From e695cb0e3b92c5effb2b4aaad85c1659caabf62d Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Sat, 22 May 2021 03:16:02 +0200
Subject: [PATCH] CRTC on CGA-like graphics chips is now on the entire 3D0 to
 3D7 port range, fixes scrolling in some old games.

---
 src/machine/m_amstrad.c | 6 ++++++
 src/machine/m_tandy.c   | 6 ++++++
 src/video/vid_cga.c     | 6 ++++++
 3 files changed, 18 insertions(+)

diff --git a/src/machine/m_amstrad.c b/src/machine/m_amstrad.c
index d9112c4c0..6996ab1da 100644
--- a/src/machine/m_amstrad.c
+++ b/src/machine/m_amstrad.c
@@ -228,6 +228,9 @@ vid_out_1512(uint16_t addr, uint8_t val, void *priv)
     amsvid_t *vid = (amsvid_t *)priv;
     uint8_t old;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x03d4:
 		vid->crtcreg = val & 31;
@@ -277,6 +280,9 @@ vid_in_1512(uint16_t addr, void *priv)
     amsvid_t *vid = (amsvid_t *)priv;
     uint8_t ret = 0xff;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x03d4:
 		ret = vid->crtcreg;
diff --git a/src/machine/m_tandy.c b/src/machine/m_tandy.c
index aa7b18ca9..0ec09c21f 100644
--- a/src/machine/m_tandy.c
+++ b/src/machine/m_tandy.c
@@ -520,6 +520,9 @@ vid_out(uint16_t addr, uint8_t val, void *priv)
     t1kvid_t *vid = dev->vid;
     uint8_t old;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x03d4:
 		vid->crtcreg = val & 0x1f;
@@ -589,6 +592,9 @@ vid_in(uint16_t addr, void *priv)
     t1kvid_t *vid = dev->vid;
     uint8_t ret = 0xff;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x03d4:
 		ret = vid->crtcreg;
diff --git a/src/video/vid_cga.c b/src/video/vid_cga.c
index 51e0ac6af..dda14f48e 100644
--- a/src/video/vid_cga.c
+++ b/src/video/vid_cga.c
@@ -58,6 +58,9 @@ cga_out(uint16_t addr, uint8_t val, void *p)
     cga_t *cga = (cga_t *) p;
     uint8_t old;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x3D4:
 		cga->crtcreg = val & 31;
@@ -100,6 +103,9 @@ cga_in(uint16_t addr, void *p)
 
     uint8_t ret = 0xff;
 
+    if ((addr >= 0x3d0) && (addr <= 0x3d7))
+	addr = (addr & 0xff9) | 0x004;
+
     switch (addr) {
 	case 0x3D4:
 		ret = cga->crtcreg;