Ken Shirriff has written an article on reverse engineering the ALU of the 8008 microprocessor:
A computer’s arithmetic-logic unit (ALU) is the heart of the processor, performing arithmetic and logic operations on data. If you’ve studied digital logic, you’ve probably learned how to combine simple binary adder circuits to build an ALU. However, the 8008’s ALU uses clever logic circuits that can perform multiple operations efficiently. And unlike most 1970’s microprocessors, the 8008 uses a complex carry-lookahead circuit to increase its performance.
The 8008 was Intel’s first 8-bit microprocessor, introduced 45 years ago.1 While primitive by today’s standards, the 8008 is historically important because it essentially started the microprocessor revolution and is the ancestor of the x86 processor family that are probably using right now.2 I recently took some die photos of the 8008, which I described earlier. In this article, I reverse-engineer the 8008’s ALU circuits from these die photos and explain how the ALU functions.
More details at righto.com.