On Sun, 24 Oct 2021 10:27:36 -0400, Eric D Rossman wrote: > ... >I do agree that you should submit an RCF to get that section cleared up. >When none of the multiple technical folks reading a definitive source >(Principles of Operations) don't understand what something means in a >given context, that says it really needs to be made clearer. > Thanks.
What I believe most important is to clarify the distinction between address generation and storage access. It's easy to overlook this distinction because 99+% of instructions performing address generation result immediately in a storage access. (Are the only exceptions LA(Y)?) All RX instructions perform address generation identically, using base. displacement, and index. Wraparound may occur twice. All RS and SS (twice) instructions perform address generation identically, using base(s) and displacement(s). Wraparound may occur on each address generated. RR instructions such as branches perform address generation by wraparound of the R2 operand value. Omissions? Exceptions? Model dependencies? No exceptions are recognized during address generation. Exceptions may be recognized subsequently, during the resulting storage access. Branch instructions are unusual in that exceptions are recognized not during the execution of the branch but in fetching the instruction at the branch target (to wit, the address saved in the interrupt OPSW is the address of the branch target, not that of the branch.) LA(Y) recognizes no exceptions during address geneeration, and no storage access results. How much of the above is important? How much is covered elsewhere? Thanks again, gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
