Editing Threaded code (section)

==Common amenities==
Separating the data and return stacks in a machine eliminates a great deal of stack management code, substantially reducing the size of the threaded code. The dual-stack principle originated three times independently: for [[Burroughs large systems]], [[Forth (programming language)|Forth]], and [[PostScript]].  It is used in some [[Java virtual machine]]s.

Three [[processor register|registers]] are often present in a threaded virtual machine. Another one exists for passing data between [[subroutine]]s ('words'). These are:
* ip or i ([[instruction pointer]]) of the virtual machine (not to be confused with the [[program counter]] of the underlying hardware implementing the VM)
* w (work pointer)
* rp or r (return [[stack (data structure)|stack]] pointer)
* sp or s ([[parameter]] stack pointer for passing parameters between words)

Often, threaded [[virtual machine]]s, such as implementations of Forth, have a simple virtual machine at heart, consisting of three ''primitives''. Those are:
# ''nest'', also called ''docol''
# ''unnest'', or ''semi_s'' (;s)
# ''next''

In an indirect-threaded virtual machine, the one given here, the operations are:
<syntaxhighlight lang="c">
 next:
   *ip++ -> w
   jump **w++
 nest:
   ip -> *rp++
   w -> ip
   next
 unnest:
   *--rp -> ip
   next
</syntaxhighlight>