The hardware exercises expect you to use a simple HDL, you write an assembler in your language of choice (I used Ruby, but others have used Python or JavaScript), and you write programs for the computer you "built" in earlier chapters using Jack (a language with Java-like syntax).
"Project 4 is written in the computer’s assembly language, and
projects 9 and 12 (a simple computer game and a basic operating system)
are written in Jack—the Java-like high-level language for which we build a
compiler in chapters 10 and 11."