diff --git a/ReC98.h b/ReC98.h
index a40c3954..3461ab3b 100644
--- a/ReC98.h
+++ b/ReC98.h
@@ -116,6 +116,13 @@ extern dots8_t *VRAM_PLANE_E;
 
 #define VRAM_OFFSET(x, y) ((x) >> 3) + (y << 6) + (y << 4)
 
+#ifdef __cplusplus
+static inline unsigned int vram_offset_shift(int x, int y)
+{
+	return VRAM_OFFSET(x, y);
+}
+#endif
+
 #define VRAM_CHUNK(plane, offset, bit_count) \
 	*(dots##bit_count##_t *)(VRAM_PLANE_##plane + offset)
 
diff --git a/th01/hardware/graph.cpp b/th01/hardware/graph.cpp
index 46e10871..08cca560 100644
--- a/th01/hardware/graph.cpp
+++ b/th01/hardware/graph.cpp
@@ -462,7 +462,7 @@ void graph_r_vline(int x, int top, int bottom, int col)
 		egc_copy_rect_1_to_0(x, top, sizeof(pattern) * 8, bottom - top);
 		return;
 	}
-	vram_row_offset = VRAM_OFFSET(x, top);
+	vram_row_offset = vram_offset_shift(x, top);
 	pattern = graph_r_pattern >> (x & 7);
 	pattern |= graph_r_pattern << (16 - (x & 7));
 
@@ -611,7 +611,7 @@ void graph_r_line(int left, int top, int right, int bottom, int col)
 		plot_loop(y_cur, h, y_direction, x_cur, w, 1);
 	}
 restore_last:
-	vram_offset = VRAM_OFFSET(x_cur, y_cur) - 1;
+	vram_offset = vram_offset_shift(x_cur, y_cur) - 1;
 	restore_at(vram_offset);
 end:
 	if(!graph_r_restore_from_1) {