would cause Number Unobtainable (NU) tone when the incorrectly threaded number was dialled and the double threading number received incorrect calls. The exchange had no way of detecting this but the programming of one of the MCUs allowed a separate program to run to detect errors and print out the locations of the duplicates. This had to be done on a regular basis. Eventually, after many more diagnostic routines had been added, this became known as Tester 299A. The MCU executed a program stored in 10 Slide in Units (SIU) located at the bottom of the MCU rack. These MTWS units (Miniature Threaded Wire Store), were matrices of eight by ten cores through which enamelled wire were threaded. Each MTWS held 500 program steps. The first 8 MTWS were used for normal operations and the last two were reserved for special routines. The 5,000 programming steps were addressed with a letters from A-E and three decimal digits e.g. B253. The letter was decided in various ways, one example was by a decision (e.g. whether some information was present, e.g. A=true, B=false resulting in either A253 or B253). Each named step consisted of eight decimal digits, depending on which cores the wire was threaded through. The first three digits (e.g. 891) told the MCU what program step to go to next. The next two digits defined the operation (e.g. 55, compare two pieces of information) and the last three told the MCU where to store the result (e.g. 020, put this information in Main Ferrite Store 10). So the whole program step would be 89155020 which would lead to the next step being A891 if the answer was true or B891 if the answer was false. Each step took 12 microseconds to execute. The program could be easily changed, on site, as developments and upgrades occurred throughout the life of the TXE4 design. 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