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+#	@(#)structures	5.4 (Berkeley) 10/4/95
+
+There are three major data structures in this package, plus a single data
+structure per screen type.  The first is a single global structure (GS)
+which contains information common to all files and screens.  It hold
+global things like the input key queues, and functions as a single place
+to hang things.  For example, interrupt routines have to be able to find
+screen structures, and they can only do this if they have a starting
+point.  The number of globals in nvi is dependent on the screen type, but
+every screen type will have at least one global, __global_list, which
+references the GS structure.
+
+The GS structure contains linked lists of screen (SCR) structures.
+Each SCR structure normally references a file (EXF) structure.
+
+The GS structure has a set of functions which update the screen and/or
+return information about the screen from the underlying screen package.
+The GS structure never goes away.  The SCR structure persists over
+instances of screens, and the EXF structure persists over references to
+files.
+
+File names have different properties than files themselves, so the name
+information for a file is held in an FREF structure which is chained from
+the SCR structure.
+
+In general, functions are always passed an SCR structure, which usually
+references an underlying EXF structure.  The SCR structure is necessary
+for any routine that wishes to talk to the screen, the EXF structure is
+necessary for any routine that wants to modify the file.  The relationship
+between an SCR structure and its underlying EXF structure is not fixed,
+and various ex commands will substitute a new EXF in place of the current
+one, and there's no way to detect this.
+
+The naming of the structures is consistent across the program.  (Macros
+even depend on it, so don't try and change it!)  The global structure is
+"gp", the screen structure is "sp", and the file structure is "ep".
+
+A few other data structures:
+
+TEXT	In nvi/cut.h.  This structure describes a portion of a line,
+	and is used by the input routines and as the "line" part of a
+	cut buffer.
+
+CB	In nvi/cut.h.	A cut buffer.  A cut buffer is a place to
+	hang a list of TEXT structures.
+
+CL	The curses screen private data structure.  Everything to
+	do standalone curses screens.
+
+MARK	In nvi/mark.h.  A cursor position, consisting of a line number
+	and a column number.
+
+MSG	In nvi/msg.h.  A chain of messages for the user.
+
+SEQ	In nvi/seq.h.  An abbreviation or a map entry.
+
+TK	The Tcl/Tk screen private data structure.  Everything to
+	do standalone Tcl/Tk screens.
+
+EXCMD   In nvi/ex/ex.h.  The structure that gets passed around to the
+	functions that implement the ex commands.  (The main ex command
+	loop (see nvi/ex/ex.c) builds this up and then passes it to the
+	ex functions.)
+
+VICMD	In nvi/vi/vi.h.  The structure that gets passed around to the
+	functions that implement the vi commands.  (The main vi command
+	loop (see nvi/vi/vi.c) builds this up and then passes it to the
+	vi functions.)