MPS = Multi Processor Specification. "MPS 1.1 was the original specification. MPS version 1.4 adds extended configuration tables for improved support of multiple PCI bus configurations and greater expandability in the future. In addition, MPS 1.4 introduces support for a secondary PCI bus without requiring a PCI bridge." -Rojakpot (I'm too lazy to re-paraphrase).
Ergo, set it to 1.4...which is probably required for HTT anyway.
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