diff -ruN mozilla.old/nsprpub/configure mozilla/nsprpub/configure
--- mozilla.old/nsprpub/configure	2003-05-19 15:05:42.000000000 -0400
+++ mozilla/nsprpub/configure	2003-09-27 16:20:55.085325144 -0400
@@ -3729,6 +3729,9 @@
     ia64)
         PR_MD_ASFILES=os_Linux_ia64.s
         ;;
+    x86_64)
+        PR_MD_ASFILES=os_Linux_x86_64.s
+        ;;
     m68k)
         CFLAGS="$CFLAGS -m68020-60"
         CXXFLAGS="$CXXFLAGS -m68020-60"
diff -ruN mozilla.old/nsprpub/configure.in mozilla/nsprpub/configure.in
--- mozilla.old/nsprpub/configure.in	2003-05-19 15:05:48.000000000 -0400
+++ mozilla/nsprpub/configure.in	2003-09-27 16:20:55.077326360 -0400
@@ -1218,6 +1218,9 @@
     ia64)
         PR_MD_ASFILES=os_Linux_ia64.s
         ;;
+    x86_64)
+        PR_MD_ASFILES=os_Linux_x86_64.s
+        ;;
     m68k)
         CFLAGS="$CFLAGS -m68020-60"
         CXXFLAGS="$CXXFLAGS -m68020-60"
diff -ruN mozilla.old/nsprpub/pr/include/md/_linux.cfg mozilla/nsprpub/pr/include/md/_linux.cfg
--- mozilla.old/nsprpub/pr/include/md/_linux.cfg	2003-02-26 18:53:38.000000000 -0500
+++ mozilla/nsprpub/pr/include/md/_linux.cfg	2003-09-27 16:20:55.053330008 -0400
@@ -182,6 +182,52 @@
 #define PR_BYTES_PER_WORD_LOG2  3
 #define PR_BYTES_PER_DWORD_LOG2 3
 
+#elif defined(__x86_64__)
+
+#define IS_LITTLE_ENDIAN 1
+#undef  IS_BIG_ENDIAN
+#define IS_64
+
+#define PR_BYTES_PER_BYTE   1
+#define PR_BYTES_PER_SHORT  2
+#define PR_BYTES_PER_INT    4
+#define PR_BYTES_PER_INT64  8
+#define PR_BYTES_PER_LONG   8
+#define PR_BYTES_PER_FLOAT  4
+#define PR_BYTES_PER_DOUBLE 8
+#define PR_BYTES_PER_WORD   8
+#define PR_BYTES_PER_DWORD  8
+
+#define PR_BITS_PER_BYTE    8
+#define PR_BITS_PER_SHORT   16
+#define PR_BITS_PER_INT     32
+#define PR_BITS_PER_INT64   64
+#define PR_BITS_PER_LONG    64
+#define PR_BITS_PER_FLOAT   32
+#define PR_BITS_PER_DOUBLE  64
+#define PR_BITS_PER_WORD    64
+
+#define PR_BITS_PER_BYTE_LOG2   3
+#define PR_BITS_PER_SHORT_LOG2  4
+#define PR_BITS_PER_INT_LOG2    5
+#define PR_BITS_PER_INT64_LOG2  6
+#define PR_BITS_PER_LONG_LOG2   6
+#define PR_BITS_PER_FLOAT_LOG2  5
+#define PR_BITS_PER_DOUBLE_LOG2 6
+#define PR_BITS_PER_WORD_LOG2   6
+
+#define PR_ALIGN_OF_SHORT   2
+#define PR_ALIGN_OF_INT     4
+#define PR_ALIGN_OF_LONG    8
+#define PR_ALIGN_OF_INT64   8
+#define PR_ALIGN_OF_FLOAT   4
+#define PR_ALIGN_OF_DOUBLE  8
+#define PR_ALIGN_OF_POINTER 8
+#define PR_ALIGN_OF_WORD    8
+
+#define PR_BYTES_PER_WORD_LOG2  3
+#define PR_BYTES_PER_DWORD_LOG2 3
+
 #elif defined(__mc68000__)
 
 #undef  IS_LITTLE_ENDIAN
diff -ruN mozilla.old/nsprpub/pr/include/md/_linux.h mozilla/nsprpub/pr/include/md/_linux.h
--- mozilla.old/nsprpub/pr/include/md/_linux.h	2003-02-26 18:53:39.000000000 -0500
+++ mozilla/nsprpub/pr/include/md/_linux.h	2003-09-27 16:20:55.053330008 -0400
@@ -49,6 +49,8 @@
 #define _PR_SI_ARCHITECTURE "alpha"
 #elif defined(__ia64__)
 #define _PR_SI_ARCHITECTURE "ia64"
+#elif defined(__x86_64__)
+#define _PR_SI_ARCHITECTURE "x86-64"
 #elif defined(__mc68000__)
 #define _PR_SI_ARCHITECTURE "m68k"
 #elif defined(__sparc__)
@@ -107,6 +109,19 @@
 #define _MD_ATOMIC_SET                _PR_ia64_AtomicSet
 #endif
 
+#if defined(__x86_64__)
+#define _PR_HAVE_ATOMIC_OPS
+#define _MD_INIT_ATOMIC()
+extern PRInt32 _PR_x86_64_AtomicIncrement(PRInt32 *val);
+#define _MD_ATOMIC_INCREMENT          _PR_x86_64_AtomicIncrement
+extern PRInt32 _PR_x86_64_AtomicDecrement(PRInt32 *val);
+#define _MD_ATOMIC_DECREMENT          _PR_x86_64_AtomicDecrement
+extern PRInt32 _PR_x86_64_AtomicAdd(PRInt32 *ptr, PRInt32 val);
+#define _MD_ATOMIC_ADD                _PR_x86_64_AtomicAdd
+extern PRInt32 _PR_x86_64_AtomicSet(PRInt32 *val, PRInt32 newval);
+#define _MD_ATOMIC_SET                _PR_x86_64_AtomicSet
+#endif
+
 #define USE_SETJMP
 #if defined(__GLIBC__) && __GLIBC__ >= 2
 #define _PR_POLL_AVAILABLE
diff -ruN mozilla.old/nsprpub/pr/src/io/prprf.c mozilla/nsprpub/pr/src/io/prprf.c
--- mozilla.old/nsprpub/pr/src/io/prprf.c	2003-02-26 18:53:42.000000000 -0500
+++ mozilla/nsprpub/pr/src/io/prprf.c	2003-09-27 16:20:55.068327728 -0400
@@ -51,7 +51,10 @@
 ** Note: on some platforms va_list is defined as an array,
 ** and requires array notation.
 */
-#if (defined(LINUX) && defined(__powerpc__)) || \
+#if (defined(LINUX) && defined(__x86_64__))
+#include <stdarg.h>
+#define VARARGS_ASSIGN(foo, bar) __va_copy((foo), (bar))
+#elif (defined(LINUX) && defined(__powerpc__)) || \
     (defined(LINUX) && defined(__s390__)) || \
     (defined(LINUX) && defined(__s390x__)) || \
     defined(WIN16) || defined(QNX) || \
diff -ruN mozilla.old/nsprpub/pr/src/md/unix/os_Linux_x86_64.s mozilla/nsprpub/pr/src/md/unix/os_Linux_x86_64.s
--- mozilla.old/nsprpub/pr/src/md/unix/os_Linux_x86_64.s	1969-12-31 19:00:00.000000000 -0500
+++ mozilla/nsprpub/pr/src/md/unix/os_Linux_x86_64.s	2003-09-27 16:20:55.068327728 -0400
@@ -0,0 +1,91 @@
+/ -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+/ 
+/ The contents of this file are subject to the Mozilla Public
+/ License Version 1.1 (the "License"); you may not use this file
+/ except in compliance with the License. You may obtain a copy of
+/ the License at http://www.mozilla.org/MPL/
+/ 
+/ Software distributed under the License is distributed on an "AS
+/ IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
+/ implied. See the License for the specific language governing
+/ rights and limitations under the License.
+/ 
+/ The Original Code is the Netscape Portable Runtime (NSPR).
+/ 
+/ The Initial Developer of the Original Code is Netscape
+/ Communications Corporation.  Portions created by Netscape are 
+/ Copyright (C) 2000 Netscape Communications Corporation.  All
+/ Rights Reserved.
+/ 
+/ Contributor(s):
+/ 
+/ Alternatively, the contents of this file may be used under the
+/ terms of the GNU General Public License Version 2 or later (the
+/ "GPL"), in which case the provisions of the GPL are applicable 
+/ instead of those above.  If you wish to allow use of your 
+/ version of this file only under the terms of the GPL and not to
+/ allow others to use your version of this file under the MPL,
+/ indicate your decision by deleting the provisions above and
+/ replace them with the notice and other provisions required by
+/ the GPL.  If you do not delete the provisions above, a recipient
+/ may use your version of this file under either the MPL or the
+/ GPL.
+/ 
+
+/ PRInt32 _PR_x86_64_AtomicIncrement(PRInt32 *val)
+/
+/ Atomically increment the integer pointed to by 'val' and return
+/ the result of the increment.
+/
+    .text
+    .globl _PR_x86_64_AtomicIncrement
+    .align 4
+_PR_x86_64_AtomicIncrement:
+    movl $1, %eax
+    lock
+    xaddl %eax, (%rdi)
+    incl %eax
+    ret
+
+/ PRInt32 _PR_x86_64_AtomicDecrement(PRInt32 *val)
+/
+/ Atomically decrement the integer pointed to by 'val' and return
+/ the result of the decrement.
+/
+    .text
+    .globl _PR_x86_64_AtomicDecrement
+    .align 4
+_PR_x86_64_AtomicDecrement:
+    movl $-1, %eax
+    lock
+    xaddl %eax, (%rdi)
+    decl %eax
+    ret
+
+/ PRInt32 _PR_x86_64_AtomicSet(PRInt32 *val, PRInt32 newval)
+/
+/ Atomically set the integer pointed to by 'val' to the new
+/ value 'newval' and return the old value.
+    .text
+    .globl _PR_x86_64_AtomicSet
+    .align 4
+_PR_x86_64_AtomicSet:
+    movl %esi, %eax
+    lock
+    xchgl %eax, (%rdi)
+    ret
+
+/ PRInt32 _PR_x86_64_AtomicAdd(PRInt32 *ptr, PRInt32 val)
+/
+/ Atomically add 'val' to the integer pointed to by 'ptr'
+/ and return the result of the addition.
+/
+    .text
+    .globl _PR_x86_64_AtomicAdd
+    .align 4
+_PR_x86_64_AtomicAdd:
+    movl %esi, %eax
+    lock
+    xaddl %eax, (%rdi)
+    addl %esi, %eax
+    ret