Comment by tragomaskhalos

There was a wonderful book called 'The Spectrum ROM disassembly' that gave a full and detailed analysis of the whole thing. A good chunk of the code was cloned from the ZX81, but as you say the tape routines were very nice, as was the stack-based calculator which was invoked via an RST call followed by opcodes for pushing numbers plus all the arithmetic and trig functions. I got knee deep into my own disassembly of the Interface 1 (?) ROM which has all the microdrive code, but never finished it :-/