Add minimal emulation of CRTC registers.

This is needed for some programs to properly detect CGA/VGA video.

3 files changed, 45 insertions, 3 deletions

diff --git a/src/main.c b/src/main.c
index 74eabda..0a6ada3 100644
--- a/src/main.c
+++ b/src/main.c
@@ -27,8 +27,8 @@ uint8_t read_port(unsigned port)
         retrace++;
         return retrace & 0x09;
     }
-    else if(port == 0x3D5)
-        return 0;
+    else if(port == 0x3D4 || port == 0x3D5)
+        return video_crtc_read(port);
     else if(port >= 0x40 && port <= 0x43)
         return port_timer_read(port);
     else if(port >= 0x60 && port <= 0x65)
@@ -41,7 +41,10 @@ void write_port(unsigned port, uint8_t value)
 {
     if(port >= 0x40 && port <= 0x43)
         return port_timer_write(port, value);
-    debug(debug_port, "port write %04x <- %02x\n", port, value);
+    else if(port == 0x03D4 || port == 0x03D5)
+        return video_crtc_write(port, value);
+    else
+        debug(debug_port, "port write %04x <- %02x\n", port, value);
 }
 
 void emulator_update(void)
diff --git a/src/video.c b/src/video.c
index d6e405b..5a42484 100644
--- a/src/video.c
+++ b/src/video.c
@@ -608,3 +608,37 @@ void int10()
         debug(debug_video, "UNHANDLED INT 10, AX=%04x\n", ax);
     }
 }
+
+// CRTC port emulation, some software use it to fix "snow" in CGA modes.
+static uint8_t crtc_port;
+static uint16_t crtc_cursor_loc;
+
+uint8_t video_crtc_read(int port)
+{
+    if(port & 1)
+    {
+        if( crtc_port == 0x0E )
+            return crtc_cursor_loc >> 8;
+        if( crtc_port == 0x0F )
+            return crtc_cursor_loc;
+        else
+            return 0;
+    }
+    else
+        return crtc_port;
+}
+
+void video_crtc_write(int port, uint8_t value)
+{
+    if(port & 1)
+    {
+        if( crtc_port == 0x0E )
+            crtc_cursor_loc = (crtc_cursor_loc & 0xFF) | (value << 8);
+        if( crtc_port == 0x0F )
+            crtc_cursor_loc = (crtc_cursor_loc & 0xFF00) | (value);
+        else
+            debug(debug_video, "CRTC port write [%02x] <- %02x\n", crtc_port, value);
+    }
+    else
+        crtc_port = value;
+}
diff --git a/src/video.h b/src/video.h
index a24a529..7661223 100644
--- a/src/video.h
+++ b/src/video.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <stdint.h>
+
 void int10(void);
 // Redraws terminal screen
 void check_screen(void);
@@ -7,3 +9,6 @@ void check_screen(void);
 int video_active(void);
 // Writes a character to the video screen
 void video_putch(char ch);
+// CRTC port read/write
+uint8_t video_crtc_read(int port);
+void video_crtc_write(int port, uint8_t value);