I'm trying to start using plain tex (in fact, the pdftex that comes with texlive2010) to write a document but I'm having some trouble with the page layout. My question is at which point is it decided what the dimensions of the page and the margins will be?
As far as I investigated the
\plainoutput routine in plain.tex only sets the height and width of the
\box255 plus headers, footers, etc, but page dimensions (like A4, letter, portrait, landscape, etc) and margins are never defined. Moreover the TEXBOOK says that by default the page layout has
1in of margin at each side (page 251) but I don't know when are these margins defined nor why in my final pdf output the document appears with A4 size,
1in of left margin and approximatley
0.7in of right margin. So the question again is when are these dimensions decided?