PC Hardware in a Nutshell, 3rd Edition

Processor Speed

The processor clock coordinates all CPU and memory operations by periodically generating a time reference signal called a clock cycle or tick. Clock frequency is specified in megahertz (MHz), which specifies millions of ticks per second, or gigahertz (GHz), which specifies billions of ticks per second. Clock speed determines how fast instructions execute. Some instructions require one tick, others multiple ticks, and some processors execute multiple instructions during one tick. The number of ticks per instruction varies according to processor architecture, its instruction set, and the specific instruction. Complex Instruction Set Computer (CISC) processors use complex instructions. Each requires many clock cycles to execute, but accomplishes a lot of work. Reduced Instruction Set Computer (RISC) processors use fewer, simpler instructions. Each takes few ticks but accomplishes relatively little work.

These differences in efficiency mean that one CPU cannot be directly compared to another purely on the basis of clock speed. For example, an AMD Athlon XP 3000+, which actually runs at 2.167 GHz, may be faster than an Intel Pentium 4 running at 3.06 GHz, depending on the application. The comparison is complicated because different CPUs have different strengths and weaknesses. For example, the Athlon is generally faster than the Pentium 4 clock for clock on both integer and floating-point operations (that is, it does more work per CPU tick), but the Pentium 4 has ...