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No processor design is perfect. The first version of a new microprocessor product is the A-0 step. Later, as improvements are made to the product for functional (bug) fixes or manufacturing improvements, the stepping number will increase.

Generally speaking, minor changes result in an increased number, (that is, A-1 to A-2) while more complex changes result in the letter being changed (that is, A-2 to B-0).

If new steppings come out with fewer bugs you are usually not eligible to RMA for a newer processor. Most of these bugs are supposed to be worked around in the motherboard design, BIOS (drivers), and applications.

Intel produces a detailed document called a "Specification Update" for each of their processor models that covers all of the various steppings, and lists exactly which bugs (they call them "errata") each one has and a description of the bug. For example you can download the Pentium IV specification Update from Intel at http://developer.intel.com/design/pentium4/specupdt/
A similar document is available for every processor they make.

AMD uses the term "revision numbers" to refer to steppings for its Athlon and other processors. To see the errata for a given AMD processorhttp://images.intellitxt.com/ast/adT...lass_10x10.gif, look up its revision guide, which lists the processor revision numbers and the errata which apply to each revision. For example, the revision guide for the AMD Athlon XP model 8 processor is available from AMD at