Another great article from Ken Shirriff, this time on reverse engineering the ARM1:
Almost every smartphone uses a processor based on the ARM1 chip created in 1985. The Visual ARM1 simulator shows what happens inside the ARM1 chip as it runs; the result (below) is fascinating but mysterious. In this article, I reverse engineer key parts of the chip and explain how they work, bridging the gap between the puzzling flashing lines in the simulator and what the chip is actually doing. I describethe overall structure of the chip and then descend to the individual transistors, showing how they are built out of silicon and work together to store and process data. After reading this article, you can look at the chip’s circuits and understand the data they store.
More details at Ken Shirriff’s blog.