diff --git a/userland/arch/x86_64/add.S b/userland/arch/x86_64/add.S
index 2c309e7..31193b9 100644
--- a/userland/arch/x86_64/add.S
+++ b/userland/arch/x86_64/add.S
@@ -3,7 +3,43 @@
 #include <lkmc.h>
 
 LKMC_PROLOGUE
+    /* Register immediate. */
     mov $1, %rax
     add $2, %rax
     LKMC_ASSERT_EQ(%rax, $3)
+
+    /* Register register. */
+    mov $1, %rax
+    mov $2, %rbx
+    add %rbx, %rax
+    LKMC_ASSERT_EQ(%rax, $3)
+
+    /* Memory and immediate. */
+.data
+    memory_immediate: .quad 0x123456789ABCDEF0
+.text
+    addq $1, memory_immediate
+    LKMC_ASSERT_EQ(memory_immediate, $0x123456789ABCDEF1)
+
+    /* Memory and register. */
+.data
+    memory_register: .quad 0x123456789ABCDEF0
+.text
+    mov $1, %rax
+    add %rax, memory_register
+    LKMC_ASSERT_EQ(memory_register, $0x123456789ABCDEF1)
+
+#if 0
+    /* Cannot do different register sizes:
+     * Error: operand type mismatch for for `add'
+     */
+    add %rax, %eax
+#endif
+
+#if 0
+    /* Cannot do memory memory.
+     * Error: too many memory references for `add'
+     */
+    addq memory_immediate, memory_register
+#endif
 LKMC_EPILOGUE