Forth virtual machine

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:

• ip (instruction pointer)
• w (word pointer)
• rp (return stack pointer)
• sp (parameter stack pointer for passing parameters between words)

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