It's strange, but when I looked at the BBC schematic for the first time I felt it was all very straightforward with no nasties. There is, IMHO, a big difference between 'lots of components' and 'complex'. A 1M byte RAM assembly using 1103s (1Kbits each, so 8192 RAM chips alone) has a lot of parts. But it;'s actually quite simple, in that it's the same circuit repeated over and over. Conversely, a PERQ 1a CPU board has under 300 ICs, but to understand that fully is a lot of work. It's complex, Getting back to the original point, I feel it's essential for the 'teacher' to have a much greater understnading of <foo> then the student (something that, alas, rarely, if ever, happeend to me at school :-(). I would not want to try to teach the internals of the C64, simply because of those undocumented chips. That's something I don't understnad, will probably never understand. The student could ask me a question that I would never be able to answer, and that's unfiar on them. Conversely, I would have no objections to teaching the internals of the PERQ CPU board to a student who already had a good grounding in digital electronics. No, I don't expect a 9-yerar-old kid to fit into that category. On sevaeral occasions I have given talks on the internals of particular machines, and I always ensure that I know much more aobut the machine than I say. If I am, say, talking about the HP9830, I might describe the CPU data path, microcode sequencer, and a bit of the microcde. But if somebody asked me about the cassette I/O system, I could flip up the right page in the schematics book and explain just what every chip did. -tony