You often hear commercial chips described in terms of transistor sizes and number of metal layers, e.g., “the SuperGizmo 5000 CPU is built in a 0.6~<f“Symbol”>mm, 3-metal CMOS process.” This description means that the minimum transistor length is 0.6~<f“Symbol”>mm, and the process has three layers of metal wiring. (The meaning of “transistor length” is explained in the design example.) Why use these two numbers as metrics? The smaller the transistor's minimum length, the more of them can be put on a chip, and the faster it—and thus, the chip—can operate. More levels of metal enable devices to connect more efficiently. To give you an idea of what's used in today's commercial microprocessors, the Pentium Pro uses a 0.35~<f“Symbol”>mm, 4-metal BiCMOS process (a combination of bipolar and CMOS technologies), while the PowerPC 604 is made in a 0.5~<f“Symbol”>mm, 4-metal CMOS process.