Understanding TeX and LaTeX horizontal rules

I just want to have a horizontal rule at the top of the text frame, closed to the top margin. I am trying to know why this is not a trivial issue.

As I understand, LaTeX \rule command makes a filled box, placed where it is called, in horizontal mode processing.

By the other hand, the TeX primitive \hrule make a horizontal filled box (of \hsize lenght) in vertical mode processing.

First of all, I am not really sure of the second affirmation. But, if it's okay, then I have two questions:

1) Why TeX insert a blank space above the first hrule it when it is invoqued at the beginning of the document? And how to avoid it?

2) Why TeX hrule implies a reduction of the space between two baselines? And how to keep it?

\documentclass{article}\usepackage{fullpage, showframe}\begin{document}\hruleOne line\hruleAnother line\end{document}