I've been taking the nand2tetris course online by myself and it's been really fun, challenging and informative. But I've gotten to chapter 6, assembly, and I noticed that the chapters after it (7-13) don't have links to their repective PDFs. Do I need to buy the book to read those chapters? I'm just confused because when I first found out about it, I read that all the resources were open and free on the site.