TeX's primitives describe the type area in terms of an offset from the upper left corner, and the width and height of the type. Some people prefer to think in terms of the margins at the top, bottom, left, and right of the page, and most composition systems other than TeX conceive of the page laid out in this way. Therefore, Eplain provides commands to directly assign and increment the margins.
\topmargin = dimen
\bottommargin = dimen
\leftmargin = dimen
\rightmargin = dimen
= and the spaces around it are optional. The control
sequences here are not TeX registers, despite appearances;
therefore, commands like \showthe\topmargin will not do what you
expect.
\advancetopmargin by dimen
\advancebottommargin by dimen
\advanceleftmargin by dimen
\advancerightmargin by dimen
Regardless of whether you use the assignment or the advance commands,
Eplain always changes the type area in response, not the other margins.
For example, when TeX starts, the left and right margins are both one
inch. If you then say \leftmargin = 2in, the right margin will
remain at one inch, and the width of the lines (i.e., \hsize)
will decrease by one inch.
When you use any of these commands, Eplain computes the old value of the particular margin, by how much you want to change it, and then resets the values of TeX's primitive parameters to correspond. Unfortunately, Eplain cannot compute the right or bottom margin without help: you must tell it the full width and height of the final output page. It defines two new parameters for this:
\paperheight
\paperwidth
If your output page has different dimensions than this, you must reassign to these parameters, as in
\paperheight = 11in \paperwidth = 17in