sinclair/next/specnext.cpp: Various improvements.

* Forced more updates on tilemap writes.
* Better handeling DivMMC CS instant switch.
* Pulled up unmapped value on 0x2ffd, 0x3ffd reads.
* next/specnext_layer2.cpp: Improved half-pixel rendering.

Author: holub

src/mame/sinclair/next/specnext.cpp

@@ -2413,9 +2413,11 @@ void specnext_state::reg_w(offs_t nr_wr_reg, u8 nr_wr_dat)
 		nr_6c_tm_default_attr_w(nr_wr_dat);
 		break;
 	case 0x6e:
+		m_screen->update_now();
 		nr_6e_tilemap_base_w(BIT(nr_wr_dat, 7), BIT(nr_wr_dat, 0, 6));
 		break;
 	case 0x6f:
+		m_screen->update_now();
 		nr_6f_tilemap_tiles_w(BIT(nr_wr_dat, 7), BIT(nr_wr_dat, 0, 6));
 		break;
 	case 0x70:
@@ -2693,7 +2695,7 @@ TIMER_CALLBACK_MEMBER(specnext_state::irq_off)
 
 TIMER_CALLBACK_MEMBER(specnext_state::irq_on)
 {
-	LOGINTVVV("<Frame IRQ>\n");
+	LOGINTVVV("<ULA/Frame IRQ>\n");
 	m_im2_ula->irq_w(ASSERT_LINE);
 	if (m_nr_c0_int_mode_pulse_0_im2_1 == 0)
 		m_irq_off_timer->adjust(m_maincpu->clocks_to_attotime(32));
@@ -2830,6 +2832,7 @@ u8 specnext_state::do_m1(offs_t offset)
 	m_divmmc->automap_nmi_delayed_on_w(0);
 
 	m_divmmc->cpu_m1_n_w(1);
+	m_divmmc->cpu_mreq_n_w(1);
 	m_divmmc->clock_w();
 	bank_update(0, 2);
 
@@ -3050,10 +3053,10 @@ void specnext_state::map_io(address_map &map)
 	}));
 
 	map(0x2001, 0x2001).mirror(0x0ffc).lr8(NAME([]() {
-		return /*m_nr_d8_io_trap_fdc_en ? ... :*/ 0x00;
+		return /*m_nr_d8_io_trap_fdc_en ? ... :*/ 0xff;
 	}));
 	map(0x3001, 0x3001).mirror(0x0ffc).lrw8(NAME([]() {
-		return /*m_nr_d8_io_trap_fdc_en ? ... :*/ 0x00;
+		return /*m_nr_d8_io_trap_fdc_en ? ... :*/ 0xff;
 	}), NAME([this](u8 data) {
 		if (m_nr_d8_io_trap_fdc_en)
 		{

src/mame/sinclair/next/specnext_layer2.cpp

@@ -121,17 +121,21 @@ void specnext_layer2_device::draw_256(screen_device &screen, bitmap_rgb32 &bitma
 	const u16 offset_v = m_offset_v - info[2];
 	clip.offset(offset_h, offset_v);
 	clip &= cliprect;
-	clip.setx(clip.left() & ~1, clip.right() | 1);
 
 	do_draw(
 			screen, bitmap, blendprio, clip, info, offset_h, offset_v,
-			[this, &blend_op] (u16 pen_base, const u8 *scr, u32 *pix, u8 *prio, u32 *bprio, u16 &hpos, u16 &vpos)
+			[this, &blend_op] (u16 pen_base, const u8 *scr, u32 *pix, u8 *prio, u32 *bprio, u16 &hpos, u16 &vpos, bool skip_second)
 			{
 				const u16 idx = pen_base + ((*scr + (m_palette_offset << 4)) % 0x100);
 				const rgb_t pen = palette().pen_color(idx);
 				const bool is_prio_color = m_pen_priority[idx];
-				blend_op(prio[0], pix[0], bprio[0], pen, is_prio_color);
-				blend_op(prio[1], pix[1], bprio[1], pen, is_prio_color);
+				if (hpos & 1)
+					hpos ^= 1;
+				else
+					blend_op(prio[0], pix[0], bprio[0], pen, is_prio_color);
+
+				if (!skip_second)
+					blend_op(prio[1], pix[1], bprio[1], pen, is_prio_color);
 			});
 }
 
@@ -149,7 +153,7 @@ void specnext_layer2_device::draw_16(screen_device &screen, bitmap_rgb32 &bitmap
 
 	do_draw(
 			screen, bitmap, blendprio, clip, info, offset_h, offset_v,
-			[this, &blend_op] (u16 pen_base, const u8 *scr, u32 *pix, u8 *prio, u32 *bprio, u16 &hpos, u16 &vpos)
+			[this, &blend_op] (u16 pen_base, const u8 *scr, u32 *pix, u8 *prio, u32 *bprio, u16 &hpos, u16 &vpos, bool skip_second)
 			{
 				if (hpos & 1)
 					hpos ^= 1;
@@ -161,6 +165,7 @@ void specnext_layer2_device::draw_16(screen_device &screen, bitmap_rgb32 &bitmap
 					blend_op(prio[0], pix[0], bprio[0], pen, is_prio_color);
 				}
 
+				if (!skip_second)
 				{
 					const u16 idx = (pen_base | (m_palette_offset << 4)) + (*scr & 0x0f);
 					const rgb_t pen = palette().pen_color(idx);
@@ -179,18 +184,34 @@ void specnext_layer2_device::do_draw(screen_device &screen, bitmap_rgb32 &bitmap
 	const u16 pen_base = m_layer2_palette_select ? m_palette_alt_offset : m_palette_base_offset;
 	u16 x_min = ((clip.left() - offset_h) >> 1) + m_scroll_x;
 	const bool x_scrollover = m_scroll_x >= info[0] && info[3] == 256;
-	if (x_scrollover) x_min -= info[0];
+	if (x_scrollover)
+		x_min -= info[0]; // scrolls over
+	else
+		x_min %= info[0]; // wraps around
+
 	for (u16 vpos = clip.top(); vpos <= clip.bottom(); vpos++)
 	{
-		const u16 y = (vpos - offset_v + m_scroll_y) % info[1];
+		u16 y = vpos - offset_v + m_scroll_y;
+		if (m_scroll_y >= info[1] && info[4] == 256)
+			y -= info[1]; // scrolls over
+		else
+			y %= info[1]; // wraps around
+
 		u16 x = x_min;
 		const u8 *scr = m_host_ram_ptr + (m_layer2_active_bank << 14) + (y * info[4]) + (x * info[3]);
 		u32 *pix = &(bitmap.pix(vpos, clip.left()));
 		u8 *prio = &(screen.priority().pix(vpos, clip.left()));
 		u32 *bprio = &(blendprio.pix(vpos, clip.left()));
+		if (clip.left() & 1)
+		{
+			pix -= 1;
+			prio -= 1;
+			bprio -= 1;
+		}
 		for (u16 hpos = clip.left(); hpos <= clip.right(); hpos += 2, pix += 2, prio += 2, bprio += 2)
 		{
-			plot_op(pen_base, scr, pix, prio, bprio, hpos, vpos);
+			// (hpos & 1) skips first pixel reading and decreases hpos reference.
+			plot_op(pen_base, scr, pix, prio, bprio, hpos, vpos, hpos == clip.right() && (~hpos & 1));
 
 			++x %= info[0];
 			if (x == 0  && !x_scrollover)