CS232, Spring 2008, JVM2 Project

How return works in the JVM2

Here are the microinstructions that implement the return instruction, with comments added to help explain what is happening.

; dispose of the subroutine's stack frame. In other words, restore sp to point to the top of the caller's stack frame.
   lv->sp
   dec4-sp
; put the return address into the pc register
   lv->mar
   mdr<-Stack[mar]
   mdr->mar
   mdr<-Stack[mar]
   mdr->pc
; restore lv to point to the bottom of the caller's stack frame
   inc4-mar
   mdr<-Stack[mar]
   mdr->lv
; if the stack is not empy, update the TOS to match the top of the stack
   sp<0?skip2
   mdr<-Stack[sp]
   mdr->tos
   End