However, this field is no longer a Checksum but looks like another “Update Version”. Since this “Update Version” is smaller than the actual “Update Version”, this should be a “Minimum Version”, meaning the minimum required version for OS kernel late loading. Intel had also added that field 2.5 years ago.
An MCE update is needed to add support for the new “Minimum Version” field in place of the old “Checksum” one.
AMD may use this part again. With the exception of one microcode, all of them were released before March. The one released on April 15 does not use the mentioned part either.
It occurred to me even then that those Checksum microcodes were just “accidents” when they appeared. Since they were not taken from the BIOS, I assume that they are not the default ones (xx xx xx xx), but ones that resemble earlier versions (00 00 00 00). I wonder what AMD has to say about this.
No, this further validates the assumption that these are “Minimum Versions” for late OS loading. The microcodes with that change were found at the Linux kernel git after all. When loading the microcode early (BIOS), this field does not matter.
Since we have now found identical variants with and without the new field, what I suspect is that AMD is releasing both, one for BIOS and another for OS kernels (Linux, Windows). Very stupid, if true, but maybe it’s because existing tooling has always known that field to be a checksum, so it’s for compatibility? No idea, would not really surprise me, though.
This is indeed interesting, especially since this is a February table. The following CPUIDs are those for which only older microcode is currently available:
