Gen Two — 1991 CRE_TRON
Parameterized, with modeling brought to the host. 

Gen Three — 1993 BTRON
Lester Longley DSP code

Gen Four — 1997 Aureal NSP
C++ on the host using Intel's Native Signal Processing (NSP) on the Pentium platform. 
 

Gen Five — 1999 Static AuSIM3D
Completely redesigned C++ with optimized core C routines, separating the DSP engine from the modeling layer.  All memory allocated at initialization for efficiency. 

Gen Six — 2003 Dynamic AuSIM3D
Extensively re-engineered C++ with heavy dynamic container use, making every feature extensible in run-time.