Firstly, the DX7 is a hardware implementation, it doesn't use a CPU for arithmetic so there is no inherent notion of resource contention. Hardware resources can

Firstly, the DX7 is a hardware implementation, it doesn't use a CPU for arithmetic so there is no inherent notion of resource contention. Hardware resources can be (and are) allocated and/or shared exactly as are needed by the requirements of the target computation. You mention "reads," but once again, in a hardware implementation the memory structure may be quite different from the cache hierarchy of a CPU (the situation that KVRists would be most familiar with). In a phase accumulator (or any numerical representation of time) it is generally desirable to have uniform precision across the full phase range. Floating point arithmetic does not have this property. On the other hand, floating point arithmetic can sometimes be more convenient than fixed-point if the hardware to hand can execute it fast. But if you're designing hardware from scratch, and especially for FM synthesis, you're probably going to find some other tricks that work even better.