The screen size of the display session depends on the attachment protocol and the model size defined in the session profile. For non-SNA DFT, HIA, and TCP/IP sessions, the default screen size is 24 rows by 80 columns. The alternate screen size is the model size configured in the HCON session profile. The screen size information is returned to the host application during a 3270 Read Partition Query function.
For SNA sessions, the session characteristics, including screen size, are defined using a LOGMODE entry within the host LOGMODE table. Users select the appropriate mode entry during session establishment time to achieve the desired screen size. (For more information, refer to "Setting Up the Host Logmode Table for HCON".) The session characteristics are transported to HCON using an SNA BIND command. Both the default and alternate screen sizes can be determined from the BIND command parameters (bytes 20 through 24) during the primary logical unit (PLU) to Secondary Logical Unit (SLU) session establishment time.
For the SSCP to LU session, the screen size is fixed at 24 rows by 80 columns. When the LU to LU session is established, the default and alternate sizes are returned to the host application during a Read Partition Query function. If the session owner changes to SSCP by pressing the System Request (SysReq) key, the screen size is set to the default.
The SNA BIND command parameters (bytes 20 to 24) and the influence upon the screen size include:
| SNA Screen Size Handling | |||
| Byte 24 | Base Screen Size | Alternate Screen Size | Display Models |
| X'00' | 24 x 80 | 24 x 80 | 2 |
| X'02' | 24 x 80 | 24 x 80 | 2 |
| X'03' | 24 x 80 | Unspecified (note 4) | 2 through 5 |
| X'7E' | Use byte 20, 21 (note 1) | Use byte 20, 21 (note 5) | 2 through 5 |
| X'7F' | Use byte 20, 21 (note 1) | Use byte 22, 23 (note 2) | 2 through 5 |
| X'01' | (note 3) | ||
Notes:
- For SNA DFT, the BIND is rejected if the base screen size is not 24 by 80. For Standalone mode, the BIND is rejected if the base screen size is less than 24 by 80 or greater than the physical display area.
- For SNA DFT, the BIND is rejected if the alternate screen size is greater than the 3270 model (rows by columns) specified in the session profile. For Standalone mode, ensure the alternate screen size fits the physical display (at least 24 by 80).
- For SNA DFT, the BIND is rejected. For Standalone mode, the BIND is accepted, but causes an irrecoverable error in HCON.
- For Standalone mode, the alternate screen size is determined by the best fit of model 2 through 5 to the physical display area and returns to the host during Read Partition Query.
- For SNA Standalone mode, Erase/Write Alternate (EWA) 3270 data stream commands are rejected with the sense code 0x1003 .