<snip> Is it possible to force an Assembler program to hit an access error if it tries to load data from the low core (page starting from address zero)? </snip> If you're talking about z/OS, in a word, "no". Such behavior would not be in accordance with z/OS requirements.
Location 0 is a valid address in z/OS and locations 0-x'7FF' are set by z/OS not to be fetch-protected. Therefore they are accessible to any program, without "access error". Programming interfaces such as FLCCVT, PSATOLD and PSAAOLD are within this range. DO NOT change control register bits. Peter Relson z/OS Core Technology Design
