IP2022 User’s Manual—Instruction Set Architecture www.ubicom.com 81 When a subroutine is called, the return address is pushed onto the
subroutine  stack,  as  shown  in  Figure  3-12.  Specifically,  each
saved  address  in  the stack is  moved to  the  next  lower  level to
make room for the new address to be saved. Stack 1 receives the
contents of the program counter. Stack 16 is overwritten with what
was in Stack 15. The contents of stack 16 are lost. Figure 3-12  Stack Operation on Subroutine Call 515-010.eps Stack 1 Stack 2 Program Counter (15:0) Stack 16 Contents are Discarded Stack 3 Stack 4 Stack 5 Stack 6 Stack 7 Stack 8 Stack 9 Stack 10 Stack 11 Stack 12 Stack 13 Stack 14 Stack 15 Stack 16