Embedded Processors 131 A key feature of processor buses is their width, which is the number of bits that can be trans- mitted at any one time. This can vary depending on both the buses implemented within the processor--for example: x86 contains bus widths of 16/32/64, 68K has 8/16/32/ 64 bit buses, MIPS 32 has 32 bit buses, and so forth--as well as the ISA register size definitions. Each bus also has a bus speed (in MHz) that impacts the performance of the processor. Buses implemented in real-world processor designs include the U, peripheral, and CPM buses in the MPC8xx family of processors and the C and X buses in the x86 Geode. To avoid redundancy, buses are covered in more detail in Chapter 4, and more examples are provided there. 3.4 Processor Performance There are several measures of processor performance, but are all based on the processor's behavior over a given length of time. One of the most common definitions of processor per- formance is a processor's throughput, the amount of work the CPU completes in a given period of time. A processor's execution is ultimately synchronized by an external system or master clock,