IBM SC33-1683-02 - Manual (Page 660)

A general note about user-written programs
The following comment applies to all user-written programs mentioned in Part
5 of this book:
v Upon return from any user-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.
628 CICS TS for OS/390: CICS Customization Guide