add directional characters display

src/lib/u64.asm

@@ -0,0 +1,18 @@
+/*  Add two unsigned 64 bit
+
+    Input:
+        r1, r2: A.high, A.low
+        r3, r4: B.high, B.low
+
+    Output:
+        r5, r6: C.high, C.low
+*/
+pub u64.add:
+    add r5, r1, r3
+    add r6, r2, r4
+    cmp r6, r2
+    jae u64.add_no_low_overflow
+    add r5, r5, 1 ; overflow
+
+    pub u64.add_no_low_overflow:
+    ret