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Vol. 3 9-59
PROCESSOR MANAGEMENT AND INITIALIZATION
Finally, before storing the proposed update in NVRAM, the BIOS must verify the
authenticity of the update via the mechanism described in Section 9.11.6, “Micro-
code Update Loader.” This includes loading the update into the current processor,
executing the CPUID instruction, reading MSR 08Bh, and comparing a calculated
value with the update revision in the proposed update header for equality.
When performing the write update function, the BIOS must record the entire update,
including the header, the update data, and the extended processor signature table (if
applicable). When writing an update, the original contents may be overwritten,
assuming the above criteria have been met. It is the responsibility of the BIOS to
ensure that more recent updates are not overwritten through the use of this BIOS
call, and that only a single update exists within the NVRAM for any processor step
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ping and platform ID.
Figure 9-8 and Figure 9-9 show the process the BIOS follows to choose an update
block and ensure the integrity of the data when it stores the new microcode update.