From 078031f62eee3519a8f3d3efeb7375da5ed1beba Mon Sep 17 00:00:00 2001
From: cracyc <cracyc@users.noreply.github.com>
Date: Wed, 13 Nov 2024 10:31:01 -0600
Subject: [PATCH] pc9801: make sure a read by itself can load the shifter

---
 src/mame/nec/pc9801.cpp   |  6 ++++--
 src/mame/nec/pc9801.h     |  3 ++-
 src/mame/nec/pc9801_v.cpp | 21 ++++++++++++++++-----
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/mame/nec/pc9801.cpp b/src/mame/nec/pc9801.cpp
index e740c370f18..12c7f23db70 100644
--- a/src/mame/nec/pc9801.cpp
+++ b/src/mame/nec/pc9801.cpp
@@ -865,7 +865,8 @@ void pc9801vm_state::egc_w(offs_t offset, uint16_t data, uint16_t mem_mask)
 			m_egc.leftover[0] = m_egc.leftover[1] = m_egc.leftover[2] = m_egc.leftover[3] = 0;
 			m_egc.count = (m_egc.regs[7] & 0xfff) + 1;
 			m_egc.first = true;
-			m_egc.init = false;
+			m_egc.start = false;
+			m_egc.loaded = false;
 			break;
 	}
 }
@@ -1116,6 +1117,7 @@ void pc9801vm_state::pc9801ux_io(address_map &map)
 	map(0x0439, 0x0439).rw(FUNC(pc9801vm_state::dma_access_ctrl_r), FUNC(pc9801vm_state::dma_access_ctrl_w));
 	map(0x043c, 0x043f).w(FUNC(pc9801vm_state::pc9801rs_bank_w)); //ROM/RAM bank
 	map(0x04a0, 0x04af).w(FUNC(pc9801vm_state::egc_w));
+	map(0x04ae, 0x04af).lr16(NAME([this]() { return m_egc.first; }));
 	map(0x3fd8, 0x3fdf).rw(m_pit, FUNC(pit8253_device::read), FUNC(pit8253_device::write)).umask16(0xff00);
 }
 
@@ -2027,7 +2029,7 @@ MACHINE_START_MEMBER(pc9801vm_state,pc9801rs)
 	save_item(NAME(m_egc.count));
 	save_item(NAME(m_egc.leftover));
 	save_item(NAME(m_egc.first));
-	save_item(NAME(m_egc.init));
+	save_item(NAME(m_egc.start));
 
 	save_item(NAME(m_grcg.mode));
 	save_item(NAME(m_vram_bank));
diff --git a/src/mame/nec/pc9801.h b/src/mame/nec/pc9801.h
index 18a94442db3..474a660db25 100644
--- a/src/mame/nec/pc9801.h
+++ b/src/mame/nec/pc9801.h
@@ -496,7 +496,8 @@ private:
 		int16_t count = 0;
 		uint16_t leftover[4]{};
 		bool first = false;
-		bool init = false;
+		bool start = false;
+		bool loaded = false;
 	} m_egc;
 
 protected:
diff --git a/src/mame/nec/pc9801_v.cpp b/src/mame/nec/pc9801_v.cpp
index ebe9138a342..c2691f498d0 100644
--- a/src/mame/nec/pc9801_v.cpp
+++ b/src/mame/nec/pc9801_v.cpp
@@ -725,7 +725,7 @@ void pc9801vm_state::egc_blit_w(uint32_t offset, uint16_t data, uint16_t mem_mas
 	int dst_off = (m_egc.regs[6] >> 4) & 0xf, src_off = m_egc.regs[6] & 0xf;
 	offset = (offset & 0x3fff) +  m_vram_bank * 0x10000;
 
-	if(!m_egc.init && (src_off > dst_off))
+	if(!m_egc.start && (src_off > dst_off))
 	{
 		if(BIT(m_egc.regs[2], 10))
 		{
@@ -733,7 +733,7 @@ void pc9801vm_state::egc_blit_w(uint32_t offset, uint16_t data, uint16_t mem_mas
 			// leftover[0] is inited above, set others to same
 			m_egc.leftover[1] = m_egc.leftover[2] = m_egc.leftover[3] = m_egc.leftover[0];
 		}
-		m_egc.init = true;
+		m_egc.start = true;
 		return;
 	}
 
@@ -741,7 +741,7 @@ void pc9801vm_state::egc_blit_w(uint32_t offset, uint16_t data, uint16_t mem_mas
 	if(m_egc.first)
 	{
 		mask &= dir ? ~((1 << dst_off) - 1) : ((1 << (16 - dst_off)) - 1);
-		m_egc.init = true;
+		m_egc.start = true;
 	}
 
 	// mask off the bits past the end of the blit
@@ -816,7 +816,8 @@ void pc9801vm_state::egc_blit_w(uint32_t offset, uint16_t data, uint16_t mem_mas
 	if(m_egc.count <= 0)
 	{
 		m_egc.first = true;
-		m_egc.init = false;
+		m_egc.start = false;
+		m_egc.loaded = false;
 		m_egc.count = (m_egc.regs[7] & 0xfff) + 1;
 	}
 }
@@ -831,7 +832,17 @@ uint16_t pc9801vm_state::egc_blit_r(uint32_t offset, uint16_t mem_mask)
 		m_egc.pat[2] = m_video_ram[1][plane_off + (0x4000 * 3)];
 		m_egc.pat[3] = m_video_ram[1][plane_off];
 	}
-	m_egc.init = true;
+
+	if(m_egc.first && !m_egc.start && !m_egc.loaded)
+	{
+		int dst_off = (m_egc.regs[6] >> 4) & 0xf, src_off = m_egc.regs[6] & 0xf;
+		if(dst_off >= src_off) // check if it needs to be shifted into the next word
+			m_egc.start = true;
+		m_egc.loaded = true;
+	}
+	else
+		m_egc.start = true;
+
 	for(int i = 0; i < 4; i++)
 		m_egc.src[i] = egc_shift(i, m_video_ram[1][plane_off + (((i + 1) & 3) * 0x4000)]);