From fde960954271070e9473b4ba61b79694c59f09e2 Mon Sep 17 00:00:00 2001
From: Aleksandar Andrejevic <aandrejevic@reactos.org>
Date: Sat, 2 Nov 2013 01:51:27 +0000
Subject: [PATCH] [NTVDM] Fix resolution calculation when scanline doubling is
 enabled.

svn path=/branches/ntvdm/; revision=60827
---
 subsystems/ntvdm/vga.c | 6 ++++++
 subsystems/ntvdm/vga.h | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/subsystems/ntvdm/vga.c b/subsystems/ntvdm/vga.c
index d4616a789b2..d788b84e669 100644
--- a/subsystems/ntvdm/vga.c
+++ b/subsystems/ntvdm/vga.c
@@ -802,6 +802,12 @@ COORD VgaGetDisplayResolution(VOID)
         if (VgaAcRegisters[VGA_AC_CONTROL_REG] & VGA_AC_CONTROL_8BIT) Resolution.X /= 2;
     }
 
+    if (VgaCrtcRegisters[VGA_CRTC_MAX_SCAN_LINE_REG] & VGA_CRTC_MAXSCANLINE_DOUBLE)
+    {
+        /* Halve the vertical resolution */
+        Resolution.Y >>= 1;
+    }
+
     /* Divide the vertical resolution by the maximum scan line (== font size in text mode) */
     Resolution.Y /= MaximumScanLine;
 
diff --git a/subsystems/ntvdm/vga.h b/subsystems/ntvdm/vga.h
index 23b5a3a98d3..4fff78e469d 100644
--- a/subsystems/ntvdm/vga.h
+++ b/subsystems/ntvdm/vga.h
@@ -80,6 +80,9 @@
 /* CRTC underline register bits */
 #define VGA_CRTC_UNDERLINE_DWORD (1 << 6)
 
+/* CRTC max scanline register bits */
+#define VGA_CRTC_MAXSCANLINE_DOUBLE (1 << 7)
+
 /* CRTC mode control register bits */
 #define VGA_CRTC_MODE_CONTROL_WRAP (1 << 5)
 #define VGA_CRTC_MODE_CONTROL_BYTE (1 << 6)