diff --git a/decode.c b/decode.c
index 3c8cc35..ccba6f3 100644
--- a/decode.c
+++ b/decode.c
@@ -244,7 +244,6 @@ prefix_end:
         // there is a REP prefix, then the 66h prefix is ignored here.
         uint8_t mandatory_prefix = prefix_rep ? prefix_rep : !!prefix_66;
         table_idx = table_walk(table_idx, mandatory_prefix, &kind);
-        prefix_rep = 0; // Don't include REP/REPNZ in instr flags
     }
 
     // Then, walk through ModR/M-encoded opcode extensions.
diff --git a/format.c b/format.c
index a719492..e844c4a 100644
--- a/format.c
+++ b/format.c
@@ -125,6 +125,7 @@ fd_format_abs(const FdInstr* instr, uint64_t addr, char* buffer, size_t len)
     const char* mnemonic = fdi_name(FD_TYPE(instr));
 
     bool prefix_rep = false;
+    bool prefix_xacq_xrel = false;
     bool prefix_addrsize = false;
     bool prefix_segment = false;
 
@@ -183,6 +184,16 @@ fd_format_abs(const FdInstr* instr, uint64_t addr, char* buffer, size_t len)
             FD_OP_REG_TYPE(instr, 0) == FD_RT_SEG)
             sizesuffix[0] = 'w';
         break;
+    case FDI_XCHG:
+        if (FD_OP_TYPE(instr, 0) == FD_OT_MEM)
+            prefix_xacq_xrel = true;
+        break;
+    case FDI_MOV:
+        // MOV C6h/C7h can have XRELEASE prefix.
+        if (FD_HAS_REP(instr) && FD_OP_TYPE(instr, 0) == FD_OT_MEM &&
+            FD_OP_TYPE(instr, 1) == FD_OT_IMM)
+            prefix_xacq_xrel = true;
+        break;
     case FDI_FXSAVE:
     case FDI_FXRSTOR:
     case FDI_XSAVE:
@@ -217,7 +228,12 @@ fd_format_abs(const FdInstr* instr, uint64_t addr, char* buffer, size_t len)
     default: break;
     }
 
-    if (prefix_rep) {
+    if (prefix_xacq_xrel || FD_HAS_LOCK(instr)) {
+        if (FD_HAS_REP(instr))
+            buf = fd_strplcpy(buf, "xrelease ", end-buf);
+        if (FD_HAS_REPNZ(instr))
+            buf = fd_strplcpy(buf, "xacquire ", end-buf);
+    } else if (prefix_rep) {
         if (FD_HAS_REP(instr))
             buf = fd_strplcpy(buf, "rep ", end-buf);
         if (FD_HAS_REPNZ(instr))
diff --git a/tests/test_decode.c b/tests/test_decode.c
index 0c574b4..548f418 100644
--- a/tests/test_decode.c
+++ b/tests/test_decode.c
@@ -90,6 +90,32 @@ main(int argc, char** argv)
     TEST64("\xf2\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]");
     TEST32("\xf3\x0f\xc7\x0f", "cmpxchg8b qword ptr [edi]");
     TEST64("\xf3\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]");
+    TEST32("\xf2\xf0\x0f\xc7\x0f", "xacquire lock cmpxchg8b qword ptr [edi]");
+    TEST64("\xf2\xf0\x0f\xc7\x0f", "xacquire lock cmpxchg8b qword ptr [rdi]");
+    TEST32("\xf3\xf0\x0f\xc7\x0f", "xrelease lock cmpxchg8b qword ptr [edi]");
+    TEST64("\xf3\xf0\x0f\xc7\x0f", "xrelease lock cmpxchg8b qword ptr [rdi]");
+    TEST32("\x87\x0f", "xchg dword ptr [edi], ecx");
+    TEST64("\x87\x0f", "xchg dword ptr [rdi], ecx");
+    TEST32("\xf2\x87\x0f", "xacquire xchg dword ptr [edi], ecx");
+    TEST64("\xf2\x87\x0f", "xacquire xchg dword ptr [rdi], ecx");
+    TEST32("\xf3\x87\x0f", "xrelease xchg dword ptr [edi], ecx");
+    TEST64("\xf3\x87\x0f", "xrelease xchg dword ptr [rdi], ecx");
+    TEST32("\xf2\xf0\x87\x0f", "xacquire lock xchg dword ptr [edi], ecx");
+    TEST64("\xf2\xf0\x87\x0f", "xacquire lock xchg dword ptr [rdi], ecx");
+    TEST32("\xf3\xf0\x87\x0f", "xrelease lock xchg dword ptr [edi], ecx");
+    TEST64("\xf3\xf0\x87\x0f", "xrelease lock xchg dword ptr [rdi], ecx");
+    TEST32("\xc6\x07\x12", "mov byte ptr [edi], 0x12");
+    TEST64("\xc6\x07\x12", "mov byte ptr [rdi], 0x12");
+    TEST32("\xf2\xc6\x07\x12", "mov byte ptr [edi], 0x12"); // no xacquire
+    TEST64("\xf2\xc6\x07\x12", "mov byte ptr [rdi], 0x12"); // no xacquire
+    TEST32("\xf3\xc6\x07\x12", "xrelease mov byte ptr [edi], 0x12");
+    TEST64("\xf3\xc6\x07\x12", "xrelease mov byte ptr [rdi], 0x12");
+    TEST32("\x66\xc7\x07\x34\x12", "mov word ptr [edi], 0x1234");
+    TEST64("\x66\xc7\x07\x34\x12", "mov word ptr [rdi], 0x1234");
+    TEST32("\x66\xf2\xc7\x07\x34\x12", "mov word ptr [edi], 0x1234"); // no xacquire
+    TEST64("\x66\xf2\xc7\x07\x34\x12", "mov word ptr [rdi], 0x1234"); // no xacquire
+    TEST32("\x66\xf3\xc7\x07\x34\x12", "xrelease mov word ptr [edi], 0x1234");
+    TEST64("\x66\xf3\xc7\x07\x34\x12", "xrelease mov word ptr [rdi], 0x1234");
     TEST("\x66", "PARTIAL");
     TEST("\xf0", "PARTIAL");
     TEST("\x0f", "PARTIAL");