On Tue, 27 Nov 2012 14:55:16 -0600, Paul Gilmartin <[email protected]> wrote:
>But what plausible use do you envision that anyone or any group at IBM might >have for the "model dependent" character? It requires that software developers >test on every available and possible future system. Simply, it's irresponsible >not to have made the behavior consistent across models. If it's going to fail >on some models, it should be designed to fail likewise on all models. Perhaps >with SIE? There is nothing new here, Paul. One of the original design points for S/360 was to separate machine behavior from the programming interface. You want to be able to change and optimize the machine behaviors over time without screwing up the interface. This is precisely why you see "model-dependent" in the documentation - so you DON'T build an expectation. And it's why you can still run programs written in 1965. Where conditional execution is involved, the machine often tries to avoid unnecessary work. This is what out-of-order execution and pipelining are all about. If the machine decides NOT to do something, then the machine MAY not detect certain conditions. Nonetheless, the instruction yields the expected result. See "Sequence of Storage References" in Ch. 5 of the POP, particularly "Divisible instruction execution." Sometimes part of the instruction can be run while a data fetch is in progress. Note that most data reference (load or store) requires intermediate fetches to get ART and DAT entries, etc. Some of this parallelism is needed to counter the distance- and cache-related effects of accessing memory. As STOC Usage Note 2 states, if the store is skipped because of the condition code, the operand might not be fetched into cache. But it is a matter of implementation as to exactly when the circuitry triggers things like PER storage-alteration events or set the page change bit. As the machines get smarter, the accuracy of those events improves. While the machine may over-indicate (a false positive) an event, it will never under-indicate an event. It would be unforgivable if the architects baked in Alan Altmark IBM ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
