TargetDescription string

In "Writing an LLVM Compiler Backend", there's some discussion of the
TargetDescription string, but it doesn't explain the examples I look
at. For instance, in the description of the PowerPC, I see

    "E-p:64:64-f64:64:64-i64:64:64-f128:64:128-n32:64"

What's "preferred alignment" versus "ABI alignment"?

What are the 3 figures following the "i"? (the documentation suggests
there will only be 2).

I see two entries for "f". Do I read these as info about "f64" and "f128"?

What is "n32:64" about?

Thanks,
Preston

http://llvm.org/docs/LangRef.html#datalayout

-Eli