OPEN

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

If you provide a character string in the str5 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.

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

0: Instructs the assembler to open the associated data data set, and supply the associated data records to the exit in later PROCESS calls.
Note: A reason code of 8 indicates that the exit intends to discard both type X'0002' and X'0090' records presented to it during the assembly.
This reason code is used by the assembler when building the assembler summary section of the listing. To calculate accurate associated data record counts the assembler must be aware of any future discards of the final two records to be written; the statistics record and the end record.

A reason code of 16 indicates a REINIT call is required.
4: Indicates that the exit writes the associated data records in later WRITE calls. If you want to provide the assembler with the values for the system variables &SYSADATA_DSN, &SYSADATA_MEMBER, and &SYSADATA_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 8 indicates that the exit intends to discard both type X'0002' and X'0090' records presented to it during the assembly.
This reason code is used by the assembler when building the assembler summary section of the listing. To calculate accurate associated data record counts the assembler must be aware of any future discards of the final two records to be written; the statistics record and the end record.

A reason code of 16 indicates a REINIT call is required.
16: Instructs the assembler to open the ADATA data set and make no further calls to the exit.