What was the last comprehensible general purpose computer?

What I mean by "comprehensible" perhaps would be better explained as a comparison.

If somebody wants to study operating systems and opens up current Linux kernel source code, even with a solid theoretical knowledge, would most likely be crushed by trying to build a mental image of what's going on there.

But if you give him a xv6 source code it something that can be done with relative easy.

So what I'm searching for is the last general purpose computer that is to current computers what xv6 is to linux kernel.

Machine that is so well laid out, documented and studied that one person can learn from it, what hardware and software decision were made by its engineers at each step of production and understand thought process behind their decisions.