The fourth edition of Computer Architecture: A Quantitative Approach may be the most significant since the first edition. Shortly before we started this revision, Intel announced that it was joining IBM and Sun in relying on multiple processors or cores per chip for high-performance designs. As the first figure in the book documents, after 16 years of doubling performance every 18 months, gle-processor performance improvement has dropped to modest annual improvements. This fork in the computer architecture road means that for the first time in history, no one is building a much faster sequential processor. If you want your program to run significantly faster, say, to justify the addition of new features, you’re going to have to parallelize your program.