The logrec recording control buffer is a “wrap-table” similar to the system trace table. The entries are variable in size. The latest entries are the most significant especially if they have not yet been written to the logrec data set. Knowing the areas of the system that have encountered errors and the actions of their associated recovery routines, information obtained from the logrec data set and from the logrec recording control buffer helps provide an overall understanding of the environment you are about to investigate.
You can find the oldest entry in the buffer by locating the end of the unused or free area, obtained from RCBFREE+RCBFLNG. (If this sum brings you to a point beyond the end of the buffer, subtract RCBTLNG from the sum.) You can also read the buffer backwards by using the entry length at the end of each entry. The latest entry appears directly before the free or unused area of the buffer.
For details about the format of the RCB, see z/OS® MVS™ Data Areas in z/OS Internet library.