diff --git a/source/lib/sysdep/arch/x86_x64/tests/test_topology.h b/source/lib/sysdep/arch/x86_x64/tests/test_topology.h
new file mode 100644
index 0000000000..1a4d266c07
--- /dev/null
+++ b/source/lib/sysdep/arch/x86_x64/tests/test_topology.h
@@ -0,0 +1,31 @@
+/* Copyright (C) 2009 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "lib/self_test.h"
+
+#include "lib/sysdep/arch/x86_x64/topology.h"
+
+class TestTopology : public CxxTest::TestSuite 
+{
+public:
+	void test_run()
+	{
+		// Just run the function, ignoring the return value, so
+		// Valgrind can check it's not doing anything very bad
+		cpu_topology_Detect();
+	}
+};
diff --git a/source/lib/sysdep/arch/x86_x64/topology.cpp b/source/lib/sysdep/arch/x86_x64/topology.cpp
index 5754e1703f..c14c770552 100644
--- a/source/lib/sysdep/arch/x86_x64/topology.cpp
+++ b/source/lib/sysdep/arch/x86_x64/topology.cpp
@@ -55,6 +55,7 @@ static size_t CoresPerPackage()
 
 		case X86_X64_VENDOR_AMD:
 			regs.eax = 0x80000008;
+			regs.ecx = 0;
 			if(x86_x64_cpuid(&regs))
 				coresPerPackage = bits(regs.ecx, 0, 7)+1;
 			break;
@@ -93,6 +94,7 @@ static size_t LogicalPerCore()
 		{
 			x86_x64_CpuidRegs regs;
 			regs.eax = 1;
+			regs.ecx = 0;
 			if(!x86_x64_cpuid(&regs))
 				DEBUG_WARN_ERR(ERR::CPU_FEATURE_MISSING);
 			const size_t logicalPerPackage = bits(regs.ebx, 16, 23);
diff --git a/source/lib/sysdep/arch/x86_x64/x86_x64.cpp b/source/lib/sysdep/arch/x86_x64/x86_x64.cpp
index daf7f850d5..8ead4790f4 100644
--- a/source/lib/sysdep/arch/x86_x64/x86_x64.cpp
+++ b/source/lib/sysdep/arch/x86_x64/x86_x64.cpp
@@ -138,6 +138,7 @@ static x86_x64_Vendors DetectVendor()
 {
 	x86_x64_CpuidRegs regs;
 	regs.eax = 0;
+	regs.ecx = 0;
 	if(!x86_x64_cpuid(&regs))
 		DEBUG_WARN_ERR(ERR::CPU_FEATURE_MISSING);
 
@@ -173,6 +174,7 @@ static void DetectSignature(size_t* model, size_t* family)
 {
 	x86_x64_CpuidRegs regs;
 	regs.eax = 1;
+	regs.ecx = 0;
 	if(!x86_x64_cpuid(&regs))
 		DEBUG_WARN_ERR(ERR::CPU_FEATURE_MISSING);
 	*model  = bits(regs.eax, 4, 7);
@@ -387,6 +389,7 @@ static void DetectCacheAndTLB()
 	x86_x64_CpuidRegs regs;
 
 	regs.eax = 0x80000005;
+	regs.ecx = 0;
 	if(x86_x64_cpuid(&regs))
 	{
 		AddTLB1Parameters(regs);
@@ -601,6 +604,7 @@ static void DetectTLB_CPUID2()
 	// extract descriptors
 	x86_x64_CpuidRegs regs;
 	regs.eax = 2;
+	regs.ecx = 0;
 	if(!x86_x64_cpuid(&regs))
 		return;
 	size_t iterations = bits(regs.eax, 0, 7);
@@ -748,6 +752,7 @@ static void DetectIdentifierString(char* identifierString, size_t maxChars)
 	{
 		x86_x64_CpuidRegs regs;
 		regs.eax = function;
+		regs.ecx = 0;
 		have_brand_string &= x86_x64_cpuid(&regs);
 		memcpy(pos, &regs, 16);
 		pos += 16;
@@ -830,6 +835,7 @@ u8 x86_x64_ApicId()
 {
 	x86_x64_CpuidRegs regs;
 	regs.eax = 1;
+	regs.ecx = 0;
 	// note: CPUID function 1 should be available everywhere, but only
 	// processors with an xAPIC (8th generation or above, e.g. P4/Athlon XP)
 	// will return a nonzero value.
@@ -870,6 +876,7 @@ void cpu_Serialize()
 {
 	x86_x64_CpuidRegs regs;
 	regs.eax = 1;
+	regs.ecx = 0;
 	x86_x64_cpuid(&regs);	// CPUID serializes execution.
 }