Models after the 8086 are often referred to by the last three digits (for example, the 286, 386, and 486 ). Many of the microprocessors come in different varieties that run at various clock rates. The 80486 architecture, for example, supports clock rates of from 33 to 66 MHz. Because Intel discovered that it couldn't trademark its CPU numbers, it shifted to a naming scheme, starting with the Pentium processors. Intel's latest and sixth-generation chip is called the Pentium Pro.
All Intel microprocessors are backward compatible, which means that they can run programs written for a less powerful processor. The 80386, for example, can run programs written for the 8086, 8088, and 80286. The 80386 and later models, however, offer special programming features not available on previous models. Software written specifically for these processors, therefore, may not run on older microprocessors. The common architecture behind all Intel microprocessors is known as the x86 architecture.
Until the late 80s, Intel was essentially the only producer of PC microprocessors. Increasingly, however, Intel is facing competition from other manufacturers who produce "Intel-compatible " chips. These chips support the Intel instruction set and are often less expensive than Intel chips. In some cases, they also offer better performance. Two of the leading manufacturers of Intel-compatible chips are Cyrix and AMD.