From a5acda1a60ed04d8a2c9b1278d30ead2c1423887 Mon Sep 17 00:00:00 2001
From: 0kmg <9137159+0kmg@users.noreply.github.com>
Date: Wed, 15 Dec 2021 08:06:00 -0900
Subject: [PATCH] machine/nes.cpp: Accomodate games that don't initialize RAM.
 (#8986)

Software list items promoted to working (nes.xml)
---------------------------------------
Minna no Taabou no Nakayoshi Daisakusen (Japan) [kmg]
---
 hash/nes.xml             | 7 +++----
 src/mame/drivers/nes.cpp | 2 +-
 src/mame/includes/nes.h  | 2 ++
 src/mame/machine/nes.cpp | 9 +++++++++
 4 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/hash/nes.xml b/hash/nes.xml
index 122ec649bc9..21093cb1898 100644
--- a/hash/nes.xml
+++ b/hash/nes.xml
@@ -25164,14 +25164,13 @@ license:CC0
 		</part>
 	</software>
 
-<!-- FIXME: This game works, but it depends on RAM at byte 0x11 being non-zero at power-up -->
-	<software name="minnatab" supported="no">
-		<description>Minna no Taabou no Nakayoshi Daisakusen (Jpn)</description>
+	<software name="minnatab">
+		<description>Minna no Taabou no Nakayoshi Daisakusen (Japan)</description>
 		<year>1991</year>
 		<publisher>Character Soft</publisher>
 		<info name="serial" value="CTS-9N"/>
 		<info name="release" value="19911122"/>
-		<info name="alt_title" value="みんなのた坊のなかよし大作戦"/>
+		<info name="alt_title" value="みんなのたあ坊のなかよし大作戦"/>
 		<part name="cart" interface="nes_cart">
 			<feature name="slot" value="cnrom" />
 			<feature name="pcb" value="HVC-CNROM" />
diff --git a/src/mame/drivers/nes.cpp b/src/mame/drivers/nes.cpp
index ee86ce570c3..20ca8933276 100644
--- a/src/mame/drivers/nes.cpp
+++ b/src/mame/drivers/nes.cpp
@@ -27,7 +27,7 @@ void nes_state::nes_vh_sprite_dma_w(address_space &space, uint8_t data)
 
 void nes_state::nes_map(address_map &map)
 {
-	map(0x0000, 0x07ff).ram().mirror(0x1800);                                               // RAM
+	map(0x0000, 0x07ff).ram().mirror(0x1800).share("mainram");                              // RAM
 	map(0x2000, 0x3fff).rw(m_ppu, FUNC(ppu2c0x_device::read), FUNC(ppu2c0x_device::write)); // PPU registers
 	map(0x4014, 0x4014).w(FUNC(nes_state::nes_vh_sprite_dma_w));                            // stupid address space hole
 	map(0x4016, 0x4016).rw(FUNC(nes_state::nes_in0_r), FUNC(nes_state::nes_in0_w));         // IN0 - input port 1
diff --git a/src/mame/includes/nes.h b/src/mame/includes/nes.h
index c05a4e5126e..72ad3edb939 100644
--- a/src/mame/includes/nes.h
+++ b/src/mame/includes/nes.h
@@ -72,6 +72,7 @@ class nes_state : public nes_base_state
 public:
 	nes_state(const machine_config &mconfig, device_type type, const char *tag) :
 		nes_base_state(mconfig, type, tag),
+		m_mainram(*this, "mainram"),
 		m_ppu(*this, "ppu"),
 		m_screen(*this, "screen"),
 		m_exp(*this, "exp"),
@@ -125,6 +126,7 @@ private:
 	uint8_t      *m_vram;
 	std::unique_ptr<uint8_t[]>    m_ciram; //PPU nametable RAM - external to PPU!
 
+	required_shared_ptr<uint8_t> m_mainram;
 
 	required_device<ppu2c0x_device> m_ppu;
 	required_device<screen_device> m_screen;
diff --git a/src/mame/machine/nes.cpp b/src/mame/machine/nes.cpp
index fd8c87848b5..8b3530cbe55 100644
--- a/src/mame/machine/nes.cpp
+++ b/src/mame/machine/nes.cpp
@@ -43,6 +43,15 @@ void nes_state::machine_start()
 {
 	address_space &space = m_maincpu->space(AS_PROGRAM);
 
+	// Fill main RAM with an arbitrary pattern (alternating 0x00/0xff) for software that depends on its contents at boot up (tsk tsk!)
+	// The fill value is a compromise since certain games malfunction with zero-filled memory, others with one-filled memory
+	// Examples: Minna no Taabou won't boot with all 0x00, Sachen's Dancing Block won't boot with all 0xff, Terminator 2 skips its copyright screen with all 0x00
+	for (int i = 0; i < 0x800; i += 2)
+	{
+		m_mainram[i] = 0x00;
+		m_mainram[i + 1] = 0xff;
+	}
+
 	// CIRAM (Character Internal RAM)
 	// NES has 2KB of internal RAM which can be used to fill the 4x1KB banks of PPU RAM at $2000-$2fff
 	// Line A10 is exposed to the carts, so that games can change CIRAM mapping in PPU (we emulate this with the set_nt_mirroring