© 2000 Scenix Semiconductor, Inc. All rights reserved. - 44 - www.scenix.com SX48BD/SX52BD/SX52BD75/SX52BD100 15.11    Subroutine Operation 15.11.1  Push Operation When   a   subroutine   is   called,   the   return   address   is pushed   onto   the   subroutine   stack.   Specifically,   each address in the stack is moved to the next lower level in order  to  make  room  for  the  new  address  to  be  stored. Stack  1  receives  the  contents  of  the  program  counter. Stack 8 is overwritten with what was in Stack 7. The con- tents of stack 8 are lost. 15.11.2  Pop Operation When  a  return  instruction  is  executed  the  subroutine stack is popped. Specifically, the contents of Stack 1 are copied into the program counter and the contents of each stack level are moved to the next higher level. For exam- ple, Stack 1 receives the contents of Stack 2, etc., until Stack 7 is overwritten with the contents of Stack 8. Stack 8 is left unchanged, so the contents of Stack 8 are dupli- cated in Stack 7. PC<11:0> STACK 1 STACK 2 STACK 3 STACK 4 STACK 5 STACK 6 STACK 7 STACK 8 PC<11:0> STACK 1 STACK 2 STACK 3 STACK 4 STACK 5 STACK 6 STACK 7 STACK 8