Once upon a time there were only 32-bit registers and 72-byte save areas. Some documentation was written during that time. It would have made no sense for it to say "we provide a 72-byte save area" because the obvious retort would be "of course you do." Some documentation has not been updated since.
So I think it is relatively* safe to assume that any standard-ish z/OS program from before 2000 or so that provides an otherwise undocumented save area provides a 72-byte save area, and by implication does not know or care about the high halves of registers. It would appear that Rexx would fall into that category. Therefore you are relatively* safe in assuming that a STM R14,R12,12(R13) is sufficient. *If you wanted to be totally prudent simply save the high halves of any register(s) where you use the high half/ves in some private non-standard save area of your own. For a tiny assembler routine such as you describe, that private save area might be the low half of some otherwise unused register. Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Dave Clark Sent: Friday, January 21, 2022 6:32 AM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Re: Saving Caller's 64-bit Regsiters "IBM Mainframe Assembler List" <ASSEMBLER-LIST@LISTSERV.UGA.EDU> wrote on 01/21/2022 09:16:30 AM: > Dave Clark wrote > <snip> > is there > some other means of knowing whether or not to save the high halves in the > caller's save area? > </snip> > > This has been covered many times. It's your interface. If you documented > that they should provide a save area suitable for saving high halves in > that area then you assume that they have met the interface requirements. > If they didn't (especially with respect to save area), bad things will > happen. It's no different than if you required a 72-byte save area and > they gave you a shorter one. Yes, I'm fully aware of all that. However, my program is being called by REXX and I'm not aware of any documentation stating what kind of register save area REXX provides. That is why I asked and was wondering if there is, perhaps, a code in the first word of the save area that indicated the format used -- and, in the absences of such a code, the 72-byte savearea is all there is.
