Me too - I wrote a Mandlebrot generator on the Amstrad CPC 6128 (3.7MHz Z80 based computer with 128kb RAM) after reading a copy of "Chaos" by James Gleick that I found in the school library. It took 3-4 hours to generate a 16 colour image of 160x120 (Mode 0?) and used some undocumented calls into the BASIC interpreter ROM for the floating point calculations. I'm sure I could have made it faster but I was producing it as a static image for a demo (demo scene).