In that scenario, offset 4 of Xs save area will no longer contain FnSA, and can thus be identified as the forward pointer of a 72-byte save area.
-- Shmuel (Seymour J.) Metz http://mason.gmu.edu/~smetz3 ________________________________________ From: IBM Mainframe Assembler List [ASSEMBLER-LIST@LISTSERV.UGA.EDU] on behalf of Mark Boonie [boo...@us.ibm.com] Sent: Wednesday, December 1, 2021 2:09 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Re: Base-less macros > The one case where there could be an issue is if the > routine calls both code expecting a 72-byte save area and code > calling a 144-byte area. > > If the linkage conventions are followed, under what circumstances > would you be unable to follow the forward chain with a little bit of work? Didn't you just identify the circumstances? Suppose routine X calls both routine Y and routine Z with a 144-byte save area: - Routine X calls routine Y which is going to use the high-order halves of some registers. Routine Y updates routine X's save area with the saved 64-bit registers and a pointer to routine Y's new save area in location 136. - Routine Y returns to routine X. - Routine X calls routine Z which is only going to use the low-order halves of the registers. Routine Z updates routine X's save area with the saved 32-bit registers and a pointer to routine Z's new save area in word 2. Before routine Z returns, the save area for routine X (the one pointed pointed to by the backward pointer from Z's current save area) has two forward pointers: one to the no longer valid save area for routine Y, and one to the valid save area for routine Z. Scanning forward doesn't allow you to tell which one is correct. Unless by "linkage conventions" you mean calling a routine with a save area that is only as large as that which is required, so that X would need two different save areas for calling Y and Z? - mb
