index-lower.
The table does not contain the value of the upper bound,
but the value of upper-lower as an unsigned integer.
-If the index is out of bounds or if the fetched pointer is 0,
-then fetch the default instruction pointer.
+The default instruction pointer is used when the index is out of bounds.
If the resulting PC is 0, then trap.
.P
CSB selects the new PC by searching.