1. A computer program product for protecting data in a computer system of a machine architecture, the computer program product comprising: a non-transitory computer storage medium readable by said computer system, said computer storage medium storing instructions for performing: obtaining for translation a virtual address of a desired block of main storage; obtaining an initial origin of a translation table in a hierarchy of translation tables, said hierarchy of translation tables comprising a segment table; based on said obtained initial origin address and said virtual address, obtaining a segment table entry from said segment table, said segment table entry configured to contain a format control field; based on an enhanced dynamic address translation (eDAT) facility being enabled, determining, by said computer system, whether said format control field in said segment table entry is enabled; and based on said format control field being enabled, performing: determining whether an access validity field in said segment table entry is enabled, said segment table entry comprising said access validity field, a segment access control field, a segment fetch protection field and a segment-frame absolute address of a large block of main storage, said large block of main storage comprising said desired block of main storage; and based on said access validity field being enabled, performing: permitting stores to said desired block of main storage, wherein the stores are associated with the virtual address, said stores permitted based on said segment access control field matching a program access key provided by any one of a Program Status Word or an operand of a program instruction being executed, wherein the stores are associated with the virtual address; and not permitting stores to said desired block of main storage based on said segment access control filed not matching said program access key, wherein the stores are associated with the virtual address.