Multi-arm ScanMux

A real IEEE 1687 ScanMux selects one of several arms into the live scan chain based on a select value: only the matched arm's own content is actually shiftable; every other arm stays physically detached until it's selected. ScanMuxNode/ScanArm model this directly.

Unlike HierarchySpec/InstrumentSpec, build_sib_plan doesn't construct a ScanMuxNode yet: build the PhysicalGraph (and its matching ModuleInstance tree) directly instead:

from warptap import (
    InstrumentDirection, InstrumentNode, ModuleInstance,
    PhysicalGraph, ScanArm, ScanMuxNode,
)

mux = ScanMuxNode(
    "mux_a", select_width=2,
    arms=(
        ScanArm(
            values=(1,),
            instrument=InstrumentNode(
                name="arm1", width=3, capture_value=0,
                direction=InstrumentDirection.WRITE,
            ),
        ),
        ScanArm(
            values=(2,),
            instrument=InstrumentNode(
                name="arm2", width=2, capture_value=0,
                direction=InstrumentDirection.WRITE,
            ),
        ),
    ),
)
graph = PhysicalGraph(chain=(mux,))
root = ModuleInstance("top", children=(ModuleInstance("arm1"), ModuleInstance("arm2")))

Each ScanArm's own values is the select value(s) that route to it (arm1 at select=1, arm2 at select=2); an arm gates exactly one of instrument/nested: the same leaf-or- hierarchy shape a plain SibNode has, so a mux arm can gate a further nested sub-network instead of a single instrument, just like HierarchySpec does.

Driving it

Targeting arm1 then arm2 retargets the mux directly from one arm to the other: no intermediate "close everything" step:

from warptap import PDLInterpreter

pdl = PDLInterpreter(graph, root)
pdl.iTarget("arm1")
pdl.iWrite(0b110)
pdl.iApply()

pdl.iTarget("arm2")     # switches the mux's own select value directly
pdl.iWrite(0b01)
pdl.iApply()

pdl.iTarget("arm1")     # switches back -- arm1's own value survives the round trip
pdl.iRead(0b110)
pdl.iApply()

A written value survives being switched away from and back, on real hardware: confirmed by cross-simulating this exact scenario against real RTL.