apple2video: fix low-memory -ramsize crash by protecting HGR reads

Author: arekkusu42

src/mame/apple/apple2.cpp

@@ -315,7 +315,8 @@ void apple2_state::machine_start()
 	save_item(NAME(m_reset_latch));
 
 	// setup video pointers
-	m_video->set_ram_pointers(m_ram_ptr, m_ram_ptr);
+	m_video->set_ram_pointers(m_ram_ptr, nullptr);
+	m_video->set_ram_masks(m_ram_size-1, 0);
 	m_video->set_char_pointer(memregion("gfx1")->base(), memregion("gfx1")->bytes());
 }
 

src/mame/apple/apple2e.cpp

@@ -990,7 +990,7 @@ void apple2e_state::machine_start()
 
 	// setup video pointers
 	m_video->set_ram_pointers(m_ram_ptr, m_aux_ptr);
-	m_video->set_aux_mask(m_aux_mask);
+	m_video->set_ram_masks(0xffff, m_aux_mask);
 	m_video->set_char_pointer(memregion("gfx1")->base(), memregion("gfx1")->bytes());
 
 	// IOU events occur at the rightmost edge of active video

src/mame/apple/apple2video.cpp

@@ -833,19 +833,34 @@ void a2_video_device::hgr_update(screen_device &screen, bitmap_ind16 &bitmap, co
 	for (int row = beginrow; row <= endrow; row++)
 	{
 		unsigned const address = start_address + (((row/8) & 0x07) << 7) + (((row/8) & 0x18) * 5) + ((row & 7) << 10);
-		uint8_t const *const vram_row = &m_ram_ptr[address];
-
-		uint16_t words[40];
-
-		unsigned last_output_bit = 0;
-
-		for (int col = std::max(0, startcol-1); col < std::min(stopcol+1, 40); col++)
+		static const uint16_t empty_words[40] =
 		{
-			unsigned word = double_7_bits[vram_row[col] & 0x7f];
-			if (vram_row[col] & bit7_mask)
-				word = (word * 2 + last_output_bit) & 0x3fff;
-			words[col] = word;
-			last_output_bit = word >> 13;
+			// empty RAM socket causes TTL input to float high
+			0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff,
+			0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff,
+			0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff,
+			0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff,
+			0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff, 0x3fff
+		};
+		uint16_t ram_words[40];
+		uint16_t *words;
+
+		if (address > m_ram_mask)
+			words = (uint16_t *)empty_words;
+		else
+		{
+			uint8_t const *const vram_row = &m_ram_ptr[address];
+			unsigned last_output_bit = 0;
+			words = ram_words;
+
+			for (int col = std::max(0, startcol-1); col < std::min(stopcol+1, 40); col++)
+			{
+				unsigned word = double_7_bits[vram_row[col] & 0x7f];
+				if (vram_row[col] & bit7_mask)
+					word = (word * 2 + last_output_bit) & 0x3fff;
+				words[col] = word;
+				last_output_bit = word >> 13;
+			}
 		}
 
 		if (rgb_monitor())

src/mame/apple/apple2video.h

@@ -67,7 +67,7 @@ class a2_video_device : public device_t, public device_palette_interface, public
 
 	void set_base_model(model base_model)       { m_base_model = base_model; }
 	void set_ram_pointers(u8 *main, u8 *aux)    { m_ram_ptr = main; m_aux_ptr = aux; }
-	void set_aux_mask(u16 aux_mask)             { m_aux_mask = aux_mask; }
+	void set_ram_masks(u16 ram_mask, u16 aux_mask) { m_ram_mask = ram_mask; m_aux_mask = aux_mask; }
 	void set_hgr2(u16 hgr2)                     { m_hgr2 = hgr2; }
 	void set_char_pointer(u8 *charptr, int size) { m_char_ptr = charptr; m_char_size = size; }
 	void setup_GS_graphics() { m_8bit_graphics = std::make_unique<bitmap_ind16>(560, 192); }
@@ -120,7 +120,7 @@ class a2_video_device : public device_t, public device_palette_interface, public
 	int monochrome_hue();
 
 	u8 *m_ram_ptr = nullptr, *m_aux_ptr = nullptr, *m_char_ptr = nullptr;
-	u16 m_aux_mask = 0xffff;
+	u16 m_ram_mask = 0xffff, m_aux_mask = 0xffff;
 	u16 m_hgr2 = 0x4000;
 	int m_char_size = 0;
 	bool m_page2 = false;

src/mame/apple/superga2.cpp

@@ -100,7 +100,7 @@ void superga2_state::machine_start()
 	save_item(NAME(m_speaker_state));
 
 	// setup video pointers
-	m_video->set_ram_pointers(m_ram_ptr, m_ram_ptr);
+	m_video->set_ram_pointers(m_ram_ptr, nullptr);
 	m_video->set_char_pointer(nullptr, 0);  // no text modes on this machine
 }
 

src/mame/apple/tk2000.cpp

@@ -150,7 +150,7 @@ void tk2000_state::machine_start()
 	save_item(NAME(m_ctrl_key));
 
 	// setup video pointers
-	m_video->set_ram_pointers(m_ram_ptr, m_ram_ptr);
+	m_video->set_ram_pointers(m_ram_ptr, nullptr);
 	m_video->set_char_pointer(nullptr, 0);  // no text modes on this machine
 	m_video->set_hgr2(0xa000);
 }