Even System/370 [0] architecture (mainframe) is a great place to start. There's a one-to-one correspondence between assembler instructions and machine instructions, which makes writing and debugging considerably easier. It's actually an incredibly robust processor architecture that's simple to understand.

[0] IBM System/370 Principles of Operation (the "big yellow book")

http://bitsavers.trailing-edge.com/pdf/ibm/370/princOps/GA22...