When a record is read from the system input file, it is first scanned to determine whether it is a BCI control command. If not, it is recognized as a user command to be passed to the product region. However, before it is passed, it is reformatted to remove leading blanks. If a command exit is active, it is called passing the command as a parameter. The command exit may then choose to modify the command, to reject it, or to insert additional commands into the input stream.
On entering the exit, register 1 contains the address of a 76-byte area as follows:
Contain a binary full word indicating the length of the following data which can be a maximum of 72 bytes.
Contain the user command and data with leading blanks removed and any unused characters to the right filled with blanks.
Before returning, the exit should set Register 15 to indicate what action BCI is to take with the returned data, as follows: