Emacs Hexl to read device files

To pass a numeric argument to a command, enter the argument first. For example, Escape 3 Left moves 3 characters left. So type ESC 3 ESC x compare-windows RET or M-3 M-x compare-windows RET.

The most frequently used commands to compare two files are those of ediff. You can invoke them from the “Tools > Compare (Ediff)” menu. There is a command called ediff-windows-linewise, though usually you would compare two buffers (ediff-buffers) or two files (ediff-files) or two revisions under a version control system (ediff-revision). Press n and p to navigate between diff regions; press ? to see more available commands.

If you want to open the whole file (which requires ), but show only part of it in the editor window, use narrowing. Select the part of the buffer you want to work on and press C-x n n (narrow-to-region). Say “yes” if you get a prompt about a disabled command. Press C-x n w (widen) to see the whole buffer again. If you save the buffer, the complete file is selected: all the data is still there, narrowing only restricts what you see.

If you want to view a part of a file, you can insert it into the current buffer with shell-command with a prefix argument (M-1 M-!); run the appropriate command to extract the desired lines, e.g. <huge.txt tail -n +57890001 | head -n 11.

There is also a Lisp function insert-file-contents which can take a byte range. You can invoke it with M-: (eval-expression):

(insert-file-contents "huge.txt" nil 456789000 456791000)

Note that you may run into the integer size limit (version- and platform-dependent, check the value of most-positive-fixnum).

In theory it would be possible to write an Emacs mode that loads and saves parts of files transparently as needed (though the limit on integer sizes would make using actual file offsets impossible on 32-bit machines). The only effort in that direction that I know of is VLF (GitHub link here).