From 14c847d681c53f2cb48f6c35fa2eedc5aac53620 Mon Sep 17 00:00:00 2001
From: AJR <ajrhacker@users.noreply.github.com>
Date: Tue, 13 Dec 2022 10:42:59 -0500
Subject: [PATCH] mc6843: Clear STRB bits when read (mostly reverts
 05803c61309319e5a6dd86d074620a3b324be7ec)

---
 src/devices/machine/mc6843.cpp | 35 ++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/src/devices/machine/mc6843.cpp b/src/devices/machine/mc6843.cpp
index 20cfccdb5bd..a961f83fcca 100644
--- a/src/devices/machine/mc6843.cpp
+++ b/src/devices/machine/mc6843.cpp
@@ -126,6 +126,10 @@ void mc6843_device::dor_w(u8 data)
 	m_dor = data;
 	m_dor_loaded = true;
 	logerror("dor_w %02x\n", m_dor);
+
+	m_strb &= ~SB_DTERR;
+	if(m_strb == 0)
+		m_isr &= 0x7;
 }
 
 u8 mc6843_device::ctar_r()
@@ -238,18 +242,22 @@ void mc6843_device::sur_w(u8 data)
 
 u8 mc6843_device::strb_r()
 {
+	u8 strb = m_strb;
 	if(!machine().side_effects_disabled()) {
-		logerror("strb_r %02x -%s%s%s%s%s%s%s%s\n", m_strb,
-				 m_strb & SB_HERR  ? " herr" : "",
-				 m_strb & SB_WERR  ? " werr" : "",
-				 m_strb & SB_FI    ? " fi" : "",
-				 m_strb & SB_SERR  ? " serr" : "",
-				 m_strb & SB_SAERR ? " saerr" : "",
-				 m_strb & SB_DMERR ? " dmerr" : "",
-				 m_strb & SB_CRC   ? " crc" : "",
-				 m_strb & SB_DTERR ? " dterr" : "");
+		logerror("strb_r %02x -%s%s%s%s%s%s%s%s\n", strb,
+				 strb & SB_HERR  ? " herr" : "",
+				 strb & SB_WERR  ? " werr" : "",
+				 strb & SB_FI    ? " fi" : "",
+				 strb & SB_SERR  ? " serr" : "",
+				 strb & SB_SAERR ? " saerr" : "",
+				 strb & SB_DMERR ? " dmerr" : "",
+				 strb & SB_CRC   ? " crc" : "",
+				 strb & SB_DTERR ? " dterr" : "");
+		m_strb &= ~(SB_HERR|SB_WERR|SB_SERR|SB_SAERR|SB_DMERR|SB_CRC|SB_DTERR);
+		if(m_strb == 0)
+			m_isr &= 0x7;
 	}
-	return m_strb;
+	return strb;
 }
 
 void mc6843_device::sar_w(u8 data)
@@ -308,10 +316,6 @@ TIMER_CALLBACK_MEMBER(mc6843_device::update_tick)
 
 void mc6843_device::command_start()
 {
-	// clear errors
-	m_strb = 0;
-	m_isr &= 0x7;
-
 	assert(m_state == S_IDLE);
 	switch(m_cmr & 0xf) {
 
@@ -346,6 +350,9 @@ void mc6843_device::command_start()
 	case C_SSR: case C_RCR: case C_SSW: case C_SWD: case C_MSR: case C_MSW:
 		m_stra |= SA_BUSY;
 		m_stra &= ~(SA_DDM|SA_TNEQ);
+		m_strb &= ~SB_DMERR;
+		if(m_strb == 0)
+			m_isr &= 0x7;
 		m_state = S_SRW_WAIT_READY;
 		break;