OPEN

The assembler calls the exit with a request type of 1 (OPEN) at the start of the assembly.

The exit sets the return code in the Exit Parameter List to one of these:

0: Instructs the assembler to open the terminal data set, and supply the terminal output records to the exit in later PROCESS calls.
The exit can set the record length for the terminal data set by setting the reason code to 4 and the buffer length field. The buffer length field can be set to any value from 1 to 255 on z/OS® and CMS, or from 1 to 125 on z/VSE®. If the value is zero or greater than 255 on z/OS and CMS, or zero or greater than 125 on z/VSE, the assembler issues message ASMA404W and does not call the exit for any further processing.

Note: A reason code of 16 indicates a REINIT call is required.
4: Indicates that the exit writes the terminal records in later WRITE calls. If you want to provide the assembler with the values for the system variables &SYSTERM_DSN, &SYSTERM_MEMBER, and &SYSTERM_VOLUME, the exit must set the reason code to 4 and place the values in the data set name, member name, and volume serial fields of the exit-specific information block. The assembler also shows this information in the Diagnostic Cross Reference and Assembler Summary section of the listing, and includes it in the associated data file Output File Information record.
Note: A reason code of 16 indicates a REINIT call is required.
16: Instructs the assembler to open the terminal data set and make no further calls to the exit.

If you provide a character string in the str6 suboption of the EXIT assembler option, the buffer pointer field points to the character string, and the buffer length contains the length of the character string. The buffer length is set to zero if there is no character string.