diff --git a/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp b/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
--- a/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
+++ b/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
@@ -127,6 +127,10 @@
     }
   };
   APInt Insn(16, support::endian::read16be(Bytes.data()));
+  // 2 bytes of data are consumed, so set Size to 2
+  // If we don't do this, disassembler may generate result even
+  // the encoding is invalid. We need to let it fail correctly.
+  Size = 2;
   Result = decodeInstruction(DecoderTable80, Instr, Insn, Address, this, STI,
                              MakeUp);
   if (Result == DecodeStatus::Success)
diff --git a/llvm/lib/Target/M68k/M68kInstrArithmetic.td b/llvm/lib/Target/M68k/M68kInstrArithmetic.td
--- a/llvm/lib/Target/M68k/M68kInstrArithmetic.td
+++ b/llvm/lib/Target/M68k/M68kInstrArithmetic.td
@@ -73,8 +73,10 @@
   let Inst = (descend
     CMD, (operand "$dst", 3),
     !cast<MxOpModeEncoding>("MxOpMode"#DST_TYPE.Size#"_"#DST_TYPE.RLet#"_EA").Value,
-    (descend /*MODE without last bit*/0b00,
-             /*REGISTER prefixed with D/A bit*/(operand "$opd", 4))
+    !cond(
+      !eq(SRC_TYPE.RLet, "r") : (descend 0b00, (operand "$opd", 4)),
+      !eq(SRC_TYPE.RLet, "d") : (descend 0b000, (operand "$opd", 3))
+    )
   );
 }
 
diff --git a/llvm/test/MC/Disassembler/M68k/arithmetic.txt b/llvm/test/MC/Disassembler/M68k/arithmetic.txt
--- a/llvm/test/MC/Disassembler/M68k/arithmetic.txt
+++ b/llvm/test/MC/Disassembler/M68k/arithmetic.txt
@@ -1,10 +1,8 @@
-# FIXME: The first two tests are disabled due to decoding conflict
-# RUN: llvm-mc -disassemble -triple m68k %s | not FileCheck -check-prefix=CHECK-CONFLICT %s
 # RUN: llvm-mc -disassemble -triple m68k %s | FileCheck %s
 
-# CHECK-CONFLICT: adda.l %a0, %a1
+# CHECK: adda.l %a0, %a1
 0xd3 0xc8
-# CHECK-CONFLICT: sub.w %d3, %d1
+# CHECK: sub.w %d3, %d1
 0x92 0x43
 
 # CHECK: add.w    (56,%a4,%d3), %d2
@@ -109,8 +107,8 @@
 # CHECK: or.l    %d6, (48,%a4,%a2)
 0x8d 0xb4 0xa8 0x30
 
-# CHECK: or.b    %a2, %d7
-0x8e 0x0a
+# CHECK: or.b  %d2, %d7
+0x8e 0x02
 
 # CHECK: or.b    (4163,%a1), %d0
 0x80 0x29 0x10 0x43
@@ -128,4 +126,91 @@
 0x93 0xc4
 
 # CHECK: sub.l   #16843009, %d3
-0x96 0xbc 0x01 0x01 0x01 0x01
\ No newline at end of file
+0x96 0xbc 0x01 0x01 0x01 0x01
+
+# CHECK: add.w %d0, %d4
+0xd8 0x40
+
+# CHECK: add.w %a2, %d3
+0xd6 0x4a
+
+# CHECK: add.l %d1, %d2
+0xd4 0x81
+
+# CHECK: add.l %a0, %d1
+0xd2 0x88
+
+# CHECK: sub.w %a5, %d1
+0x92 0x4d
+
+# CHECK: sub.w %d2, %d4
+0x98 0x42
+
+# CHECK: sub.l %d2, %d5
+0x9a 0x82
+
+# CHECK: sub.l %a2, %d5
+0x9a 0x8a
+
+# CHECK: subx.w %d2, %d4
+0x99 0x42
+
+# CHECK: subx.l %d5, %d3
+0x97 0x85
+
+# CHECK: suba.l %d4, %a0
+0x91 0xc4
+
+# CHECK: suba.l #12576, %a2
+0x95 0xfc 0x00 0x00 0x31 0x20
+
+# CHECK: suba.l (%sp), %a0
+0x91 0xd7
+
+# CHECK: and.b (256,%a2), %d0
+0xc0 0x2a 0x01 0x00
+
+# CHECK: and.w %d0, %d7
+0xce 0x40
+
+# CHECK: and.w %a1, %d3
+0xc6 0x49
+
+# CHECK: and.w (32,%a0,%d1), %d1
+0xc2 0x70 0x18 0x20
+
+# CHECK: and.w (288,%a2), %d4
+0xc8 0x6a 0x01 0x20
+
+# CHECK: and.l %d1, (49,%a0,%sp)
+0xc3 0xb0 0xf8 0x31
+
+# CHECK: and.l #100, (1,%a1,%d4)
+0x02 0xb1 0x00 0x00 0x00 0x64 0x48 0x01
+
+# CHECK: and.l %d0, %d1
+0xc2 0x80
+
+# CHECK: and.l %a1, %d2
+0xc4 0x89
+
+# CHECK: or.b #7, (%a1)
+0x00 0x11 0x00 0x07
+
+# CHECK: or.w %d2, %d5
+0x8a 0x42
+
+# CHECK: or.w %a0, %d2
+0x84 0x48
+
+# CHECK: or.w #0, (0,%a2,%a0)
+0x00 0x72 0x00 0x00 0x88 0x00
+
+# CHECK: or.l (15,%pc,%a1), %d0
+0x80 0xbb 0x98 0x0f
+
+# CHECK: or.l %a0, %d0
+0x80 0x88
+
+# CHECK: or.l %d1, %d6
+0x8c 0x81