AMD Athlon 6 - Manual (Page 13)

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24332G—October 2003 AMD Athlon™ Processor Model 6 Revision Guide
Preliminary Information
22 Real Mode RDPMC with Illegal ECX May Cause Unpredictable Operation
Products Affected. A0, A2, A5
Normal Specified Operation. Illegal values of ECX (that is, ECX>3) for the RDPMC (Read Performance
Monitor Counter) instruction cause the processor to take a general protection exception.
Non-conformance. If the RDPMC is executed in real mode with a specific illegal value of ECX=4, then the
processor may incorrectly enter the GP fault handler as if it were in 32-bit real mode.
Potential Effect on System. Incorrect instruction decode leading to unpredictable system failure.
Suggested Workaround. When in real mode, restrict use of the RDPMC instruction to legal counter values
(0-3). This circumstance is not expected to occur in normal operation and has only been detected in a
simulation environment.
Resolution Status. No fix planned.