On Tue, 2 Sept 2025 at 13:05, Steve Smith < 00001b5163999d8a-dmarc-requ...@listserv.uga.edu> wrote:
Is there such a thing as an implicit index register? ;-) > Yes, there is. All RX instructions have an index register - it's not in any sense optional at the machine code level - if you don't explicitly provide one in the assembler language then the assembler will generate the instruction using 0. (And of course register 0 when used as an index is deemed to have the value 0.) It's extremely common for RX instructions to be used in contexts where no index register is needed. But I'm sure you know all this. Now if you had asked "is there such a thing as specifying an implicit index register?", then that would have a different answer. ;-) Tony H. > On Tue, Sep 2, 2025 at 7:11 AM Jonathan Scott < > 00001b5498fc732f-dmarc-requ...@listserv.uga.edu> wrote: > > > This technique relies on the rule that including an explicit index > > register in an address expression suppresses the alignment warning, even > if > > that index register is zero (which may originally have been an > oversight). > > HLASM's own source code uses this technique, so it should be safe to use. > > I thought I had submitted a documentation update request a few years ago > to > > ensure this was properly documented, probably as part of the explanation > of > > message ASMA033I (which already shows how to use an explicit base > > register), but I can't find any mention in the current documentation, and > > having retired from IBM I no longer have access to the request queue so I > > can't tell whether it is still in the queue. > > > > Jonathan Scott > > > > -----Original Message----- > > From: IBM Mainframe Assembler List <ASSEMBLER-LIST@LISTSERV.UGA.EDU> On > > Behalf Of Peter Morrison > > Sent: 02 September 2025 11:10 > > To: ASSEMBLER-LIST@LISTSERV.UGA.EDU > > Subject: Using (0) to suppress alignment checks in HLASM > > Importance: High > > > > Hello, > > > > Has anyone else used (0) at the end of an RX (or RXY) > > instruction to suppress the assembler's alignment check? > > > > For Example: > > > > DC X'00' force next field to be > > unaligned > > X_UNAL DC AL4(0) Declare an unaligned fullword > > > > L R0,X_UNAL > > gets a warning > > L R0,X_UNAL(0) > > same generated code (ix reg is 0) but no warning > > > > If so, where is it documented? (this is easier than a > > <PUSH ACONTROL/ACONTROL NOALIGN/instr/POP ACONTROL> sequence (and takes > no > > extra lines)) > > > > Peter Morrison > > >
