Oooh! Just left a top-level comment but I thought I'd reiterate somewhere you might see it - this is a stellar writeup. It's a rare pleasure to find such a technically comprehensive breakdown of embedded firmware on HN.
8051 is the cockroach of embedded systems. Bizarrely resilient, and absolutely disgusting. My very first firmware job (erm, internship) was taking a Cortex-M0+ and making it serve as a pin-for-pin replacement for an 8051. Many crimes against software humanity were committed.