diff --git a/internal/gen/loadertest.go b/internal/gen/loadertest.go
index 039ba05..55d267d 100644
--- a/internal/gen/loadertest.go
+++ b/internal/gen/loadertest.go
@@ -86,18 +86,18 @@ func arg(t string) string {
 		"imm32": fmt.Sprintf("$%d", math.MaxInt32), // <xs:enumeration value="imm32" />
 		"imm64": fmt.Sprintf("$%d", math.MaxInt64), // <xs:enumeration value="imm64" />
 
-		"al":  "AL",  // <xs:enumeration value="al" />
-		"cl":  "CL",  // <xs:enumeration value="cl" />
-		"r8":  "CH",  // <xs:enumeration value="r8" />
-		"ax":  "AX",  // <xs:enumeration value="ax" />
-		"r16": "SI",  // <xs:enumeration value="r16" />
-		"eax": "AX",  // <xs:enumeration value="eax" />
-		"r32": "DX",  // <xs:enumeration value="r32" />
-		"rax": "AX",  // <xs:enumeration value="rax" />
-		"r64": "R15", // <xs:enumeration value="r64" />
-		// <xs:enumeration value="mm" />
-		"xmm0": "X0", // <xs:enumeration value="xmm0" />
-		"xmm":  "X7", // <xs:enumeration value="xmm" />
+		"al":   "AL",  // <xs:enumeration value="al" />
+		"cl":   "CL",  // <xs:enumeration value="cl" />
+		"r8":   "CH",  // <xs:enumeration value="r8" />
+		"ax":   "AX",  // <xs:enumeration value="ax" />
+		"r16":  "SI",  // <xs:enumeration value="r16" />
+		"eax":  "AX",  // <xs:enumeration value="eax" />
+		"r32":  "DX",  // <xs:enumeration value="r32" />
+		"rax":  "AX",  // <xs:enumeration value="rax" />
+		"r64":  "R15", // <xs:enumeration value="r64" />
+		"mm":   "M5",  // <xs:enumeration value="mm" />
+		"xmm0": "X0",  // <xs:enumeration value="xmm0" />
+		"xmm":  "X7",  // <xs:enumeration value="xmm" />
 		// <xs:enumeration value="xmm{k}" />
 		// <xs:enumeration value="xmm{k}{z}" />
 		"ymm": "Y13", // <xs:enumeration value="ymm" />
@@ -122,7 +122,7 @@ func arg(t string) string {
 		// <xs:enumeration value="m64{k}{z}" />
 		"m128": "128(AX)(CX*2)", // <xs:enumeration value="m128" />
 		// <xs:enumeration value="m128{k}{z}" />
-		// <xs:enumeration value="m256" />
+		"m256": "256(AX)(CX*2)", // <xs:enumeration value="m256" />
 		// <xs:enumeration value="m256{k}{z}" />
 		// <xs:enumeration value="m512" />
 		// <xs:enumeration value="m512{k}{z}" />
diff --git a/internal/load/load.go b/internal/load/load.go
index 013c66b..d456a95 100644
--- a/internal/load/load.go
+++ b/internal/load/load.go
@@ -97,7 +97,7 @@ func (l Loader) include(f opcodesxml.Form) bool {
 	for _, isa := range f.ISA {
 		switch isa.ID {
 		// AMD-only.
-		case "TBM", "CLZERO", "FMA4", "XOP", "SSE4A":
+		case "TBM", "CLZERO", "FMA4", "XOP", "SSE4A", "3dnow!", "3dnow!+":
 			return false
 		// Incomplete support for some prefetching instructions.
 		case "PREFETCH", "PREFETCHW", "PREFETCHWT1", "CLWB":
@@ -113,6 +113,17 @@ func (l Loader) include(f opcodesxml.Form) bool {
 		}
 	}
 
+	// Go appears to have skeleton support for MMX instructions. See the many TODO lines in the testcases:
+	// Reference: https://github.com/golang/go/blob/649b89377e91ad6dbe710784f9e662082d31a1ff/src/cmd/asm/internal/asm/testdata/amd64enc.s#L3310-L3312
+	//
+	//		//TODO: PALIGNR $7, (BX), M2            // 0f3a0f1307
+	//		//TODO: PALIGNR $7, (R11), M2           // 410f3a0f1307
+	//		//TODO: PALIGNR $7, M2, M2              // 0f3a0fd207
+	//
+	if f.MMXMode == "MMX" {
+		return false
+	}
+
 	// x86 csv contains a number of CMOV* instructions which are actually not valid
 	// Go instructions. The valid Go forms should have different opcodes from GNU.
 	// Therefore a decent "heuristic" is CMOV* instructions that do not have