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AMD recently released a ‘BIOS fix’ to motherboard makers to address Errata 109, in which REP MOVS instructions caused subsequent instructions to be skipped under specific pipeline conditions.Previously it was not clear if and how AMD even supported microcode updates in the K8 family until this announcement. It also gives background information on the K8 microcode design and provides information on altering the microcode and loading the altered update for those who are interested in microcode hacking.
The primary purpose of microcode patching is to modify or disable defective functionality, rather than add new features.Microcode Update Procedure: The update procedure expects the 64-bit virtual address of the update data, including the 64 byte header, to be in edx:eax: edx = high 32 bits of 64-bit virtual address eax = low 32 bits of 64-bit virtual address ecx = 0xc0010020 (MSR to trigger update) Execute wrmsr with these register values.If the address and update block data are valid, wrmsr completes successfully. The microcode does not appear to update MSR 0x8B with the new update signature as it does on Intel processors, despite the fact that some BIOS code that was analyzed does seem to check this field.The latest microcode update blocks are included in the driver.’ ‘The information has been provided by Anonymous.’ ‘Background: Modern x86 microprocessors from Intel and AMD contain a feature known as ‘microcode update’, or as the vendors prefer to call it, ‘BIOS update’.Essentially the processor can reconfigure parts of its own hardware to fix bugs (‘errata’) in the silicon that would normally require a recall.