\def\sidemargin{\afterassignment\sidemarginII\hoffset}

\def\sidemarginII{\advance\hoffset -1true in
\advance\hsize -2\hoffset}

\def\vertmargin{\afterassignment\vertmarginII\voffset}

\def\vertmarginII{\advance\voffset -1true in
\advance\vsize -2\voffset}