Simple answer: 3270 hardware function called "Read Partion Query" which returns the screen size (plus lots of other info about what the specific terminal can do, such as code pages). Some programs are capable of issuing the RPQ command and parsing the complex RPQ response. Other programs are lazy and don't do that, defaulting to the logmode screen size. You can read all about the RPQ command in the 3270 Data Stream Programming Reference http://bitsavers.trailing-edge.com/pdf/ibm/3270/GA23-0059-4_3270_Data_Stream_Programmers_Reference_Dec88.pdf
On Mon, Dec 6, 2021 at 9:08 PM Christian Svensson < [email protected]> wrote: > Hi folks, > > I am trying to go back to basics a bit and figure out some things that I > have always treated as magic. > Right now it is how a 3270 terminal connected to a z/OS determines the size > of the terminal. In this case through OSA-ICC (zBC12, OSA-Express5) and not > TCPIP, > > On systems where I have configured VTAM with TCPIP the terminal size seems > to almost always "just work". > Let's take a look at an example. I have configured X3270 with model > "3279-4-E" and oversize set to "133x60". > > 1) What does the z/OS VTAM screen look like? > Over OSA-ICC: 80x24 > Over TCPIP: 80x24 > > So far so good. > > 2) What does the TSO screen look like? > Over OSA-ICC: 80x24 > Over TCPIP: 80x24 > > Ok, no change. > > 3) What does the ISPF screen look like? > Over OSA-ICC: 80x24 > Over TCPIP: 133x60 > > Oh! But why? I do not know! Also, after I exit ISPF back to TSO, the TSO > screen is now correctly filling the full screen! > > I also note that z/VM's login screen seems to behave similarly. Over > OSA-ICC the size seems to be fixed at 80x24, but over TCPIP it follows what > I configured X3270 to use. > Dialing a z/OS VTAM terminal through the z/OS login screen (connected via > TCPIP) seems to result in a 133x60 screen both in TSO and ISPF. However if > I ATTACH an OSA-ICC terminal straight to a z/OS guest, ISPF gets 133x60 but > TSO only 80x24 - *but only if I reconnect to an existing user session.* If > I log out and try to log in again I get IKT00405I "Screen erase caused by > error recovery procedure". > > As you can see I am a mess when it comes to understanding how this all fits > together! It seems pretty much random to me, except that TCPIP "almost > always works" and OSA-ICC "almost never works". > I would love to learn more, but I don't even know where to begin. > > Regards, > > ---------------------------------------------------------------------- > 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
