skyfox.cpp : Improve background based on PCB (#5976)

* skyfox.cpp : Improve background based on PCB

The previous background implementation was completely wrong.
I found stars position stored in memory.
I used this to rewrite the background drawing code.

Reference PCB video:
- https://www.youtube.com/watch?v=oyE1CfcVrDc
- https://www.youtube.com/watch?v=5QhTNrNH-CU

* Remove debug code.

* Organize code and adjust display

- Split background VRAM and work RAM from addresses map
- Rename DECLARE_WRITE8_MEMBER
- Remove unused interupt declarations
- Adjust background star display

* Fixed problems with flipscreen

- Adjust sprite position
- Remove background flip

src/mame/drivers/skyfox.cpp

@@ -33,7 +33,7 @@ Verified Dip locations and recommended settings with manual
 
 ***************************************************************************/
 
-WRITE8_MEMBER(skyfox_state::skyfox_vregs_w)
+WRITE8_MEMBER(skyfox_state::output_w)
 {
 	switch (offset)
 	{
@@ -55,11 +55,12 @@ void skyfox_state::skyfox_map(address_map &map)
 	map(0x0000, 0xbfff).rom();
 	map(0xc000, 0xcfff).ram();
 	map(0xd000, 0xd3ff).ram().share("spriteram");
-	map(0xd400, 0xdfff).ram(); // ?
+	map(0xd400, 0xd4ff).ram().share("bgram"); // For background stars
+	map(0xd500, 0xdfff).ram();
 	map(0xe000, 0xe000).portr("INPUTS");
 	map(0xe001, 0xe001).portr("DSW0");
 	map(0xe002, 0xe002).portr("DSW1");
-	map(0xe008, 0xe00f).w(FUNC(skyfox_state::skyfox_vregs_w));
+	map(0xe008, 0xe00f).w(FUNC(skyfox_state::output_w));
 	map(0xf001, 0xf001).portr("DSW2");
 }
 
@@ -204,21 +205,13 @@ GFXDECODE_END
 
 /* Scroll the background on every vblank (guess). */
 
-INTERRUPT_GEN_MEMBER(skyfox_state::skyfox_interrupt)
-{
-	/* Scroll the bg */
-	m_bg_pos += (m_bg_ctrl >> 1) & 0x7; // maybe..
-}
-
 void skyfox_state::machine_start()
 {
-	save_item(NAME(m_bg_pos));
 	save_item(NAME(m_bg_ctrl));
 }
 
 void skyfox_state::machine_reset()
 {
-	m_bg_pos = 0;
 	m_bg_ctrl = 0;
 }
 
@@ -227,7 +220,6 @@ void skyfox_state::skyfox(machine_config &config)
 	/* basic machine hardware */
 	Z80(config, m_maincpu, XTAL(8'000'000)/2); /* Verified at 4MHz */
 	m_maincpu->set_addrmap(AS_PROGRAM, &skyfox_state::skyfox_map);
-	m_maincpu->set_vblank_int("screen", FUNC(skyfox_state::skyfox_interrupt));
 
 	Z80(config, m_audiocpu, XTAL(14'318'181)/8); /* Verified at 1.789772MHz */
 	m_audiocpu->set_addrmap(AS_PROGRAM, &skyfox_state::skyfox_sound_map);
@@ -242,7 +234,7 @@ void skyfox_state::skyfox(machine_config &config)
 	m_screen->set_palette(m_palette);
 
 	GFXDECODE(config, m_gfxdecode, m_palette, gfx_skyfox);
-	PALETTE(config, m_palette, FUNC(skyfox_state::skyfox_palette), 256+256); // 256 static colors (+256 for the background??)
+	PALETTE(config, m_palette, FUNC(skyfox_state::skyfox_palette), 256); // 256 static colors
 
 	/* sound hardware */
 	SPEAKER(config, "mono").front_center();

src/mame/includes/skyfox.h

@@ -25,7 +25,8 @@ public:
 		m_screen(*this, "screen"),
 		m_palette(*this, "palette"),
 		m_soundlatch(*this, "soundlatch"),
-		m_spriteram(*this, "spriteram")
+		m_spriteram(*this, "spriteram"),
+		m_bgram(*this, "bgram")
 	{ }
 
 	void skyfox(machine_config &config);
@@ -43,16 +44,15 @@ private:
 	required_device<palette_device> m_palette;
 	required_device<generic_latch_8_device> m_soundlatch;
 	required_shared_ptr<uint8_t> m_spriteram;
+	required_shared_ptr<uint8_t> m_bgram;
 
-	int m_bg_pos;
 	int m_bg_ctrl;
 
-	DECLARE_WRITE8_MEMBER(skyfox_vregs_w);
+	DECLARE_WRITE8_MEMBER(output_w);
 	virtual void machine_start() override;
 	virtual void machine_reset() override;
 	void skyfox_palette(palette_device &palette) const;
 	uint32_t screen_update_skyfox(screen_device &screen, bitmap_ind16 &bitmap, const rectangle &cliprect);
-	INTERRUPT_GEN_MEMBER(skyfox_interrupt);
 	void draw_sprites( bitmap_ind16 &bitmap, const rectangle &cliprect );
 	void draw_background(bitmap_ind16 &bitmap, const rectangle &cliprect);
 

src/mame/video/skyfox.cpp

@@ -83,10 +83,6 @@ void skyfox_state::skyfox_palette(palette_device &palette) const
 
 	compute_res_net_all(rgb, color_prom, skyfox_decode_info, skyfox_net_info);
 	palette.set_pen_colors(0, rgb);
-
-	// Grey scale for the background??? is wrong
-	for (int i = 0; i < 256; i++)
-		palette.set_pen_color(i + 256, rgb_t(i, i, i));
 }
 
 
@@ -153,8 +149,8 @@ void skyfox_state::draw_sprites( bitmap_ind16 &bitmap, const rectangle &cliprect
 
 		if (m_bg_ctrl & 1) // flipscreen
 		{
-			x = width  - x - (n - 1) * 8;
-			y = height - y - (n - 1) * 8;
+			x = width  - x - n * 8;
+			y = height - y - n * 8;
 			flipx = !flipx;
 			flipy = !flipy;
 		}
@@ -196,26 +192,31 @@ void skyfox_state::draw_background(bitmap_ind16 &bitmap, const rectangle &clipre
 {
 	uint8_t *rom = memregion("gfx2")->base();
 
-	/* The foreground stars (sprites) move at twice this speed when
-	   the bg scroll rate [e.g. (m_bg_ctrl >> 1) & 7] is 4 */
-	int pos = (m_bg_pos >> 4) & (512 * 2 - 1);
+	/* Flashing stops until the first star moves after turning on the power */
+	bool shining = (m_bg_ctrl & 0x8);
+	/* Maybe star pattern change. This will change at some intervals or when restart */
+	int pattern = (m_bg_ctrl & 0x6) >> 1;
 
 	for (int i = 0; i < 0x1000; i++)
 	{
-		int offs = (i * 2 + ((m_bg_ctrl >> 4) & 0x3) * 0x2000) % 0x8000;
+		/* contains the position of stars from 0xd4e0 in RAM */
+		int ramoffset = 0xe0 + (i & 0xf) * 2;
+		int pos = m_bgram[ramoffset + 1] * 2 + ((m_bgram[ramoffset] & 0x80) ? 1 : 0);
 
+		/* ROM offset of star pattern */
+		int offs = (i * 2) % 0x2000 + pattern * 0x2000;
+
+		/* Looks like good, but was written with intuition, Probably not perfect */
 		int pen = rom[offs];
-		int x = rom[offs + 1] * 2 + (i & 1) + pos + ((i & 8) ? 512 : 0);
-		int y = ((i / 8) / 2) * 8 + (i % 8);
+		int x = rom[offs + 1] * 2 + ((i >> 4) & 1) + pos;
+		int y = (i >> 4);
+		x += 0x60; // Adjustment based on PCB display
 
-		if (m_bg_ctrl & 1) // flipscreen
-		{
-			x = 512 * 2 - (x % (512 * 2));
-			y = 256     - (y % 256);
-		}
+		// When flipscreen is enabled, direction of background scroll is reversed by the in-game subroutine.
+		// This PCB seems does not support background flip.
 
-		for (int j = 0; j <= ((pen & 0x80) ? 0 : 3); j++)
-			bitmap.pix16((((j / 2) & 1) + y) % 256, ((j & 1) + x) % 512) = 256 + (pen & 0x7f);
+		if (((m_bg_ctrl >> 4) & 3) != (pen & 3) || !shining)
+			bitmap.pix16(y % 256, x % 512) = pen;
 	}
 }