IBM SC34-6814-04 - Manual (Page 710)

A general note about user-written programs:
On 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.
688 Customization Guide