Pascal is simple and clear, and can be translated easily to anything from LISP, Fortran, Python to C or C++ (in fact, subsets of Pascal are often used as sample language in books about compilers, including in Pascal inventor N. Wirth's own compiler book (which, unlike Knuth's, was completed timely):
Wirth, Niklaus, Compilers (1996), 101pp., 2rd revision, 2017, online: https://people.inf.ethz.ch/wirth/CompilerConstruction/Compil..., last accessed 2026-07-07).
It does not matter that Pascal is not much in use anymore, because due to its readability, it's timeless. It nearly reads like English prose, yet is automatically executable. It has also been standardized, and there is a book-sized language description available, as are several -- commercial and open source -- implementations.
In contrast, his pseudo-assembler is arcane. Whenever I wanted to implement an algorithm following Knuth TACOP, I had to work off his English pseudo-code description rather than the associated pseudo-assembler code.
given the timespan and the focus on complete analysis of running times and not just asymptotics, in the end maybe it wasn't so terrible a choice.
And unfortunately, for a lot of modern algorithms, you're going to have dive into SIMD-like algorithms, something MMIX doesn't have. Also, a lot of modern processors have a decent suite of bitwise operations (e.g., count leading/trailing zeros/ones, popcount) that is also missing from MMIX.
The programming languages that are in favor may change from decade to decade, but so to does most of the assembly language techniques.
Don's first computer was the IBM 650 https://en.wikipedia.org/wiki/IBM_650?useskin=vector see also http://ed-thelen.org/comp-hist/KnuthIBM650Appreciation.pdf so MIX was a simplified version of the 650 because, well, it's well-defined and simple -- and Don knew a popular IBM machine very well. And there's this, in Vol 1:
This series of books is affectionately dedicated to the Type 650 computer once installed at Case Institute of Technology, in remembrance of many pleasant evenings.
MMIX is for all you youngsters who think RISC is all the rage ;-) and I think he does an admirable job creating a fully-defined machine that does use more modern hardware techniques. The fact that he fully defines his underlying machine is exactly correct, because it lays the foundation for precisely expressing the algorithms, and for giving Time and Space (runtime) estimates.
I believe it's fundamentally incorrect to think of these abstract machines as 'assembly language' but rather, I think, they define a stable foundation onto which accurately described algorithms can be expressed. You're supposed to 'play computer' and follow along -- step by step -- to understand the deep details of the algorithms.
They were all the rage for a while, because they make procedure calls fast but turn out to have subtle issues in highly-multithreaded scenarios.