Sometimes within a single project there is a requirement for both an FPGA and a classical CPU, so that complex program flows can be implemented simply. One solution in such situations is a processor realized within an FPGA. While it is possible to purchase such ‘soft cores’, they are not usually particularly cheap, and so in this article we look at a simple homebrew CPU in an FPGA. Thanks to the open-source code it is possible to modify the design to suit your own purposes. To this we add a DIY C compiler, which can also be extended. And, as usual, the series of articles describing the project will expand on the theory by including as many practical examples as possible.