In my book, that's not *really* a self-modifying channel program. (It's just using the same data area in more than one CCW. I know of at least one ISV product in use today that writes from a buffer and later in that same channel program, uses the same buffer for input.)
The only *real* self-modifying channel program I can recall, was an ISAM-like access method back in the OS/360 days, which read an actual CCW op code from disk to dynamically modify a Seek farther down the chain, making it either a Seek Head, Seek Cylinder or Full Seek, depending on where the desired record resided. As I recall, a Full Seek violated the File Mask and thus was an indication that the desired record was on a different pack. In a non-virtual system, this kind of thing wasn't too hard to do. Today, true, self-modifying channel programs would require the use of a Start I/O driver since it is the real (not virtual) channel program that actually executes. The closest thing I've seen to that in MVS are specialized drivers that use PCIs to monitor the progress of a channel program and then maybe make an on-the-fly change to a CCW depending on some status or item of input data. Such programs always have to allow for the possibility that the CCW to be modified may have already been pre-fetched from storage by the IOP (thus the change may not be in time). Typically the change is simple, like changing a NOP to a TIC, and if the change doesn't get made in time, the NOP just ends the channel program. In any event, ECKD (for disk anyway) probably makes doing such things much harder and considering the overall speed of the hardware today, its difficult to imagine any circumstances where it might be worth the effort. At 02:32 PM 3/8/2007, Bill Fairchild wrote: >In a message dated 3/7/2007 4:47:23 P.M. Central Standard Time, >[EMAIL PROTECTED] writes: >>recent thread discussing patching CCWTRANS to handle ISAM and other >self-modifying channel programs > >Is ISAM still used anywhere under MVS/ESA/OS-390/z? Does anyone know of any >self-modifying DASD channel programs other than from ISAM? Just curious. I >remember seeing a BDAM chan pgm from Bob Rannie in which an R0 data field >was read, updated, and rewritten all in one chan pgm. Would depend on the >BDAM >options being used. ================================================== Art Celestini Celestini Development Services Phone: 201-670-1674 Wyckoff, NJ ============= http://celestini.com ============= Mail sent to the "From" address used in this post will be rejected by our server. Please send off- list email to: ibmmain<at-sign>celestini<dot>com. ================================================== ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
