If the CPU is a model 1, then the value of idxb is irrelevant. For
tidiness, it should always be 0, but on a model 1, idxe (index
enable) is always 0, so idxb is not looked at. Thus,
- on a model 1, idxb is set to 0, which is harmless and probably a good
thing to do.
- on a model 2, idxe is set to 1, and idxb is set to 0, which is the correct
behavior.
Note that case 0 doesn't need to check for model 2, because clearing
idxe and idxb yields the correct settings on a model 1: indexing is off.
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