This is not a real computer. It is a prototypical computer – the model for a computational machine that can simulate any other computational or logical machine. It was devised in 1936, in a thought-experiment by the English mathematician, cryptographer, and early computer scientist Alan Turing. The experiment assumes an unlimited paper tape (or film-strip in the illustration above), divided into segments (or frames above), which can run back and forth through a read-write head (eraser and writer above). Importantly (and this is Turing’s brilliant insight), the film/tape can hold data as well as instructions (both as symbols). There is a logic unit that can read the symbols, and act upon them (erase them and write new symbols), onto the film. The film frames can contain symbols (such as numbers and arithmetic symbols). There is also a memory unit that can store recent symbols. This is how Turing imagined the logical computation machine – a computer that had input and output (the tape or film and read/write head in this diagram), a memory, and a logical processing unit. And this is precisely what a modern PC has – input and output via keyboard, network, disc, printer, monitor, memory in the form of RAM and hard-disk; and a logical processing unit in the form of a microprocessor chip.