The 1802 had sixteen general purpose 16-bit registers. Any one of these could be selected as the program counter. In a tiny embedded system (which is what the part was meant for), you might choose to designate one as the ‘normal’ PC and reserve a few others for important subroutines, which could then be invoked with a one-byte ‘SEP n’ instruction. Similarly you could implement coroutines or simple task switching by switching the selected PC between a pair of registers.

On the other hand, there was no conventional call instruction. The SCRT (“Standard Call and Return Technique”) for recursive or reentrant subroutines essentially involved defining (non-reentrant) coroutines to perform the ‘recursive call’ and ‘recursive return’ operations.