On Fri, 17 Dec 2010 07:50:48 -0500, Peter Relson wrote: >Bill Fairchild wrote: >>At some time later, IBM also added a special protection feature for bytes >>512-2047 of page 0. This feature, described in the Principles of >>Operation, and which is also both hardware and software, causes a program >>interrupt by any attempt for a non-key-0 program to fetch any storage in >>the first virtual 2K, where many register save areas are located, except >>for bytes 0-511, which may be fetched by a non-key-0 program. This allows >>the normal program to do a L instruction at location 16, which is where >>the address of the CVT is located. If you try to display bytes 0-511 with >>TSO, you should be able to, but not bytes 512-2047. And you can display >>bytes 2048-4095 with TSO. > >Close, But not right. It is the 2nd 2K, bytes 2048-4095, that have this >special attribute.
Peter's reply prompted me to look it up. If I'm looking at the right part of the POO, the reference is to the Fetch-Protection-Override Control. This applies to the first 2K of memory. "Bit 38 of control register 0 is the fetch-protection override control. When the bit is one, fetch protection is ignored for locations at effective addresses 0-2047." This allows the first page of memory to be fetch protected while allowing fetch access to the first 2K of the PSA. -- Tom Marchant ---------------------------------------------------------------------- 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
