John, I recall the same my knowledge is old..over 20 yrs working on VSE
Scott ford www.identityforge.com from my IPAD 'Infinite wisdom through infinite means' > On Dec 10, 2013, at 10:59 AM, John Eells <[email protected]> wrote: > > Skip Robinson wrote: >> Not sure what's meant by having to chain CCWs. I just finished updating an >> old RYO program that writes 80 byte records to any device specified >> because I needed to test >32K blocks on tape. I merely added a DCBE with >> BLKSIZE=0 and pointed to it in the existing DCB. No other changes. Created >> a tape file with BLKSIZE=261760 (256K) (as reported by RMM) by specifying >> that block size in JCL. No attempt at chaining, which I would not know how >> to do anyway. > <snip> > > There are two CCW bits, Chain Command (CC) and Chain Data (CD), that are used > when chaining CCWs to construct a channel program. If I recall correctly, CC > causes the next CCW's command to run and CD causes the data address part of > the next CCW to be used to continue the operation started in a preceding CCW. > No new SSCH is needed for the channel to process the chained CCWs. An > entire channel program can run to completion with single start. > > There are a variety of reasons to chain CCWs, among them performance (no > redispatch is required to issue the next CCW), particularly for disk. > "Simplicity" probably isn't one of them; thank goodness for access methods! > > You can also used Program Controlled Interrupts (PCI) to dynamically append > to a running channel program. Program Fetch does this for load modules, > which have text records that vary in length. If the system has enough cycles > to spare, you can get an entire load module with a single channel program > this way, as the lengths are retrieved and used to build CCWs on the fly. If > the system is too busy, the channel program ends and a new one is built to > pick up where it left off until the entire module has been fetched. > > I have not yet bothered to learn how any of this is done in transfer mode > (with zHPF), which doesn't actually use CCWs... > > -- > John Eells > IBM Poughkeepsie > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
