- IBM is pretty much committed to even-halfword instructions because Jump only jumps even halfwords.
- You want a confession? You know one reason why I got in the habit of not using DS 0H in code? Because when I started out with punched card decks, 24MB hard drives and Assembler D, every transition from assembled data to DS and back forced a new TXT card and wasted cards and/or DASD space. You may laugh now. FWIW, DC 0H'0' avoided the problem but is trickier 029-jockeying than EQU *, and every typo cost you your daily shot back in those days. - I have a house rule to use J (not B!) *+n only to jump over a single instruction, never more than one. Yeah, it may be a problem waiting to happen, especially now with machine instruction length a little less intuitive (change A to AG and there goes your J *+8). What I like about it is that labels invite the question "who jumps here?"* so if I can avoid a label I do. It's a tradeoff. No one ever said assembler coding was for the faint-hearted. *A better solution probably is the structured assembler macros but by the time they came along I was not writing much assembler, so this old dog never learned that new trick. See you in STL? Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Gord Tomlin Sent: Wednesday, August 1, 2018 3:23 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Re: EQU * considered harmful On 2018-08-01 16:41, Charles Mills wrote: > "Avoid instructions (executable code) and operand data (working storage or > stack storage) in the same cache lines; which > can be costly due to moving cache lines between the separated (split) local > caches (instruction/data L1/L2)" > > -- C. Kevin Shum, Distinguished Engineer, IBM z Systems Microprocessor > Development (March 2016) > > Charles Exactly. "Mixing executable code and operand data considered harmful" And if you always avoid mixing instructions and operand data, using EQU * for labels in code is no longer potentially harmful. We're on pretty safe ground if we assume IBM will always only create instructions that are an even number of bytes in size. I prefer, and always use, DS 0H for labels in code, but if EQU * causes problems in your code you have other things to clean up. Here's one to rail about: branching to a hard coded offset from the current location, e.g., B *+12 This is a tire fire waiting to happen.
