 Of the 20 bits allocated for each program instruction, 10 were used to hold the instruction code, which allowed for 1,024 (210) different instructions. The machine had 26 initially, increasing to 30 when the function codes to programmatically control the data transfer between the magnetic drum and the cathode ray tube (CRT) main store were added. On the Intermediary Version programs were input by key switches, and the output was displayed as a series of dots and dashes on a cathode ray tube known as the output device, just as on the SSEM from which the Mark 1 had been developed. However, the Final Specification machine, completed in October 1949, benefitted from the addition of a teleprinter with a 5-hole paper-tape reader and punch.
