As noticed above, creators admitted that creating OS written in Assembly was a challenge for them. However, I totally agree about optimizing capabilities of modern compilers. Interesting fact on the topic, my friend, Principal Developer of well-known tech giant has a practice of picking up candidates, who mentioned "fluent Assembly" skill in their CV, and challenge them by suggesting to create a small, highly optimized application. After that he compiles the same application in C++ with maximum optimization, and in 90% of cases generated code is more optimal, that written manually. However, before becoming all sceptical about obsolete skills of learning Assembly, I would point out on the rest 10% of his candidates.