On Fri, 29 Jul 2005 15:25:49 -0700, Edward E. Jaffe <[EMAIL PROTECTED]> wrote:
>Tom Schmidt wrote: > >>My reaction is similar to yours, Ed. I had been wondering about the need >>for some kind of "come from" display when IBM brought branch relative long >>instructions to the architecture. The R15/R14 game seems like it falls >>apart if the arrival at location 0 (or just arbitrary low storage) came >>about because of a branch relative (jump) or worse, a BR-long. >> >> > >I'm glad you agree with me, Tom. But, I honestly don't see how the >advent of relative branch has compounded this long-standing issue in any >way. I consider the possibility of a relative branch becoming "wild" to >be quite remote. (One of their chief benefits!) Remember, they don't >depend on a GPR being loaded correctly and/or an associated USING being >properly specified. They are always relative to the current PSW. > >For example, ,'BC 15,0(0,0)' will definitely go to location zero and >leave no linkage trail in a register. How would you even code (on >purpose or by accident) a BRC or BRCL to branch to location zero? >Likewise, 'BAS xx,0(0,0)' will definitely go to location zero, but there >will be a linkage register trail. Again, I don't see how you would even >code the equivalent relative instructions BRAS or BRASL to branch there. Never say never: Consider a badly constructed base for an EXecute instruction that "happens" to address an X'C0' "somewhere in virtual storage". The X'C0' is interpretted by the processor as a valid BRC instruction and... off we go! Granted this should be a rare event, but then the non-rare events are supposed to be handled by the applications folks themselves thus leaving these WTF-events to us to marvel over. I have not come up with any cases where your favorite future instruction - EXR - could cause such weirdness, by the way. (There may be more wonderous cases to be found - I don't know (yet).) -- Tom Schmidt Madison, WI ---------------------------------------------------------------------- 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
