diff --git a/include/vm_area.h b/include/vm_area.h
new file mode 100644
index 0000000..1b40bbd
--- /dev/null
+++ b/include/vm_area.h
@@ -0,0 +1,45 @@
+#ifndef _VM_AREA_H_
+#define _VM_AREA_H_
+
+typedef struct {
+    /*
+     * These are the x and y coordinate offsets in the logical dimension
+     * established in a vm_screen. An offset of (0, 0) would be in the
+     * top-left; (5, 5) would be 5 pixels down, and 5 pixels to the
+     * right, of that top-left corner.
+     */
+    int xoff;
+    int yoff;
+
+    /*
+     * These are the width and height of the area we're defining. A
+     * single pixel in the logical area would have a width and height of
+     * (1, 1); use larger numbers to indicate a larger square (if the
+     * two are equal) or rectangle (if unequal).
+     */
+    int width;
+    int height;
+} vm_area;
+
+/*
+ * Set the contents of an SDL_Rect to the equivalent fields contained in
+ * a vm_area.
+ */
+#define SET_SDL_RECT(name, a) \
+    (name).x = (a).xoff; \
+    (name).y = (a).yoff; \
+    (name).w = (a).width; \
+    (name).h = (a).height
+
+/*
+ * Much like SET_SDL_RECT(), except this will (as a side-effect!)
+ * declare an SDL_Rect variable (`name`) and pass that into the SET
+ * macro.
+ */
+#define MAKE_SDL_RECT(name, a) \
+    SDL_Rect name; \
+    SET_SDL_RECT(name, a)
+
+extern void vm_area_set(vm_area *, int, int, int, int);
+
+#endif
diff --git a/include/vm_screen.h b/include/vm_screen.h
index 1151025..fb1379f 100644
--- a/include/vm_screen.h
+++ b/include/vm_screen.h
@@ -4,25 +4,7 @@
 #include <stdbool.h>
 #include <SDL.h>
 
-typedef struct {
-    /*
-     * These are the x and y coordinate offsets in the logical dimension
-     * established in a vm_screen. An offset of (0, 0) would be in the
-     * top-left; (5, 5) would be 5 pixels down, and 5 pixels to the
-     * right, of that top-left corner.
-     */
-    int xoff;
-    int yoff;
-
-    /*
-     * These are the width and height of the area we're defining. A
-     * single pixel in the logical area would have a width and height of
-     * (1, 1); use larger numbers to indicate a larger square (if the
-     * two are equal) or rectangle (if unequal).
-     */
-    int width;
-    int height;
-} vm_area;
+#include "vm_area.h"
 
 typedef struct {
     /*
@@ -49,27 +31,7 @@ typedef struct {
     int ycoords;
 } vm_screen;
 
-/*
- * Set the contents of an SDL_Rect to the equivalent fields contained in
- * a vm_area.
- */
-#define SET_SDL_RECT(name, a) \
-    (name).x = (a).xoff; \
-    (name).y = (a).yoff; \
-    (name).w = (a).width; \
-    (name).h = (a).height
-
-/*
- * Much like SET_SDL_RECT(), except this will (as a side-effect!)
- * declare an SDL_Rect variable (`name`) and pass that into the SET
- * macro.
- */
-#define MAKE_SDL_RECT(name, a) \
-    SDL_Rect name; \
-    SET_SDL_RECT(name, a)
-
 extern bool vm_screen_active(vm_screen *);
-extern void vm_area_set(vm_area *, int, int, int, int);
 extern int vm_screen_add_window(vm_screen *, int, int);
 extern int vm_screen_init();
 extern int vm_screen_xcoords(vm_screen *);
diff --git a/sources.cmake b/sources.cmake
index 9c604e0..647c42b 100644
--- a/sources.cmake
+++ b/sources.cmake
@@ -15,6 +15,7 @@ set(erc_sources
 	mos6502.stat.c
 	objstore.c
 	option.c
+	vm_area.c
 	vm_bitfont.c
 	vm_screen.c
 	vm_segment.c
diff --git a/src/vm_area.c b/src/vm_area.c
new file mode 100644
index 0000000..3990ee3
--- /dev/null
+++ b/src/vm_area.c
@@ -0,0 +1,18 @@
+/*
+ * vm_area.c
+ */
+
+#include "vm_area.h"
+
+/*
+ * Assign the values of an area, which are an x and y offset plus a
+ * width and height.
+ */
+inline void
+vm_area_set(vm_area *area, int xoff, int yoff, int width, int height)
+{
+    area->xoff = xoff;
+    area->yoff = yoff;
+    area->width = width;
+    area->height = height;
+}
diff --git a/src/vm_screen.c b/src/vm_screen.c
index 9ca1002..738c004 100644
--- a/src/vm_screen.c
+++ b/src/vm_screen.c
@@ -210,16 +210,3 @@ vm_screen_draw_rect(vm_screen *screen, vm_area *area)
 
     SDL_RenderFillRect(screen->render, &rect);
 }
-
-/*
- * Assign the values of an area, which are an x and y offset plus a
- * width and height.
- */
-inline void
-vm_area_set(vm_area *area, int xoff, int yoff, int width, int height)
-{
-    area->xoff = xoff;
-    area->yoff = yoff;
-    area->width = width;
-    area->height = height;
-}
diff --git a/tests/vm_area.c b/tests/vm_area.c
new file mode 100644
index 0000000..5571540
--- /dev/null
+++ b/tests/vm_area.c
@@ -0,0 +1,14 @@
+#include <criterion/criterion.h>
+
+#include "vm_area.h"
+
+Test(vm_area, set)
+{
+    vm_area area;
+
+    vm_area_set(&area, 1, 2, 3, 4);
+    cr_assert_eq(area.xoff, 1);
+    cr_assert_eq(area.yoff, 2);
+    cr_assert_eq(area.width, 3);
+    cr_assert_eq(area.height, 4);
+}