You should choose the NORENT option for the greatest compatibility with your old modules.
The NORENT WRITABLE options allow the compiler to use a static pointer
Under the NOWRITABLE option, the compiler will not use a static pointer for either of these purposes, but it has to generate more lines of code to provide the same function.
But you must use the NOWRITABLE option if both of the following are true:
However, the NOWRITABLE(FWS) option can have a potentially very strong negative impact on performance, so do not use it if either of the above items does not apply to you.