MVSFORUMS.com

[ReinerZ]

Hi all,

I'm new to z/OS MVS, but have worked for many years in a VSE/VM-Shop. The last ten years I have worked in the "Windows World". Because of a new job I have to refresh my mainframe-skills immediately (especially Assembler language).

To accomplish this, I have begun to write a little Program (in Assembler-Language), which extracts data of a few (of the many!!!) MVS-Data-Area's. In particular this are the ASCB, TCB, JSCB, JCT Control Blocks. Starting from an ASCB I navigate (with the help of embedded pointers) down the chain. Reading the documentation of the various areas I saw an acronym named "SVA". For example the field JSCBJCTA is a field in the JSCB that points to the JCT. This field is classfied as a CHARACTER-Field with a length of 3 Bytes (24-Bit Address). I have inserted this 3 bytes via the ICM-Instruction in a GP-Register and used this Reg. as a Base-Register for the JCT-DSECT. This worked fine, with a "little" restriction that the address in my DSECT-Reg. points 16 Bytes BEFORE the beginning of the JCT-Area!! Adding F'16' to the DSECT-Reg. after the ICM points now exact to the beginning of the JCT! But this seems somewhat strange to me .

My question is now:
What does the acronym "SVA" exactly mean? "System Virtual Address"? If this is correct, what is the difference between a "System virtual address" and a usual ("normal") virtual address?

Thanks in advance and greetings from Germany!

Reiner

PS: The Comment field for the JSCBJCTA field states, that I should use the SWAREQ-Macro to convert this field to an address. But SWAREQ is an "Authorized-Service" and I cant't use those services currently, because I'm workin via TN3270 only as a "Guest" on the z900-Machine of the University of Leipzig, Germany.

[bob_buxton]

SVA actually means SWA virtual addres where SWA is the Scheduler Work Area which is a generic term for the control blocks which represent your JCL and other data.

For historic reasons many of these pointers are 3-byte fields but the associated control blocks may be above the 16Mb line and so require a 4-byte address.
The SWAREQ macro converts the 3 byte token into a real 4 byte address.

Although the SWAREQ macro is documented in the Authorised Services book it does have an UNAUTH=YES option and can be used by any program without special authority. It is a fast service and will not impact your programs performance
_________________
Bob Buxton
Ex Websphere MQ for zOS development

[ReinerZ]

Bob,

allright, good answer! Thank you!

Reiner

[kolusu]

ReinerZ,

SVA = Shared Virtual Area which is a major element of VSE/ESA virtual storage both above and below the 16MB line. The storage areas that make up the SVA contain all the common reentrant modules shared by the system. The SVA provides economy of real storage by sharing one copy of the modules, protection because SVA code cannot be overwritten except by key 0 programs, and reduced pathlength because the modules can be branched to. The SVA is duplicated above the 16MB line and is often referred to as the 31-bit SVA.

Also if you ever have any questions regarding any acronyms or terms of mainframe then refer this link

http://mainframes.com/

Hope this helps...

Cheers

Kolusu
_________________
Kolusu
www.linkedin.com/in/kolusu

[ReinerZ]

Hi,

sorry, but I know what the acronym "SVA" means under VSE! I have worked with VSE/VM for more than 15 years...

What I'm talking about in this posting is: The meaning of "SVA" under z/OS resp. MVS...


PS: Thanks for the link! It's very useful!

Best regards

Reiner