On Mar 2, 2013, at 20:04, Charles Mills wrote: > I recall distinctly the hardware having fetch protection but there being no > apparent OS support for it. > That matches my old recollection of an Old Timer's recounting his astonishment at having read a dump in which a Protection Exception appeared to have been taken on a fetch instruction.
I believe (with no good evidence) that it was controlled by a bit in the page key. It may have been model-dependent. A matching PSW key always allowed read-write access; a different PSW key might have either no access or read-only access depending on the bit's setting. Truly the Bad Old Days, when there was no privacy enforced between jobs. And IBM OS technology has always trailed the hardware technology. To wit, nowadays, the absence in z/OS of complete support for 64-bit virtual. (The less said of COBOL the better; it's not part of the OS.) Jim Mulder's explanation is most plausible; at some point there may have been understandable reluctance to load user code in key 0, only partly because there would have been no way to guarantee confidentiality of such code. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
