There are some people working on the 386 microcode. Dumping the Pentium microcode ROM from the die photos would be straightforward (but tedious). The hard part is to figure out what all the bits mean.
The Pentium's ROM appears to be slightly scrambled (see footnote 6 in my article). ROMs are often a bit permuted for electrical reasons. For example, instead of columns ordered ABABABAB..., they will be ordered ABBAABBA... and then the A and B select lines can be shared by two columns. But the columns in the Pentium appear to be permuted in an irregular way. I'm not sure if this was for obfuscation or if automated layout software decided this was better.
That register could be a Model-Specific Register; I haven't looked at it closely enough to see what it does. The Pentium is very complicated with 3.1 million transistors, so my reverse-engineering of it is essentially bits and pieces here and there.
