The contents of R0 may be relevant, but not in the OP's context. As others have repeatedly pointed out, a base or index field of 0 specifies a value of 0, not the contents of R0. Further, unless you're, e.g., constructing page tables, running DAT-off, prefixing is also irrelevant.
-- Shmuel (Seymour J.) Metz http://mason.gmu.edu/~smetz3 ________________________________________ From: IBM Mainframe Assembler List <[email protected]> on behalf of Steve Thompson <[email protected]> Sent: Wednesday, November 6, 2019 6:08 PM To: [email protected] Subject: Re: Questionable Instructions in Obtaining EAX documentation Yes, and I am, by your definition, one of the .01%. But my point was, the contents of R0 may be relevant. Implying it never matters is not quite right. However I must agree with you that most people working on/with z/OS will never need to work at this level. But depending on what they are doing with VM and/or a z Linux distro.... BTW, I went and looked it up (I have quite a few copies of the PoOP from different points in time). This knowledge was once quite important to me because a long time ago I worked on MDF at AMDAHL. And I was once working on an emulator for S/370-XA but I think it was Don Higgins (regardless he lived across the bay from me when I was in Tampa before Amdahl) beat me to it. And I think that is where Hercules came from (hazy memory these days). If you get into the depths of MVS (as in MVS 3.8J or 3.8O I think it was), you might need that info, so maybe it is more than 0.01% that will be exposed to this during their lives. Regards, Steve Thompson On 11/6/19 1:44 PM, Steve Smith wrote: > Prefixing is absolutely* irrelevant to this discussion, and to 99.99% of > assembler programmers for their entire lives. It's well-documented if > you're interested, but you probably aren't. Any discussion here is very > likely to be spurious, specious, and superficial. > > *I guess it's "really" irrelevant too. :-) > > sas > > On Wed, Nov 6, 2019 at 1:30 PM Seymour J Metz <[email protected]> wrote: > >> The contents of R0 is irrelevant to forming the effective address of an RS >> or RX instruction. the contents of the prefix register are only relevant >> for real addresses. >> >> Now,, if you're running DAT off and you use an instruction that takes an >> address from a register, e.g., MVCL, then it is subject to prefixing. But >> even there it's in terms of the storage assigned to the LPAR. >>
