When an exit routine is given control using a 32-bit exit parameter list, Register 13 points to a 19-word area. The 19th word of this area can be used to pass information between Assembler exits. For example, when the COMMAREA PARM is used, the 19th word can be set to point to the exit communication area COMMAREA provides. The first 2 bytes of this communication area give the length of the area. You are free to change the entire communication area, including the initial halfword.
For COBOL or C exits, the address and length of this area are passed in the COBOL or C program’s parameter list. In this case, there is no halfword preface – the address points directly to the communication area.