If an exit requires additional storage, use a subpool other than 0, 240 or 250. Storage allocated in subpool 0 (or in subpools 240 and 250, which are converted to subpool 0 requests) are given a storage key of 0 and SHARED with the jobstep TCB. This can cause any program running in a key other than 0 under the jobstep TCB to experience protection exceptions (abend0c4 rc04) if the program obtained storage in subpool 0 and attempt to modify it.