diff --git a/vga/README.md b/vga/README.md
index f7fc705..d22d89f 100644
--- a/vga/README.md
+++ b/vga/README.md
@@ -50,6 +50,12 @@ Then, load the bitstream to the board. The following pattern should be shown in
 
 ![VGA demo pattern](vga_demo.png)
 
+### Test
+
+Sources in [test/hdl/](test/hdl) provide a *Virtual VGA screen* based on the [VGA (RGB image buffer)](https://ghdl.github.io/ghdl-cosim/vhpidirect/examples/arrays.html#vga-rgb-image-buffer) from [ghdl.github.io/ghdl-cosim](https://ghdl.github.io/ghdl-cosim). A VGA monitor (VHDL) reads the output of the UUT and draws frames in a buffer, packing the colours of each pixel in a 32 bit `integer`. After each frame is complete, foreign subprogram `save_screenshot` is executed. Just before finishing the simulation, foreign subprogram `sim_cleanup` is executed.
+
+[test/hdl/imagemagick/](test/hdl/imagemagick) provides a backend for the virtual screen based on [Imagemagick](https://www.imagemagick.org/). `save_screenshot` saves each frame to a binary file in RGB24 format. Then, `convert` from Imagemagick is used for generating a PNG screenshot. In `sim_cleanup`, `convert` is used for merging all the PNGs into an animated GIF. Images are saved to `test/hdl/imagemagick/out/`.
+
 ## Development
 
 Currently, the only supported build system is a [Makefile](Makefile). However, we are willing to support the following managers/runners:
diff --git a/vga/src/VGA_config_pkg.vhd b/vga/src/VGA_config_pkg.vhd
index d58cdcc..0f8bf99 100644
--- a/vga/src/VGA_config_pkg.vhd
+++ b/vga/src/VGA_config_pkg.vhd
@@ -1,6 +1,6 @@
 package VGA_config is
 
-type VGA_config is record
+type VGA_config_t is record
   width  : natural; -- W: width
   height : natural; -- H: height
   rate   : natural; -- R: refresh rate
@@ -19,7 +19,7 @@ end record;
 --   __                     __
 -- _|  |XXXXXXX________XXXX|
 
-type VGA_configs_t is array (natural range <>) of VGA_config;
+type VGA_configs_t is array (natural range <>) of VGA_config_t;
 constant VGA_configs : VGA_configs_t(0 to 61) := (
 --           W      H     R        C    hS    hF    hB  vS    vF   vB      hP      vP
    0 => (  640,   350,   70,   25175,   96,   48,   16,  2,   60,  37,  false,   true ),
diff --git a/vga/src/VGA_sync_gen_cfg.vhd b/vga/src/VGA_sync_gen_cfg.vhd
index 0ed2aa0..90f4af1 100644
--- a/vga/src/VGA_sync_gen_cfg.vhd
+++ b/vga/src/VGA_sync_gen_cfg.vhd
@@ -1,11 +1,11 @@
 library ieee;
 context ieee.ieee_std_context;
 
-use work.VGA_config.all;
+use work.VGA_config.VGA_config_t;
 
 entity VGA_sync_gen_cfg is
   generic (
-    CONFIG: VGA_config
+    CONFIG: VGA_config_t
   );
   port (
     CLK:   in  std_logic;
diff --git a/vga/src/VGA_sync_gen_idx.vhd b/vga/src/VGA_sync_gen_idx.vhd
index 507adbc..d46e951 100644
--- a/vga/src/VGA_sync_gen_idx.vhd
+++ b/vga/src/VGA_sync_gen_idx.vhd
@@ -35,7 +35,7 @@ begin
   begin
     if rising_edge(CLK) then
       if RST then
-        st <= 0;
+        st <= 2;
       else
         if EN and p_tc then
           st <= 0 when st = 3 else st + 1;
diff --git a/vga/src/demo.vhd b/vga/src/demo.vhd
index 93f8008..cc5c7fb 100644
--- a/vga/src/demo.vhd
+++ b/vga/src/demo.vhd
@@ -5,7 +5,7 @@ use work.VGA_config.all;
 
 architecture demo of Design_Top is
 
-  constant cfg : VGA_config := VGA_configs(G_SCREEN);
+  constant cfg : VGA_config_t := VGA_configs(G_SCREEN);
 
   constant ymid : integer range -1 to cfg.height := cfg.height/2;
   constant xmid : integer range -1 to cfg.width  := cfg.width/2;
@@ -30,9 +30,7 @@ architecture demo of Design_Top is
 begin
 
   i_sync: entity work.VGA_sync_gen_cfg
-  generic map (
-    CONFIG => cfg
-  )
+  generic map ( cfg )
   port map (
     CLK   => CLK,
     EN    => EN,
diff --git a/vga/test/.gitignore b/vga/test/.gitignore
new file mode 100644
index 0000000..4f81b70
--- /dev/null
+++ b/vga/test/.gitignore
@@ -0,0 +1,2 @@
+!/imagemagick/out/.gitkeep
+/imagemagick/out/
\ No newline at end of file
diff --git a/vga/test/hdl/screen.vhd b/vga/test/hdl/VGA_screen.vhd
similarity index 71%
rename from vga/test/hdl/screen.vhd
rename to vga/test/hdl/VGA_screen.vhd
index 0092b71..a8cb651 100644
--- a/vga/test/hdl/screen.vhd
+++ b/vga/test/hdl/VGA_screen.vhd
@@ -1,9 +1,9 @@
 library ieee;
 context ieee.ieee_std_context;
 
-use work.vga_cfg_pkg.all;
+use work.VGA_config.all;
 
-entity vga_screen is
+entity VGA_screen is
   generic (
     G_SCREEN  : natural;
     G_WWIDTH  : natural := 0; -- X11 window width
@@ -16,8 +16,8 @@ entity vga_screen is
     RGB:   in std_logic_vector(2 downto 0);
     VID:   in std_logic
   );
-  constant cfg: VGA_config := VGA_configs(G_SCREEN);
-  package tb_pkg is new work.pkg
+  constant cfg: VGA_config_t := VGA_configs(G_SCREEN);
+  package tb_pkg is new work.VGA_screen_pkg
     generic map (
       G_WIDTH  => cfg.width,
       G_HEIGHT => cfg.height
@@ -25,7 +25,7 @@ entity vga_screen is
   use tb_pkg.all;
 end entity;
 
-architecture arch of vga_screen is
+architecture arch of VGA_screen is
 
   constant clk_period : time := (1.0/real(cfg.clk)) * 1 ms;
 
@@ -36,36 +36,10 @@ architecture arch of vga_screen is
 
 begin
 
-  proc_init: process
-  begin
-    -- X11 window size
-    if G_WWIDTH /= 0 and G_WHEIGHT /= 0 then
-      sim_init(G_WWIDTH, G_WHEIGHT);
-    else
-      sim_init(cfg.width, cfg.height);
-    end if;
-    wait;
-  end process;
-
-  proc_clk: process
-  begin
-    clk <= '0'; wait for clk_period/2;
-    clk <= '1'; wait for clk_period/2;
-  end process;
+  clk <= not clk after clk_period/2;
 
-  i_sync: entity work.vga_sync_gen
-    generic map (
-      G_HPULSE => cfg.hpulse,
-      G_HFRONT => cfg.hfront,
-      G_WIDTH  => cfg.width,
-      G_HBACK  => cfg.hback,
-      G_HPOL   => cfg.hpol,
-      G_VPULSE => cfg.vpulse,
-      G_VFRONT => cfg.vfront,
-      G_HEIGHT => cfg.height,
-      G_VBACK  => cfg.vback,
-      G_VPOL   => cfg.vpol
-    )
+  i_sync: entity work.VGA_sync_gen_cfg
+    generic map ( cfg )
     port map (
       CLK   => clk,
       EN    => '1',
@@ -115,6 +89,7 @@ begin
       if rising_edge(clk) then
         sync(1) <= sync(0);
         if sync="01" then
+          report "save screenshot " & to_string(frame) severity note;
           save_screenshot(
             screen,
             screen'length(2), --width
diff --git a/vga/test/hdl/pkg.vhd b/vga/test/hdl/VGA_screen_pkg.vhd
similarity index 82%
rename from vga/test/hdl/pkg.vhd
rename to vga/test/hdl/VGA_screen_pkg.vhd
index 226e5a9..6c53963 100644
--- a/vga/test/hdl/pkg.vhd
+++ b/vga/test/hdl/VGA_screen_pkg.vhd
@@ -1,7 +1,7 @@
 library ieee;
 context ieee.ieee_std_context;
 
-package pkg is
+package VGA_screen_pkg is
   generic (
     G_WIDTH  : natural := 640;
     G_HEIGHT : natural := 480
@@ -13,9 +13,6 @@ package pkg is
 
   shared variable screen: screen_t;
 
-  procedure sim_init (width, height: natural);
-  attribute foreign of sim_init : procedure is "VHPIDIRECT sim_init";
-
   -- TODO: pass a fat pointer, so that width and height need not to be passed explicitly
   procedure save_screenshot (
     variable ptr: screen_t;
@@ -31,13 +28,9 @@ package pkg is
 
   function RGB_to_integer (rgb: std_logic_vector(2 downto 0)) return integer;
 
-end pkg;
-
-package body pkg is
+end VGA_screen_pkg;
 
-  procedure sim_init (width, height: natural) is
-  begin report "VHPIDIRECT sim_init" severity failure;
-  end procedure;
+package body VGA_screen_pkg is
 
   procedure save_screenshot (
     variable ptr: screen_t;
@@ -65,4 +58,4 @@ package body pkg is
     return to_integer(unsigned(raw24));
   end function;
 
-end pkg;
+end VGA_screen_pkg;
diff --git a/vga/test/hdl/tb.vhd b/vga/test/hdl/VGA_tb.vhd
similarity index 79%
rename from vga/test/hdl/tb.vhd
rename to vga/test/hdl/VGA_tb.vhd
index 70761e5..303fcc2 100644
--- a/vga/test/hdl/tb.vhd
+++ b/vga/test/hdl/VGA_tb.vhd
@@ -4,15 +4,17 @@ use std.env.stop;
 library ieee;
 context ieee.ieee_std_context;
 
+use work.VGA_config.all;
+
 entity tb_vga is
   generic (
-    G_SCREEN : natural := 4
+    G_SCREEN : natural := 22
   );
 end entity;
 
 architecture arch of tb_vga is
 
-  constant clk_period : time := 20 ns;
+  constant clk_period : time := (1.0/real(VGA_configs(G_SCREEN).clk)) * 1 ms;
   signal clk, rst, save_video: std_logic := '0';
 
   type vga_t is record
@@ -25,11 +27,7 @@ architecture arch of tb_vga is
 
 begin
 
-  proc_clk: process
-  begin
-    clk <= '0'; wait for clk_period/2;
-    clk <= '1'; wait for clk_period/2;
-  end process;
+  clk <= not clk after clk_period/2;
 
   proc_main: process
   begin
@@ -37,7 +35,7 @@ begin
     rst <= '1';
     wait for 10*clk_period;
     rst <= '0';
-    wait for 35 ms;
+    wait for 100 ms;
     report "end simulation" severity note;
     save_video <= '1';
     wait for 200 ns;
@@ -45,8 +43,7 @@ begin
     wait;
   end process;
 
-
-  VIRT_VGA: entity work.vga_screen
+  VIRT_VGA: entity work.VGA_screen
     generic map (
       G_SCREEN => G_SCREEN
     )
@@ -58,8 +55,7 @@ begin
       VID   => save_video
     );
 
-
-  UUT: entity work.vga_pattern
+  UUT: entity work.Design_Top(demo)
     generic map (
       G_SCREEN => G_SCREEN
     )
diff --git a/vga/test/hdl/vga_clk.vhd b/vga/test/hdl/vga_clk.vhd
deleted file mode 100644
index 812eb9e..0000000
--- a/vga/test/hdl/vga_clk.vhd
+++ /dev/null
@@ -1,27 +0,0 @@
-library ieee;
-context ieee.ieee_std_context;
-
-entity vga_clk is
-  generic (
-    G_PERIOD: time
-  );
-  port (
-    RST:  in  std_logic;
-    CIN:  in  std_logic;
-    EIN:  in  std_logic;
-    COUT: out std_logic;
-    EOUT: out std_logic
-  );
-end entity;
-
-architecture arch of vga_clk is
-begin
-  proc_clk: process
-  begin
-    COUT <= '0';
-    wait for G_PERIOD/2;
-    COUT <= '1';
-    wait for G_PERIOD/2;
-  end process;
-  EOUT <= EIN;
-end architecture;
\ No newline at end of file
diff --git a/vga/test/caux.c b/vga/test/imagemagick/caux.c
similarity index 89%
rename from vga/test/caux.c
rename to vga/test/imagemagick/caux.c
index 89159a3..5d6cbd9 100644
--- a/vga/test/caux.c
+++ b/vga/test/imagemagick/caux.c
@@ -4,10 +4,6 @@
 #include <stdint.h>
 #include <string.h>
 
-void sim_init(uint32_t width, uint32_t height) {
-  printf("sim_init is not required, thus empty, in this implementation\n");
-}
-
 void integer_to_raw24(int32_t *ptr, uint32_t width, uint32_t height, uint8_t *raw24) {
   int x, y;
   uint32_t *q = ptr;
@@ -23,6 +19,10 @@ void integer_to_raw24(int32_t *ptr, uint32_t width, uint32_t height, uint8_t *ra
 }
 
 void save_screenshot(int32_t *ptr, uint32_t width, uint32_t height, int id) {
+  if ( id == 0 ) {
+    return;
+  }
+
   assert(ptr != NULL);
 
   uint8_t *raw24 = malloc(width * height * 3);
diff --git a/vga/test/out/.gitkeep b/vga/test/imagemagick/out/.gitkeep
similarity index 100%
rename from vga/test/out/.gitkeep
rename to vga/test/imagemagick/out/.gitkeep
diff --git a/vga/test/run.sh b/vga/test/imagemagick/run.sh
similarity index 69%
rename from vga/test/run.sh
rename to vga/test/imagemagick/run.sh
index 445ce04..b097743 100644
--- a/vga/test/run.sh
+++ b/vga/test/imagemagick/run.sh
@@ -5,10 +5,12 @@ set -e
 cd $(dirname "$0")
 
 echo "> Analyze ../src/*.vhd and ./hdl/*.vhd"
-ghdl -i --std=08 -frelaxed ../src/*.vhd ./hdl/*.vhd
+ghdl -i --std=08 -frelaxed ../../src/*.vhd ../hdl/*.vhd
 
 echo "> Build tb (with caux.c)"
 ghdl -m --std=08 -frelaxed -Wl,caux.c -o tb tb_vga
 
+rm *.o *.cf
+
 echo "> Execute tb (save wave.ghw)"
-./tb --wave=wave.ghw
+./tb --wave=out/wave.ghw