The Z80 was software compatible with the Intel 8080. The only "difference" was that the assembler used different names for the instructions and registers. This was the only thing that Intel could have called copyright on. There was no copyright protection on the silicon itself unless Zilog copied the chip design verbatim. The first true "clean room" clone :)
Anyhow over the years many 3rd party tools have accepted either the Zilog or Intel assembler as they were a 1 to 1 translation. Can make reading source written by multiple parties a complete pain :(
Thank you for coming to my Ted talk :P
