This is the step where ATLAS performs all its empirical tuning, and then uses the discovered kernels to build all required libraries. It uses the BLDdir created by the configure step, and is invoked from the BLDdir with the make build command, or simply by make. This step can be quite long, depending on your platform and whether or not you use architectural defaults. For a system like the Core2Duo with architectural defaults, the build step may take 10 or 20 minutes, while in order to complete a full ATLAS search on a slower platform (eg. MIPS) could take anywhere between a couple of hours and a full day.