diff --git a/README.adoc b/README.adoc
index 85e2652..7eaa00b 100644
--- a/README.adoc
+++ b/README.adoc
@@ -12397,6 +12397,7 @@ Bibliography:
 * Integer typecasts
 ** link:userland/arch/x86_64/movzx.S[]: MOVZX
 ** link:userland/arch/x86_64/movsx.S[]: MOVSX
+* link:userland/arch/x86_64/bswap.S[]: BSWAP: convert between little endian and big endian
 
 ==== x86 CQTO and CLTQ instructions
 
diff --git a/userland/arch/x86_64/bswap.S b/userland/arch/x86_64/bswap.S
new file mode 100644
index 0000000..c6e56fa
--- /dev/null
+++ b/userland/arch/x86_64/bswap.S
@@ -0,0 +1,13 @@
+# # bswap
+
+    # Little endian <=> big endian.
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+
+    mov $0x12345678, %eax
+    bswapl %eax
+    LKMC_ASSERT_EQ_32(%eax, $0x78563412)
+
+LKMC_EPILOGUE