Sir and sir. Is there a difference?

[Sir Nate]

Sir as in "over here sir"
And sir as in Sir Nate.
Is there any major difference.
Or are both just a proper way to call someone. Perhaps people would call you sir simply because they don't know you and want to be respectful and the other sir shows respect to the knight as well as his name.

[Ian]

In modern English 'sir' is just a form of address to show respect generally to someone of a superior status.  Now if you were knighted by the queen of England, like the actor Ian McKellen (Gandalf), then it would be appended to your name as Sir Ian McKellen.

The word 'sir' is just a relaxed version of the word 'sire' and become common as a form of address toward a knight in the 13th or 14th century.  And sire has it's root in the same word in French 'messire'

[Sir James A]

Sir Ian, as usual, covered it very well.

From what I have seen, and reflective of what Sir Ian is saying, is that the lowercase is formality, whereas the capitalized is for a title, along the same lines as Doctor, Captain, etc.