Berkeley Yacc reflects its origins.  The reason so many routinesuse exactly six register variables is that Berkeley Yacc wasdeveloped on a VAX using PCC.  PCC placed at most six variablesin registers.  I went to considerable effort to find which sixvariables most belonged in registers.  Changes in machines andcompilers make that effort worthless, perhaps even harmful.The code contains many instances where address calculations areperformed in particular ways to optimize the code for the VAX.