Portal | References | Downloads | Info | Programs | JCLs | Mainframe wiki | Quick Ref
IBM Mainframe Forum Index
 
Register
 
IBM Mainframe Forum Index Mainframe: Search IBM Mainframe Forum: FAQ Memberlist Profile Log in to check your private messages Log in
 
Learning about Systems Assembler programming

 
Post new topic   Reply to topic    IBMMAINFRAMES.com Support Forums -> PL/I & Assembler
View previous topic :: :: View next topic  
Author Message
WilliamDownie

New User


Joined: 01 Jul 2020
Posts: 3
Location: UK

PostPosted: Mon Jul 20, 2020 3:27 pm    Post subject: Learning about Systems Assembler programming
Reply with quote

Hello,

I've been coding Assembler for several years now as an application programmer. At the moment I have some time and access to a mainframe (the IBM MtM machine) and would like to try some of the facilities an Assembler systems program would use. This is to give me a broader understanding of what Assembler can do.

I was wondering if anyone could give any pointers on what would be useful to learn and if there are any good resources on the internet or books on the subject. Note, I don't have access to a transaction manager such as CICS.

I've got a book called "Advanced Assembler Language and MVS interfaces" written by Carmine Cannatello and this has chapters on Supervisor Services and on Cross Memory Services, both of which look like something I'd like to understand better.

Thanks
Back to top
View user's profile Send private message

steve-myers

Active Member


Joined: 30 Nov 2013
Posts: 860
Location: The Universe

PostPosted: Mon Jul 20, 2020 9:03 pm    Post subject:
Reply with quote

Cross memory services are something that Assembler programmers rarely exercise. I only wrote - from scratch - one cross memory application, and that was some 40 years ago and was rather limited. These days a different approach would be used, one not possible then as the hardware (and software) didn't support what is now called cross memory services.

There are five huge manuals - MVS Programming: Authorized Assembler Services Guide, z/OS MVS Programming: Authorized Assembler Reference ALE - DYN, z/OS MVS Programming: Authorized Assembler Reference EDT - IXG, z/OS MVS Programming: Authorized Assembler Reference LLA-SDU, and z/OS MVS Programming: Authorized Assembler Reference SET - WTO. You'll hit the Guide manual pretty hard, as it is the primary description of dynamic allocation, and cherry pick from the other four as required.

At least 90% - probably more - of dynamic allocation can be exercised by anyone in any program. I never understood why it was placed as an "authorized" service, but there it is. The little piece that is "authorized" is not really marked out in the text.

Back in the days when I did product support for an ISV - never mind the product or ISV, we had one product that utilized the data in virtual piece of cross memory services. I got to understand the programming aspects of accessing the data in its data space, but, in part because we never had any trouble with that piece of the product, I never understood how to prepare the application to connect a "data space" to a VSAM Linear Data Set. To this day I think of it as some sort of voodoo exercise! One detail the ISV never adequately communicated to its customers (or for that matter to us grunts that supported the product) is that Data in Virtual amounts to a weird sort of BDAM data set in storage that uses what amounts to MVS paging services to read data from the LDS, but you have to explicitly write altered pages back to the LDS. When you use multiple gigabyte LDS data sets (as our product did then, and I presume it still does) it amounts to a multiple gigabyte storage requirement! In the 1990s and early 2000s this kind of real storage was kind of unusual. I understand that DB2 uses Linear Data sets, but I don't think it uses the Data in Virtual piece.
Back to top
View user's profile Send private message
Gary Adair

New User


Joined: 31 Oct 2013
Posts: 2
Location: USA

PostPosted: Mon Aug 31, 2020 1:38 pm    Post subject: Reply to: Learning about Systems Assembler programming
Reply with quote

Back when I was doing dynamic allocations, 10 plus years ago, you needed to have Protect Key 0 and Supervisor State to change the DCB before executing the DYNALLOC. That is why it is an Authorized Service.
Back to top
View user's profile Send private message
steve-myers

Active Member


Joined: 30 Nov 2013
Posts: 860
Location: The Universe

PostPosted: Mon Aug 31, 2020 6:21 pm    Post subject:
Reply with quote

Bulls****. I have been using MVS dynamic allocation and before that the MVT IKJDAIR service, which called some form of dynamic allocation. In all that time, never once did I change to key 0. The only change to the DCB is to copy a system determined DD name to the DCB after completing the allocation, or to copy a DD name to dynamic allocation data before using allocation to unallocate a DD.
Back to top
View user's profile Send private message
Gary Adair

New User


Joined: 31 Oct 2013
Posts: 2
Location: USA

PostPosted: Tue Sep 01, 2020 9:46 am    Post subject: Reply to: Learning about Systems Assembler programming
Reply with quote

No need for the bulls*** comment sir, not appreciated. I am saying what I had to do many times in order for application programs to modify the DCB and use DYNALLOC. If not it would abend with a S0C4 Protection Exception.
Back to top
View user's profile Send private message
steve-myers

Active Member


Joined: 30 Nov 2013
Posts: 860
Location: The Universe

PostPosted: Tue Sep 01, 2020 4:53 pm    Post subject:
Reply with quote

In most application programs the DCB is in user key storage. You do not need to be in key 0 to alter it. Period. End of story.

I grant that I had one user key program that wanted to alter the DEB to simulate dynamic allocation. Foolish. This was in VS2 Release 1 (SVS). That program had to go to key 0. I have user key programs to look at the LINKLIB DCB, but just to look, not alter. I still have programs that look at JOBLIB and TASKLIB DCBs. Just look, not alter. TASKLIB DCBs should be user key. I honestly do not know about JOBLIB DCBs.

Real system programmers do every thing possible to run in user key, and only go to privilege state when absolutely required. It is much safer that way.
Back to top
View user's profile Send private message
View previous topic :: :: View next topic  
Post new topic   Reply to topic    IBMMAINFRAMES.com Support Forums -> PL/I & Assembler All times are GMT + 6 Hours
Page 1 of 1

 

Search our Forum:

Similar Topics
Topic Author Forum Replies Posted
No new posts CICS file processing using applicatio... Satish_gsv CICS 3 Sat Jul 11, 2020 4:18 am
No new posts TSO and the STAX Assembler Macro mxgdontmics PL/I & Assembler 43 Mon Jun 15, 2020 10:33 am
No new posts COBOL Student learning COBOL File Han... torrind COBOL Programming 3 Sat May 09, 2020 3:40 am
No new posts Mainframe ASSEMBLER LEARNING MANUAL w... upendrasri PL/I & Assembler 2 Tue May 05, 2020 11:42 am
No new posts LA instruction in assembler janishyathi bonam PL/I & Assembler 4 Tue May 05, 2020 8:17 am

Back to Top
 
Job Vacancies | Forum Rules | Bookmarks | Subscriptions | FAQ | Polls | Contact Us