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diff --git a/src/atiwonderio.c b/src/atiwonderio.c
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+++ b/src/atiwonderio.c
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+/* $XFree86: xc/programs/Xserver/hw/xfree86/drivers/ati/atiwonderio.c,v 1.4 2003/01/01 19:16:35 tsi Exp $ */
+/*
+ * Copyright 2000 through 2003 by Marc Aurele La France (TSI @ UQV), tsi@xfree86.org
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting documentation, and
+ * that the name of Marc Aurele La France not be used in advertising or
+ * publicity pertaining to distribution of the software without specific,
+ * written prior permission.  Marc Aurele La France makes no representations
+ * about the suitability of this software for any purpose.  It is provided
+ * "as-is" without express or implied warranty.
+ *
+ * MARC AURELE LA FRANCE DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS.  IN NO
+ * EVENT SHALL MARC AURELE LA FRANCE BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "ati.h"
+#include "atichip.h"
+#include "atiwonderio.h"
+
+#ifndef AVOID_CPIO
+
+/*
+ * ATIModifyExtReg --
+ *
+ * This function is called to modify certain bits in an ATI extended VGA
+ * register while preserving its other bits.  The function will not write the
+ * register if it turns out its value would not change.  This helps prevent
+ * server hangs on older adapters.
+ */
+void
+ATIModifyExtReg
+(
+    ATIPtr      pATI,
+    const CARD8 Index,
+    int         CurrentValue,
+    const CARD8 CurrentMask,
+    CARD8       NewValue
+)
+{
+    /* Possibly retrieve the current value */
+    if (CurrentValue < 0)
+        CurrentValue = ATIGetExtReg(Index);
+
+    /* Compute new value */
+    NewValue &= (CARD8)(~CurrentMask);
+    NewValue |= CurrentValue & CurrentMask;
+
+    /* Check if value will be changed */
+    if (CurrentValue == NewValue)
+        return;
+
+    /*
+     * The following is taken from ATI's VGA Wonder programmer's reference
+     * manual which says that this is needed to "ensure the proper state of the
+     * 8/16 bit ROM toggle".  I suspect a timing glitch appeared in the 18800
+     * after its die was cast.  18800-1 and later chips do not exhibit this
+     * problem.
+     */
+    if ((pATI->Chip <= ATI_CHIP_18800) && (Index == 0xB2U) &&
+       ((NewValue ^ 0x40U) & CurrentValue & 0x40U))
+    {
+        CARD8 misc = inb(R_GENMO);
+        CARD8 bb = ATIGetExtReg(0xBBU);
+
+        outb(GENMO, (misc & 0xF3U) | 0x04U | ((bb & 0x10U) >> 1));
+        CurrentValue &= (CARD8)(~0x40U);
+        ATIPutExtReg(0xB2U, CurrentValue);
+        ATIDelay(5);
+        outb(GENMO, misc);
+        ATIDelay(5);
+        if (CurrentValue != NewValue)
+            ATIPutExtReg(0xB2U, NewValue);
+    }
+    else
+        ATIPutExtReg(Index, NewValue);
+}
+
+#endif /* AVOID_CPIO */