diff --git a/decode.c b/decode.c
index 0129fb4..ad880de 100644
--- a/decode.c
+++ b/decode.c
@@ -608,6 +608,21 @@ fd_decode(const uint8_t* buffer, size_t len_sz, int mode_int, uintptr_t address,
         }
     }
 
+    if (instr->type == FDI_XCHG_NOP)
+    {
+        // Only 4890, 90, and 6690 are true NOPs.
+        if (instr->operands[0].reg == 0 && instr->operands[1].reg == 0)
+        {
+            instr->operands[0].type = FD_OT_NONE;
+            instr->operands[1].type = FD_OT_NONE;
+            instr->type = FDI_NOP;
+        }
+        else
+        {
+            instr->type = FDI_XCHG;
+        }
+    }
+
     if ((prefixes & PREFIX_LOCK) && !desc->lock)
         return FD_ERR_UD;
     if ((prefixes & PREFIX_LOCK) && instr->operands[0].type != FD_OT_MEM)
diff --git a/instrs.txt b/instrs.txt
index 64d9445..5c6926d 100644
--- a/instrs.txt
+++ b/instrs.txt
@@ -143,7 +143,9 @@
 8d                    RM    GP     GP     -      -      LEA MUSTMEM
 8e                    RM    SREG   GP     -      -      MOV_G2S
 8f/0                  M     GP     -      -      -      POP DEF64
-90                    NP    -      -      -      -      NOP
+# Against frequent belief, only, XCHG (r/e)AX, (r)AX with 90 is NOP.
+# As a lacking REX.B cannot be specified here, this is hardcoded.
+90                    OA    GP     GP     -      -      XCHG_NOP
 91                    OA    GP     GP     -      -      XCHG
 92                    OA    GP     GP     -      -      XCHG
 93                    OA    GP     GP     -      -      XCHG
diff --git a/tests/prefixes.txt b/tests/prefixes.txt
index 71fda57..dc6eb5d 100644
--- a/tests/prefixes.txt
+++ b/tests/prefixes.txt
@@ -11,3 +11,6 @@ decode   f2660f10c1                     [SSE_MOVSD reg8:r0 reg8:r1]
 decode   f3660f10c1                     [SSE_MOVSS reg4:r0 reg4:r1]
 decode   f3f2660f10c1                   [SSE_MOVSD reg8:r0 reg8:r1]
 decode   f266f3660f10c1                 [SSE_MOVSS reg4:r0 reg4:r1]
+decode64 4890                           [NOP]
+decode64 4990                           [XCHG reg8:r8 reg8:r0]
+decode64 6690                           [NOP]