binary-overwrite-mode is an interactive compiled Lisp function in `simple.el'.

(binary-overwrite-mode &optional ARG)

Toggle Binary Overwrite mode.
With a prefix argument ARG, enable Binary Overwrite mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.

When Binary Overwrite mode is enabled, printing characters typed
in replace existing text. Newlines are not treated specially, so
typing at the end of a line joins the line to the next, with the
typed character between them. Typing before a tab character
simply replaces the tab with the character typed.
C-q replaces the text at the cursor, just as
ordinary typing characters do.

Note that Binary Overwrite mode is not its own minor mode; it is
a specialization of overwrite mode, entered by setting the
`overwrite-mode' variable to `overwrite-mode-binary'.