In this article, I reverse-engineer the priority encoder in the ARM1 processor. By examining the chip layout provided by the Visual ARM1 project, I have determined how this circuit works and created a schematic.
The ARM1 chip is the ancestor of the extremely popular ARM processors used in most smart phones. The ARM1 is a good choice for reverse engineering since it was designed in 1985 and its simple RISC silicon circuits are easier to understand than modern processors. This article jumps into the chip details; if you want an overview of the ARM1 internals, start with my first article on reverse engineering the ARM1.
More details at Ken Shirriff’s blog.