1. A computer program product for protecting data in a computer system of a machine architecture, the computer program product comprising: a 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 address 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; in response to an enhanced dynamic address translation (eDAT) facility being enabled, determining whether said format control field in said segment table entry is enabled; and in response to said format control field being enabled when key controlled protection applies, 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 in response to said access validity field being enabled, performing not permitting fetches from said desired block of main storage in response to the segment fetch protection field being enabled and a program access key being not equal to said segment access control field, the program access key provided by any one of a Program Status Word or an operand of a program instruction, wherein the fetches are associated with the virtual address; permitting fetches from said desired block of main storage in response to the segment fetch protection field being enabled and said program access key being equal to said segment access control field, the program access key provided by any one of a Program Status Word or the operand of the program instruction; and permitting fetches from said desired block of main storage in response to the segment fetch protection field not being enabled.