diff --git a/ppc64/ppc64asm/ext_test.go b/ppc64/ppc64asm/ext_test.go
index f9242e1..806701b 100644
--- a/ppc64/ppc64asm/ext_test.go
+++ b/ppc64/ppc64asm/ext_test.go
@@ -40,7 +40,7 @@
 // from an external disassembler's output.
 type ExtInst struct {
 	addr uint32
-	enc  [4]byte
+	enc  [8]byte
 	nenc int
 	text string
 }
@@ -200,20 +200,25 @@
 	defer w.Flush()
 	size = 0
 	generate(func(x []byte) {
-		if len(x) > 4 {
-			x = x[:4]
+		if len(x) != 4 && len(x) != 8 {
+			panic(fmt.Sprintf("Unexpected instruction %v\n", x))
+		}
+		izeros := zeros
+		if len(x) == 4 {
+			// Only pad to 4 bytes for a 4 byte instruction word.
+			izeros = izeros[4:]
 		}
 		if debug {
-			fmt.Printf("%#x: %x%x\n", start+size, x, zeros[len(x):])
+			fmt.Printf("%#x: %x%x\n", start+size, x, izeros[len(x):])
 		}
 		w.Write(x)
-		w.Write(zeros[len(x):])
-		size += len(zeros)
+		w.Write(izeros[len(x):])
+		size += len(izeros)
 	})
 	return file, f, size, nil
 }
 
-var zeros = []byte{0, 0, 0, 0}
+var zeros = []byte{0, 0, 0, 0, 0, 0, 0, 0}
 
 // pad pads the code sequence with pops.
 func pad(enc []byte) []byte {
diff --git a/ppc64/ppc64asm/objdumpext_test.go b/ppc64/ppc64asm/objdumpext_test.go
index 37aa257..033f670 100644
--- a/ppc64/ppc64asm/objdumpext_test.go
+++ b/ppc64/ppc64asm/objdumpext_test.go
@@ -64,7 +64,7 @@
 		reading bool
 		next    uint32 = start
 		addr    uint32
-		encbuf  [4]byte
+		encbuf  [8]byte
 		enc     []byte
 		text    string
 	)
@@ -88,15 +88,19 @@
 				text = "error: unknown instruction"
 				enc = nil
 			}
-			if len(enc) == 4 {
-				// prints as word but we want to record bytes
-				enc[0], enc[3] = enc[3], enc[0]
-				enc[1], enc[2] = enc[2], enc[1]
+			// Prefixed instructions may not decode as expected if
+			// they are an invalid form. Some are tested in decode.txt.
+			// objdump treats these like two instructions.
+			//
+			// Look for primary opcode 1 and advance an exta 4 bytes if
+			// this failed to decode.
+			if strings.HasPrefix(text, ".long") && enc[0]>>2 == 1 {
+				next += 4
 			}
 			ext.Dec <- ExtInst{addr, encbuf, len(enc), text}
-			encbuf = [4]byte{}
+			encbuf = [8]byte{}
+			next += uint32(len(enc))
 			enc = nil
-			next += 4
 		}
 	}
 	var textangle = []byte("<.text>:")