It's been a long time but once upon a time DS 0H was used for labels in code for two reasons. TSO TEST liked it better than EQU *, and when a person mixed data and code, EQU * could possibly refer to an odd address.
And yes EQU * is a good way to mark a location in a record/control block that is either variable length or liable to change in length. richard -----Original Message----- From: IBM Mainframe Assembler List [mailto:[email protected]] On Behalf Of Gary Weinhold Sent: Wednesday, August 01, 2018 2:40 PM To: [email protected] Subject: Re: EQU * considered harmful To avoid the problem Dan illustrates but retain the advantages Charles cites of not labeling specific instructions, we use label DS 0H instead of label EQU * But i think some of the point of the original post was lost, since the question was whether label EQU * was ever beneficial, where the "*" indicates current location rather than meaning generically any value. On 2018-08-01 2:23 PM, Dan Greiner wrote: > I too disagree (rather strongly). > > As an example, consider where EQU is used to give names to bits of a field in > memory. > > FLAGS DS X > F_OPEN EQU X'80' > F_CLOSE EQU X'40' > F_FUBAR EQU X'20' > ... > TM FLAGS,F_FUBAR > JO TOTALLY_HOSED > > Furthermore, you can assign a "length" to each bit, and use that as an offset > in the field, e.g: > > FLAGS DS XL4 > F_OPEN EQU X'80',0 > F_CLOSE EQU X'40',0 > F_FUBAR EQU X'02',3 > ... > TM FLAGS+L'F_FUBAR,F_FUBAR > > (apologies if the syntax is not precise ... I'm doing this from memory at > home). > > As to Charles' comment about using EQU as a branch target, I'm a little bit > less comfortable. If — by chance or accident — there happens to be code > before the EQU that knocks the location off a halfword boundary, this could > spell trouble. E.g: > > LA 7,ITS_ON > TM BYTE,BIT > BCR X'01',7 > ... > other instructions > HI_MOM DC C'Hello' > ITS_ON EQU * > > Since the constant "Hello" is 5 bytes long, this knocks the label ITS_ON onto > an odd boundary. If the branch had been directly to the location (as opposed > to BCR), HLASM would have flagged an error. But in this case, the error may > go undetected until execution — at which point the hardware will slap you > with a PIC-0006 (PIC-0006). Gary Weinhold Senior Application Architect DATAKINETICS | Data Performance & Optimization Phone +1.613.523.5500 x216 Email: [email protected] Visit us online at www.DKL.com E-mail Notification: The information contained in this email and any attachments is confidential and may be subject to copyright or other intellectual property protection. If you are not the intended recipient, you are not authorized to use or disclose this information, and we request that you notify us by reply mail or telephone and delete the original message from your mail system. ----------------------------------------- The information contained in this communication (including any attachments hereto) is confidential and is intended solely for the personal and confidential use of the individual or entity to whom it is addressed. The information may also constitute a legally privileged confidential communication. If the reader of this message is not the intended recipient or an agent responsible for delivering it to the intended recipient, you are hereby notified that you have received this communication in error and that any review, dissemination, copying, or unauthorized use of this information, or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this communication in error, please notify us immediately by e-mail, and delete the original message. Thank you
