Due to the nature of low-level programming two well-written pieces of code that perform the same function there will inevitably be some degree of similarity. As such it would be impossible for Phoenix to defend itself on the grounds that no part of its BIOS matched IBM's. Phoenix developed a "clean room" technique that isolated the engineers who had been contaminated by reading the IBM source listings in the IBM Technical Reference Manuals. The contaminated engineers wrote specifications for the BIOS APIs and provided the specifications to "clean" engineers who had not been exposed to IBM BIOS source code. Those "clean" engineers developed code from scratch to mimic the BIOS APIs.
Now adays Microsoft and its large partners don't even bother with the clean room anymore.