diff --git a/th05/main/boss/b4_both.cpp b/th05/main/boss/b4_both.cpp
index 8f30a727..252c6b62 100644
--- a/th05/main/boss/b4_both.cpp
+++ b/th05/main/boss/b4_both.cpp
@@ -13,6 +13,7 @@
 #include "th05/sprites/main_pat.h"
 #include "th04/main/playfld.hpp"
 #include "th05/main/boss/boss.hpp"
+#include "th05/main/boss/impl.hpp"
 
 #define mai boss
 #define yuki boss2
diff --git a/th05/main/boss/boss.hpp b/th05/main/boss/boss.hpp
index 60d0786d..f7b237a3 100644
--- a/th05/main/boss/boss.hpp
+++ b/th05/main/boss/boss.hpp
@@ -36,66 +36,6 @@ static const int BOSS_FLYSTEP_RANDOM_FRAMES = 28;
 // function returns true.
 bool pascal near boss_flystep_random(int frame);
 
-#define flystep_random_for( \
-	boss, \
-	next_y_direction, \
-	speed, \
-	frames, \
-	clamp_left, \
-	clamp_right, \
-	clamp_top, \
-	clamp_bottom, \
-	sprite_left, \
-	sprite_right, \
-	sprite_still, \
-	frame \
-) \
-	if(frame == 0) { \
-		if(boss.pos.cur.x.v < BOSS_FLYSTEP_RANDOM_FIELD_LEFT) { \
-			boss.angle = (randring2_next16_mod(0x60) - 0x30); \
-		} else if(boss.pos.cur.x.v > BOSS_FLYSTEP_RANDOM_FIELD_RIGHT) { \
-			boss.angle = (randring2_next16_and(0x60) + 0x30); \
-		} else { \
-			boss.angle = randring2_next16(); \
-		} \
-		if( \
-			(next_y_direction == Y_UP  ) && (boss.angle <  0x80) || \
-			(next_y_direction == Y_DOWN) && (boss.angle >= 0x80) \
-		) { \
-			boss.angle = -boss.angle; \
-		} \
-		next_y_direction = Y_ANY; \
-	} \
-	if(frame >= 0) { \
-		vector2_near( \
-			boss.pos.velocity, boss.angle, (to_sp(speed) - (frame * 2)) \
-		); \
-		boss.pos.cur.x.v += boss.pos.velocity.x.v; \
-		boss.pos.cur.y.v += boss.pos.velocity.y.v; \
-		if(boss.pos.velocity.x.v < 0) { \
-			boss.sprite = sprite_left; \
-		} else { \
-			boss.sprite = sprite_right; \
-		} \
-		if(boss.pos.cur.y.v < clamp_top) { \
-			boss.pos.cur.y.v = clamp_top; \
-			next_y_direction = Y_DOWN; \
-		} else if(boss.pos.cur.y.v > clamp_bottom) { \
-			boss.pos.cur.y.v = clamp_bottom; \
-			next_y_direction = Y_UP; \
-		} \
-		if(boss.pos.cur.x.v < clamp_left) { \
-			boss.pos.cur.x.v = clamp_left; \
-		} else if(boss.pos.cur.x.v > clamp_right) { \
-			boss.pos.cur.x.v = clamp_right; \
-		} \
-		if(frame >= frames) { \
-			boss.sprite = sprite_still; \
-			return true; \
-		} \
-	} \
-	return false;
-
 // Steps the [boss] from its current position towards the target point, moving
 // it by a hardcoded fraction of the distance. Returns true once the [boss]
 // has reached the target point.
diff --git a/th05/main/boss/impl.hpp b/th05/main/boss/impl.hpp
new file mode 100644
index 00000000..56c8d23d
--- /dev/null
+++ b/th05/main/boss/impl.hpp
@@ -0,0 +1,62 @@
+// Shared boss implementation macros.
+
+#define flystep_random_for( \
+	boss, \
+	next_y_direction, \
+	speed, \
+	frames, \
+	clamp_left, \
+	clamp_right, \
+	clamp_top, \
+	clamp_bottom, \
+	sprite_left, \
+	sprite_right, \
+	sprite_still, \
+	frame \
+) { \
+	if(frame == 0) { \
+		if(boss.pos.cur.x.v < BOSS_FLYSTEP_RANDOM_FIELD_LEFT) { \
+			boss.angle = (randring2_next16_mod(0x60) - 0x30); \
+		} else if(boss.pos.cur.x.v > BOSS_FLYSTEP_RANDOM_FIELD_RIGHT) { \
+			boss.angle = (randring2_next16_and(0x60) + 0x30); \
+		} else { \
+			boss.angle = randring2_next16(); \
+		} \
+		if( \
+			(next_y_direction == Y_UP  ) && (boss.angle <  0x80) || \
+			(next_y_direction == Y_DOWN) && (boss.angle >= 0x80) \
+		) { \
+			boss.angle = -boss.angle; \
+		} \
+		next_y_direction = Y_ANY; \
+	} \
+	if(frame >= 0) { \
+		vector2_near( \
+			boss.pos.velocity, boss.angle, (to_sp(speed) - (frame * 2)) \
+		); \
+		boss.pos.cur.x.v += boss.pos.velocity.x.v; \
+		boss.pos.cur.y.v += boss.pos.velocity.y.v; \
+		if(boss.pos.velocity.x.v < 0) { \
+			boss.sprite = sprite_left; \
+		} else { \
+			boss.sprite = sprite_right; \
+		} \
+		if(boss.pos.cur.y.v < clamp_top) { \
+			boss.pos.cur.y.v = clamp_top; \
+			next_y_direction = Y_DOWN; \
+		} else if(boss.pos.cur.y.v > clamp_bottom) { \
+			boss.pos.cur.y.v = clamp_bottom; \
+			next_y_direction = Y_UP; \
+		} \
+		if(boss.pos.cur.x.v < clamp_left) { \
+			boss.pos.cur.x.v = clamp_left; \
+		} else if(boss.pos.cur.x.v > clamp_right) { \
+			boss.pos.cur.x.v = clamp_right; \
+		} \
+		if(frame >= frames) { \
+			boss.sprite = sprite_still; \
+			return true; \
+		} \
+	} \
+	return false; \
+}
diff --git a/th05/main/boss/move.cpp b/th05/main/boss/move.cpp
index 8d78aa5c..4d0396df 100644
--- a/th05/main/boss/move.cpp
+++ b/th05/main/boss/move.cpp
@@ -1,3 +1,5 @@
+#include "th05/main/boss/impl.hpp"
+
 extern y_direction_t boss_flystep_random_next_y_direction;
 
 bool pascal near boss_flystep_random(int frame)