1234949SbaptBerkeley Yacc reflects its origins. The reason so many routines 2234949Sbaptuse exactly six register variables is that Berkeley Yacc was 3234949Sbaptdeveloped on a VAX using PCC. PCC placed at most six variables 4234949Sbaptin registers. I went to considerable effort to find which six 5234949Sbaptvariables most belonged in registers. Changes in machines and 6234949Sbaptcompilers make that effort worthless, perhaps even harmful. 7234949Sbapt 8234949SbaptThe code contains many instances where address calculations are 9234949Sbaptperformed in particular ways to optimize the code for the VAX. 10