Appendix B. Mark I and Mark I* Software Details

To modern eyes, the programming conventions of the early Ferranti computers seem both primitive and complex, the complexity arising from the fact that coding was based on the 5-bit teleprinter characters. Users were expected to learn the 5-digit binary equivalents by heart, with the added complication that ‘backwards binary’ was employed with the least-significant digit at the left-hand end so as to be compatible with the engineer’s serial waveforms where time flowed from left to right. All this, and much more, is explained in this chapter—to the point where the bold reader can write a small program. The bootstrapping, routine-calling and monitoring processes are also described, along with the scheme for memory management, the integration of the small but fast primary store with the slower but larger magnetic drum secondary store. We detail the differences between the instruction sets for the Mark I and for the Mark I*—the latter being much easier to use.