From c24080c92a6984e37223e9c7592919913fbce91e Mon Sep 17 00:00:00 2001
From: Volker Ruppert <Volker.Ruppert@t-online.de>
Date: Sun, 8 Apr 2012 14:21:18 +0000
Subject: [PATCH] - more accurate horizontal retrace emulation (based on the
 DOSBox implementation)

---
 bochs/iodev/vgacore.cc | 12 ++++++++++--
 bochs/iodev/vgacore.h  |  2 ++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/bochs/iodev/vgacore.cc b/bochs/iodev/vgacore.cc
index c46530680..680c60dd6 100644
--- a/bochs/iodev/vgacore.cc
+++ b/bochs/iodev/vgacore.cc
@@ -427,7 +427,7 @@ void bx_vgacore_c::determine_screen_dimensions(unsigned *piHeight, unsigned *piW
 void bx_vgacore_c::calculate_retrace_timing()
 {
   Bit32u dot_clock[4] = {25175000, 28322000, 25175000, 25175000};
-  Bit32u htotal, clock, cwidth, vtotal, vrstart, vrend, hfreq, vfreq;
+  Bit32u htotal, hbstart, hbend, clock, cwidth, vtotal, vrstart, vrend, hfreq, vfreq;
 
   htotal = BX_VGA_THIS s.CRTC.reg[0] + 5;
   htotal <<= BX_VGA_THIS s.x_dotclockdiv2;
@@ -435,6 +435,11 @@ void bx_vgacore_c::calculate_retrace_timing()
   clock = dot_clock[BX_VGA_THIS s.misc_output.clock_select];
   hfreq = clock / (htotal * cwidth);
   BX_VGA_THIS s.htotal_usec = 1000000 / hfreq;
+  hbstart = BX_VGA_THIS s.CRTC.reg[2];
+  BX_VGA_THIS s.hbstart_usec = (1000000 * hbstart * cwidth) / clock;
+  hbend = (BX_VGA_THIS s.CRTC.reg[3] & 0x1f) + ((BX_VGA_THIS s.CRTC.reg[5] & 0x80) >> 2);
+  hbend = hbstart + ((hbend - hbstart) & 0x3f);
+  BX_VGA_THIS s.hbend_usec = (1000000 * hbend * cwidth) / clock;
   vtotal = BX_VGA_THIS s.CRTC.reg[6] + ((BX_VGA_THIS s.CRTC.reg[7] & 0x01) << 8) +
            ((BX_VGA_THIS s.CRTC.reg[7] & 0x20) << 4) + 2;
   vrstart = BX_VGA_THIS s.CRTC.reg[16] + ((BX_VGA_THIS s.CRTC.reg[7] & 0x04) << 6) +
@@ -512,7 +517,8 @@ Bit32u bx_vgacore_c::read(Bit32u address, unsigned io_len)
         retval |= 0x01;
       } else {
         line_usec = display_usec % BX_VGA_THIS s.htotal_usec;
-        if (line_usec >= (BX_VGA_THIS s.htotal_usec - 2)) {
+        if ((line_usec >= BX_VGA_THIS s.hbstart_usec) &&
+            (line_usec <= BX_VGA_THIS s.hbend_usec)) {
           retval |= 0x01;
         }
       }
@@ -1201,6 +1207,8 @@ void bx_vgacore_c::write(Bit32u address, Bit32u value, unsigned io_len, bx_bool
         switch (BX_VGA_THIS s.CRTC.address) {
           case 0x00:
           case 0x02:
+          case 0x03:
+          case 0x05:
           case 0x06:
           case 0x10:
             calculate_retrace_timing();
diff --git a/bochs/iodev/vgacore.h b/bochs/iodev/vgacore.h
index bcd222f04..9b9f20962 100644
--- a/bochs/iodev/vgacore.h
+++ b/bochs/iodev/vgacore.h
@@ -209,6 +209,8 @@ protected:
     bx_bool y_doublescan;
     // h/v retrace timing
     Bit32u htotal_usec;
+    Bit32u hbstart_usec;
+    Bit32u hbend_usec;
     Bit32u vtotal_usec;
     Bit32u vblank_usec;
     Bit32u vrstart_usec;