IBM SC34-6814-04 - Manual (Page 802)

A general note about user-written programs:
The following comment applies to all user-written programs mentioned in Part 6 of
this book:
v Upon return from any customer-written program, CICS must always receive
control in primary-space translation mode, with the original contents of all access
registers restored, and with all general purpose registers restored (except for
those which provide return codes or linkage information).
For information about translation modes, refer to the IBM ESA/370 Principles of
Operation manual.
780 Customization Guide