I had been doing system programming for a few years, read several books about operating systems, and written a couple of device drivers for UNIX before I encountered this book. I was comfortable working on existing operating systems, but writing a complete operating system seems like it would be overwhelming. Reading Xinu, and later doing labs on a PDP-11? gave me confidence to take on much more challenging projects and led me to conclude that if I needed to, I could write an OS from the ground up.