Often, Forth implementations have a simple virtual machine at heart, consisting of three primitives. Those are:

  • nest, also called docol
  • unnest, or semi_s (;s)
  • next

three registers are used to implement the virtual machine, another one exists for passing data between words, which use the vm. These are: different models exist to implement the forth vm, the one given here implements indirect threaded code, or ITC 
next:   (ip)+ ->   w    ;  jmp (w)+
nest:    ip   -> -(rp)  ;  w -> ip  ;  next
unnest: (rp)+ ->   ip   ;  next
other models are:
  • DTC (Direct Threaded Code)
  • STC (Subroutine Threaded Code)
  • TTC (Token Threaded Code)

less often used are
  • return threading
  • string threading
  • call threading