Assembly language
Much of the content of "Zen of Assembly Language" was updated in "Zen of Code Optimization: The Ultimate Guide to Writing Software That Pushes PCs to the Limit" (1994), along with new material. The presentation of stepwise program refinement empirically demonstrated how algorithm re-design could improve performance up to a factor of 100. Assembly language re-coding, on the other hand, may only improve performance by a factor of 10. Abrash also showed how elusive performance improvement can be, and improving performance in one subroutine can expose bottlenecks in other routines. Finally, he demonstrated processor-dependent assembly-based performance improvements by comparing assembly language optimizations across x86 generations, including how some micro-optimizations disappeared or even made a program slower.