Named sub-field addressing

Real ICL lets an instrument declare named Alias sub-ranges within its own register: a control/status word split into meaningfully-named fields rather than one opaque integer. Declare them on the InstrumentSpec, then address them by name from iWrite/iRead instead of writing/reading the whole register.

from warptap import Alias, InstrumentSpec, PDLInterpreter, build_sib_plan

specs = [
    InstrumentSpec(
        "status_reg", width=4, capture_value=0,
        aliases=(Alias("lo", 0, 1), Alias("hi", 2, 3)),
    )
]
graph, root = build_sib_plan(specs)

pdl = PDLInterpreter(graph, root)
pdl.iTarget("status_reg")
pdl.iWrite(0b01, field="lo")   # bits[1:0]
pdl.iWrite(0b11, field="hi")   # bits[3:2] -- doesn't clobber lo's own bits
pdl.iApply()

field=None (the default on both iWrite and iRead) addresses the whole instrument, exactly as without any Alias declared at all. An unknown field name raises PDLError naming the bad field, rather than silently addressing the wrong bits.

Fields and separate applies

Writing one field in a fresh iApply cycle, with no other field of the same instrument also queued that same batch, preserves the other fields' own last-committed values instead of zeroing them: PDLInterpreter tracks each WRITE-direction instrument's own last-known committed value across calls, so a sub-field write only ever touches the bits it names. iRead(expected, field=...) works the same way for the read side, masking in only the named sub-range so the rest of the instrument's content is don't-care for that check.