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 was originated three times independently: for Burroughs large systems, Forth and PostScript, and is used in some Java virtual machines.
Three registers are often present in a threaded virtual machine. Another one exists for passing data between subroutines ('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 pointer)
- sp or s (parameter stack pointer for passing parameters between words)
Often, threaded virtual machines 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:
next: (ip)+ -> w ; jmp (w)+ nest: ip -> -(rp) ; w -> ip ; next unnest: (rp)+ -> ip ; nextThis is perhaps the simplest and fastest interpreter or virtual machine.
Read more about this topic: Threaded Code
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