z/OS Block Mode Screen Design › 3270 Information Display System › Initial Write Compared to Subsequent Write

Initial Write Compared to Subsequent Write

The initial write of a screen is the case where the screen must clear what may exist on the screen and write the first instance of its image on the screen. After it is written, the image is retained in the device and control unit buffers until it is changed or erased. The CA Gen implementation rewrites the content of fields that change. The software does not redefine the fields, nor does it rewrite the entire screen.

Additionally, on the initial write, CA Gen uses either the Erase/Write or the Erase/Write Alternate device commands. These commands first clear the buffer of all formatting information and set all display positions to nulls, which are displayed as blanks. Then, the data stream is processed to write new formatting and data into the buffer.