This edition is a minor revision of the highly acclaimed first edition. It retains the essential structure of the first edition. Specifically, it presents and investigates a simple computer model (the H1). The H1 is not a static model. It has flaws (major flaws, in fact) that the reader fixes. Thus, the flaws in the H1 are not flaws in the book. Rather, they are assets the book uses to engage the reader in the design and implementation process. The H1 software package supports Windows, Mac OS X, Linux, and Raspbian. It has been substantially ungraded since the first edition. It is now available on the web as a free download.