Cascading FM synthesis operations
the code on the bottom looks like a triple-nested FM oscillator if i understand how the AdvanceOscillator() method works.  likely the SFreeSpinningOscillator class has a method of setting the initial phase of the oscillator.  you may get wildly different tone timbres with exactly the same code and different phases for the 4 oscillators.  

if you can get your hands on a DX7 alg chart





you can see that most of their algs are sum of two chains.  alg 2 has a triple nested oscillator chain where your d is the DX7 osc #3.  but i can't tell you what the initial phases of their oscillators are.  there are so many parameters that affect the timbre, sometimes in unpredictable ways.  so doing software to emulate a DX7 will involve some tweeking of knobs