Index: /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp
===================================================================
--- /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp	(revision 61650)
+++ /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp	(revision 61651)
@@ -8908,4 +8908,6 @@
 #define IEM_MC_STORE_XREG_U128(a_iXReg, a_u128Value) \
     do { pIemCpu->CTX_SUFF(pCtx)->CTX_SUFF(pXState)->x87.aXMM[(a_iXReg)].xmm = (a_u128Value); } while (0)
+#define IEM_MC_STORE_XREG_U64(a_iXReg, a_u64Value) \
+    do { pIemCpu->CTX_SUFF(pCtx)->CTX_SUFF(pXState)->x87.aXMM[(a_iXReg)].au64[0] = (a_u64Value); } while (0)
 #define IEM_MC_STORE_XREG_U64_ZX_U128(a_iXReg, a_u64Value) \
     do { pIemCpu->CTX_SUFF(pCtx)->CTX_SUFF(pXState)->x87.aXMM[(a_iXReg)].au64[0] = (a_u64Value); \
Index: /trunk/src/VBox/VMM/VMMAll/IEMAllInstructions.cpp.h
===================================================================
--- /trunk/src/VBox/VMM/VMMAll/IEMAllInstructions.cpp.h	(revision 61650)
+++ /trunk/src/VBox/VMM/VMMAll/IEMAllInstructions.cpp.h	(revision 61651)
@@ -1549,9 +1549,53 @@
             IEM_MC_END();
         }
-        return VINF_SUCCESS;
-    }
-
-    IEMOP_BITCH_ABOUT_STUB();
-    return VERR_IEM_INSTR_NOT_IMPLEMENTED;
+    }
+    else if (pIemCpu->fPrefixes == IEM_OP_PRF_REPNZ)
+    {
+        IEMOP_MNEMONIC("movsd Wsd,Vsd");
+        uint8_t bRm; IEM_OPCODE_GET_NEXT_U8(&bRm);
+        if ((bRm & X86_MODRM_MOD_MASK) == (3 << X86_MODRM_MOD_SHIFT))
+        {
+            /*
+             * Register, register.
+             */
+            IEMOP_HLP_DONE_DECODING_NO_LOCK_PREFIX();
+            IEM_MC_BEGIN(0, 1);
+            IEM_MC_LOCAL(uint64_t,                  uSrc);
+
+            IEM_MC_MAYBE_RAISE_SSE2_RELATED_XCPT();
+            IEM_MC_ACTUALIZE_SSE_STATE_FOR_CHANGE();
+            IEM_MC_FETCH_XREG_U64(uSrc, ((bRm >> X86_MODRM_REG_SHIFT) & X86_MODRM_REG_SMASK) | pIemCpu->uRexReg);
+            IEM_MC_STORE_XREG_U64((bRm & X86_MODRM_RM_MASK) | pIemCpu->uRexB, uSrc);
+
+            IEM_MC_ADVANCE_RIP();
+            IEM_MC_END();
+        }
+        else
+        {
+            /*
+             * Memory, register.
+             */
+            IEM_MC_BEGIN(0, 2);
+            IEM_MC_LOCAL(uint64_t,                  uSrc);
+            IEM_MC_LOCAL(RTGCPTR,                   GCPtrEffSrc);
+
+            IEM_MC_CALC_RM_EFF_ADDR(GCPtrEffSrc, bRm, 0);
+            IEMOP_HLP_DONE_DECODING_NO_LOCK_PREFIX();
+            IEM_MC_MAYBE_RAISE_SSE2_RELATED_XCPT();
+            IEM_MC_ACTUALIZE_SSE_STATE_FOR_READ();
+
+            IEM_MC_FETCH_XREG_U64(uSrc, ((bRm >> X86_MODRM_REG_SHIFT) & X86_MODRM_REG_SMASK) | pIemCpu->uRexReg);
+            IEM_MC_STORE_MEM_U64(pIemCpu->iEffSeg, GCPtrEffSrc, uSrc);
+
+            IEM_MC_ADVANCE_RIP();
+            IEM_MC_END();
+        }
+    }
+    else
+    {
+        IEMOP_BITCH_ABOUT_STUB();
+        return VERR_IEM_INSTR_NOT_IMPLEMENTED;
+    }
+    return VINF_SUCCESS;
 }
 
Index: /trunk/src/VBox/VMM/testcase/tstIEMCheckMc.cpp
===================================================================
--- /trunk/src/VBox/VMM/testcase/tstIEMCheckMc.cpp	(revision 61650)
+++ /trunk/src/VBox/VMM/testcase/tstIEMCheckMc.cpp	(revision 61651)
@@ -482,4 +482,5 @@
 #define IEM_MC_FETCH_XREG_U32(a_u32Value, a_iXReg)          do { (a_u32Value) = 0; CHK_TYPE(uint32_t, a_u32Value); (void)fSseRead; } while (0)
 #define IEM_MC_STORE_XREG_U128(a_iXReg, a_u128Value)        do { CHK_TYPE(uint128_t, a_u128Value); (void)fSseWrite; } while (0)
+#define IEM_MC_STORE_XREG_U64(a_iXReg, a_u64Value)          do { CHK_TYPE(uint64_t,  a_u64alue);   (void)fSseWrite; } while (0)
 #define IEM_MC_STORE_XREG_U64_ZX_U128(a_iXReg, a_u64Value)  do { CHK_TYPE(uint64_t,  a_u64Value);  (void)fSseWrite; } while (0)
 #define IEM_MC_STORE_XREG_U32_ZX_U128(a_iXReg, a_u32Value)  do { CHK_TYPE(uint32_t,  a_u32Value);  (void)fSseWrite; } while (0)