From 8a7e1eb7a8bc1997f69026ccd0bc6742bac5f1a0 Mon Sep 17 00:00:00 2001
From: cracyc <cracyc@users.noreply.github.com>
Date: Wed, 13 Nov 2024 16:28:14 -0600
Subject: pc9801: bitswap egc mask just like the vram

---
 src/mame/nec/pc9801.cpp   | 9 +++++++--
 src/mame/nec/pc9801.h     | 1 +
 src/mame/nec/pc9801_v.cpp | 2 +-
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/mame/nec/pc9801.cpp b/src/mame/nec/pc9801.cpp
index 58a24ccbfb9..08fa0316f8a 100644
--- a/src/mame/nec/pc9801.cpp
+++ b/src/mame/nec/pc9801.cpp
@@ -855,10 +855,14 @@ void pc9801vm_state::pc9801rs_a0_w(offs_t offset, uint8_t data)
 
 void pc9801vm_state::egc_w(offs_t offset, uint16_t data, uint16_t mem_mask)
 {
-	if(!(m_egc.regs[1] & 0x6000) || (offset != 4)) // why?
-		COMBINE_DATA(&m_egc.regs[offset]);
+	if((m_egc.regs[1] & 0x6000) && (offset == 4)) // why?
+		return;
+	COMBINE_DATA(&m_egc.regs[offset]);
 	switch(offset)
 	{
+		case 4:
+			m_egc.mask = bitswap<16>(m_egc.regs[4],8,9,10,11,12,13,14,15,0,1,2,3,4,5,6,7);
+			break;
 		case 2:
 		case 6:
 		case 7:
@@ -2029,6 +2033,7 @@ MACHINE_START_MEMBER(pc9801vm_state,pc9801rs)
 	save_item(NAME(m_egc.leftover));
 	save_item(NAME(m_egc.first));
 	save_item(NAME(m_egc.start));
+	save_item(NAME(m_egc.mask));
 
 	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 474a660db25..21161435668 100644
--- a/src/mame/nec/pc9801.h
+++ b/src/mame/nec/pc9801.h
@@ -494,6 +494,7 @@ private:
 		uint16_t pat[4]{};
 		uint16_t src[4]{};
 		int16_t count = 0;
+		uint16_t mask;
 		uint16_t leftover[4]{};
 		bool first = false;
 		bool start = false;
diff --git a/src/mame/nec/pc9801_v.cpp b/src/mame/nec/pc9801_v.cpp
index c2691f498d0..429ec4498bd 100644
--- a/src/mame/nec/pc9801_v.cpp
+++ b/src/mame/nec/pc9801_v.cpp
@@ -720,7 +720,7 @@ uint16_t pc9801vm_state::egc_do_partial_op(int plane, uint16_t src, uint16_t pat
 
 void pc9801vm_state::egc_blit_w(uint32_t offset, uint16_t data, uint16_t mem_mask)
 {
-	uint16_t mask = m_egc.regs[4] & mem_mask, out = 0;
+	uint16_t mask = m_egc.mask & mem_mask, out = 0;
 	bool dir = !(m_egc.regs[6] & 0x1000);
 	int dst_off = (m_egc.regs[6] >> 4) & 0xf, src_off = m_egc.regs[6] & 0xf;
 	offset = (offset & 0x3fff) +  m_vram_bank * 0x10000;
-- 
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