<snip>
RENT: MVS protects your module's virtual storage so that your module
cannot be modified - and REFR implies RENT.
</snip>

z/OS ignores "refreshable" except when REFRPROT is in effect.
Thus if "REFR implies RENT" then it is important that the RENT indicator 
be set when REFR is specified without RENT. I presume that that is what 
happens, but I have not tried it.

"cannot be modified" is all a matter of degree. Anything can be modified 
if you are authorized enough. Some of the rules that apply (the rules are 
more complex, so this is not completely accurate):
-- RENT modules not from an APF-authorized library are not placed in key 0 
storage so they "can" be modified by an unauthorized program 
-- RENT modules from an APF-authorized library are placed in key 0 storage 
so they "can" be modified by a key 0 program. The same is true for a 
TCBKEY9 task
-- REFR modules with REFPROT are page-protected. When page-fixed, they 
"can" be modified by using real addresses

Peter Relson
z/OS Core Technology Design


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