In a message dated 11/8/2007 6:21:04 P.M. Central Standard Time, [EMAIL PROTECTED] writes: >In the MVS world, if a Problem State program attempts to modify 00000xxx (where x is 0-512 decimal and regardless of the content of the current base register) and LAP is on... So it is not truly available (except to the SCP). I know what you meant, but what you wrote is not technically correct. A problem state program can modify bytes 0-511 of PSA if it alters control register 0 and sets protect key 0 (while in supervisor state), then sets itself to problem state, and then alters that area. Problem state has nothing to do with the various protection mechanisms in the processor architecture when a single instruction's behavior is under consideration. Obviously, if you are in problem state you can't alter CR 0 or set protect key 0, as doing that requires privileged instructions. If you are APF authorized, you can put yourself in supervisor state and then make those changes. But that is a series of instructions, not the one instruction involved in "if a Problem State program attempts to modify...". Also the SCP does not have a monopoly on the STCTL instruction. Any authorized program can do it. The wisdom of and necessity for doing it is another matter.
>Otherwise, as I recall [MVS environments], that page is Key0 non-fetch protected. Not true any more. The upper half of virtual page 0 (aka PSA) is fetch protected by yet another different, independent protection mechanism. This is so that non-key 0 programs cannot look at the upper half of page 0, in which many register save areas are defined in the z/OS PSA DSECT. There might be clear text, passwords, or who knows what in a register that would be visible to an unauthorized program if such save areas were not fetch-protected somehow. The lower half of page 0 is in key 0 and not fetch protected. Bill Fairchild Franklin, TN ************************************** See what's new at http://www.aol.com ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
