Designing and building a CPU from scratch is still possible; in fact it's easier now (using an FPGA and a hardware design language) than it was a few years ago

Designing and building a CPU from scratch is still possible; in fact it's easier now (using an FPGA and a hardware design language) than it was a few years ago (using 7400-series TTL chips and wire wrap). It's old, but I highly recommend the book Understanding Digital Computers by Forrest M. Mims (1987). It teaches the architecture of a four-bit computer completely and is sufficient to gain a foothold from which to read Hennessey and Patterson. From there, read Gordon Bell's description of the VAX (and The Soul of a New Machine). For perspective, learn about the architecture of Burroughs and the IBM AS/400. I can think of no better way to grok the underlying hardware. Write a simple assembler. Next, begin writing an OS.