diff --git a/src/Makefile b/src/Makefile
index dd2d6177..d4890fb1 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -808,7 +808,7 @@ OBJS = main.o init.o graph.o $(APIOBJ) misc.o special.o \
        pxtall2.o pxtall3.o pxwide2.o pxquad.o \
        windows.o brush.o realpath.o mountlist.o input.o hotkeys.o \
        transform.o pversion.o factory.o $(PLATFORMOBJ) \
-	   loadsave.o loadsavefuncs.o \
+       loadsave.o loadsavefuncs.o \
        fileformats.o miscfileformats.o libraw2crtc.o \
        brush_ops.o buttons_effects.o layers.o \
        oldies.o tiles.o colorred.o unicode.o gfx2surface.o \
@@ -819,10 +819,11 @@ endif
 
 TESTSOBJS = $(patsubst %.c,%.o,$(wildcard tests/*.c)) \
             miscfileformats.o fileformats.o oldies.o libraw2crtc.o \
-			loadsavefuncs.o packbits.o \
-			unicode.o \
+            loadsavefuncs.o packbits.o \
+            op_c.o colorred.o \
+            unicode.o \
             io.o realpath.o version.o pversion.o \
-			gfx2surface.o \
+            gfx2surface.o \
             gfx2log.o gfx2mem.o
 
 OBJ = $(addprefix $(OBJDIR)/,$(OBJS))
diff --git a/src/op_c.c b/src/op_c.c
index 9e0ee8fa..e9366638 100644
--- a/src/op_c.c
+++ b/src/op_c.c
@@ -45,7 +45,9 @@
 // are sorted by length of the diagonal
 //#define GRAFX2_QUANTIZE_CLUSTER_SORT_BY_VOLUME
 
-int Convert_24b_bitmap_to_256_fast(T_Bitmap256 dest,T_Bitmap24B source,int width,int height,T_Components * palette);
+#if defined(__GP2X__) || defined(__gp2x__) || defined(__WIZ__) || defined(__CAANOO__)
+static int Convert_24b_bitmap_to_256_fast(T_Bitmap256 dest,T_Bitmap24B source,int width,int height,T_Components * palette);
+#endif
 
 /// Convert RGB to HSL.
 /// Both input and output are in the 0..255 range to use in the palette screen
@@ -1471,8 +1473,9 @@ int Convert_24b_bitmap_to_256(T_Bitmap256 dest,T_Bitmap24B source,int width,int
 
 extern void Set_palette_fake_24b(T_Palette palette);
 
+#if defined(__GP2X__) || defined(__gp2x__) || defined(__WIZ__) || defined(__CAANOO__)
 /// Really small, fast and dirty convertor(just for handhelds)
-int Convert_24b_bitmap_to_256_fast(T_Bitmap256 dest,T_Bitmap24B source,int width,int height,T_Components * palette)
+static int Convert_24b_bitmap_to_256_fast(T_Bitmap256 dest,T_Bitmap24B source,int width,int height,T_Components * palette)
 {
   int size;
 
@@ -1491,3 +1494,4 @@ int Convert_24b_bitmap_to_256_fast(T_Bitmap256 dest,T_Bitmap24B source,int width
   }
   return 0;
 }
+#endif
diff --git a/src/tests/testlist.h b/src/tests/testlist.h
index 3278f1ef..0f6842b9 100644
--- a/src/tests/testlist.h
+++ b/src/tests/testlist.h
@@ -4,3 +4,4 @@
 TEST(MOTO_MAP_pack)
 TEST(CPC_compare_colors)
 TEST(Packbits)
+TEST(Convert_24b_bitmap_to_256)
diff --git a/src/tests/testop_c.c b/src/tests/testop_c.c
new file mode 100644
index 00000000..eb61bc81
--- /dev/null
+++ b/src/tests/testop_c.c
@@ -0,0 +1,60 @@
+/* vim:expandtab:ts=2 sw=2:
+*/
+/*  Grafx2 - The Ultimate 256-color bitmap paint program
+
+    Copyright 2018-2019 Thomas Bernard
+    Copyright 2011 Pawel Góralski
+    Copyright 2009 Petter Lindquist
+    Copyright 2008 Yves Rizoud
+    Copyright 2008 Franck Charlet
+    Copyright 2007-2011 Adrien Destugues
+    Copyright 1996-2001 Sunset Design (Guillaume Dorme & Karl Maritaud)
+
+    Grafx2 is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation; version 2
+    of the License.
+
+    Grafx2 is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with Grafx2; if not, see <http://www.gnu.org/licenses/>
+*/
+///@file testop_c.c
+/// Unit tests.
+///
+#include <stdio.h>
+#include <string.h>
+#include "../op_c.h"
+#include "../gfx2log.h"
+
+int Test_Convert_24b_bitmap_to_256(void)
+{
+  T_Palette palette;
+  byte dest[256];
+  T_Components source[256];
+  int i;
+
+  // first try if a 256 colors picture is converted OK
+  for (i = 0; i < 256; i++)
+  {
+    source[i].R = i;
+    source[i].G = i;
+    source[i].B = i;
+  }
+  if (Convert_24b_bitmap_to_256(dest, source, 16, 16, palette) != 0)
+  {
+    return 0;
+  }
+  GFX2_LogHexDump(GFX2_DEBUG, "", dest, 0, 256);
+  for (i = 0; i < 256; i++)
+  {
+    if (memcmp(&source[i], &palette[dest[i]], sizeof(T_Components)) != 0)
+      return 0;
+  }
+  // TODO: test a real reduction
+  return 1;
+}