The next several sections describe the format in which the 3270 transmits data, which you need to understand if you are using terminal control commands. If you are using BMS, you can skip to Unformatted mode, because BMS translates the input for you.
Each modified field comes in as follows:
Only the non-null characters in the field are transmitted; nulls are skipped, wherever they appear. Thus if an entry does not fill the field, and the field was initially nulls, only the characters keyed are transmitted, reducing the length of the inbound data. Nulls (X'00') are not the same as blanks (X'40'), even though they are indistinguishable on the screen. Blanks get transmitted, and hence you normally initialize fields to nulls rather than to blanks, to minimize transmission.
A 3270 read command can specify that the terminal should return the attribute values along with the field contents, but CICS® does not use this option. Consequently, the buffer address is the location of the first byte of field data, not the preceding attributes byte (as it is in the corresponding outbound data stream).