I would allow it as long as the comments were clear. Just as I would allow the BXH/BXLE hack for bit testing with adequate comment.
-- Shmuel (Seymour J.) Metz http://mason.gmu.edu/~smetz3 עַם יִשְׂרָאֵל חַי נֵ֣צַח יִשְׂרָאֵ֔ל לֹ֥א יְשַׁקֵּ֖ר ________________________________________ From: IBM Mainframe Assembler List <ASSEMBLER-LIST@LISTSERV.UGA.EDU> on behalf of Charles Mills <charl...@mcn.org> Sent: Saturday, August 2, 2025 9:54 AM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU <ASSEMBLER-LIST@LISTSERV.UGA.EDU> Subject: Re: Execute-Type Instructions External Message: Use Caution I think I recall code back in the day that did clever "computed branch" logic by making a BC or BCR instruction the target of EX. I would defenestrate any programmer that did that sort of thing today. Charles -----Original Message----- From: IBM Mainframe Assembler List <ASSEMBLER-LIST@LISTSERV.UGA.EDU> On Behalf Of Dan Greiner Sent: Friday, August 1, 2025 8:39 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Execute-Type Instructions As is well known to subscribers of the Assembler List, the EXECUTE instruction (EX) – and the (relatively) new variant of EXECUTE RELATIVE LONG (EXRL) – provide an extremely powerful means of altering the behavior of a target instruction by ORing the contents of a the rightmost bits of the first operand of the execute-type instruction into bits 8-15 of the target instruction. Common uses include modifying the length(s) of an SS-format instruction, the register(s), mask, or immediate field of RR, RX, SI, and many other formats of instructions (it is also sufficiently complex that it drives CPU designers slightly nuts). One reason an execute-type instruction is particularly tricky is that certain instruction formats contain part of the operation code in bits 8-15, thus the actual target instruction executed may not be that which appears in the memory. This can occur when the target instruction format is IE, RI, RIL, RRD, RRE, RRF, S, SIL, SSE, and SSF. My question is (aside from IBM diagnostics) does anybody actually exploit this sort of chicanery/guile/subterfuge in their code?
