One of the biggest revelations I had during my (ongoing) journey to discover how a processor really works is to find out how that mystical microcode in a CPU is implemented. Things culminated in the 4-Bit Nibbler Do-It-Yourself CPU project that uses a microcode ROM in the control part. For understanding how microcode works it can’t get much better than that. Except…
One thing that was missing for me until recently was how microcode is used in a ‘real’ commercial processor. This very cool post by Ken Shirriff explains just that by analyzing the internals of the ARM-1 processor, the great great great grandfather of today’s ARM CPUs that are put into almost any electronic device including your smartphone. The article won’t necessarily help you to understand what microcode is as such but once you do the article is a treasure trove because it shows how theory was put into practice in a real design. A highly recommended read!