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FA-89236 / Digital logic simulation / Open access

Unknown select corrupts every decoder line · case 01

A decoder with one unknown select bit drives x on lines that cannot be selected.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The unknown case does not restrict itself to candidate lines.

THE FAILURE

The unknown case does not restrict itself to candidate lines.

Unsuccessful approach: An unknown enable still spreads x onto non-candidate lines.

Case contract

Input [kind, sel, data]: sel is MSB-first over 0/1/x/z; an x or z select bit makes both values of that bit candidates. mux: the output is the common value of all candidate data inputs if they agree and are 0 or 1, else 'x'. dec: data[0] is the enable; enable 0 gives all '0'; otherwise candidate lines are '1' only when there is a single candidate and the enable is 1, else 'x'; non-candidate lines are '0'.

Why this case matters

Unknown-select pessimism rules in RTL simulation decide whether X on a control input corrupts datapath outputs.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(*args):
    kind, sel, data = args
    cands = [0]
    for s in sel:
        if s in '01':
            cands = [c * 2 + int(s) for c in cands]
        else:
            cands = [c * 2 + b for c in cands for b in (0, 1)]
    if kind == 'mux':
        vals = sorted({data[c] for c in cands})
        return vals[0] if len(vals) == 1 and vals[0] in '01' else 'x'
    en = data[0]
    out = []
    for i in range(2 ** len(sel)):
        if en == '0':
            out.append('0')
        elif len(cands) > 1 or en != '1':
            out.append('x')
        elif i in cands:
            out.append('1')
        else:
            out.append('0')
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x0', ['1', '0', '1', '0']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '01', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '00', ['x']], ['x', '0', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x0', ['1', '0', '1', '0']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '01', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '00', ['x']], ['x', '0', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
mux unknown select differing dataxxPassed
mux unknown select agreeing data11Passed
mux4 unknown low select bit11Passed
mux4 unknown high select bit11Passed
mux unknown select floating dataxxPassed
mux4 known select11Passed
decoder unknown select bit['x', 'x', 'x', 'x']['0', '0', 'x', 'x']Failed
decoder unknown enable['x', 'x', 'x', 'x']['0', 'x', '0', '0']Failed
decoder enabled['0', '0', '1', '0']['0', '0', '1', '0']Passed

SHA-256 / 228370b1c80770ff94c8fba344f8c658dd030f86413fa16bdb489ca7cdcf2077

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(*args):
    kind, sel, data = args
    cands = [0]
    for s in sel:
        if s in '01':
            cands = [c * 2 + int(s) for c in cands]
        else:
            cands = [c * 2 + b for c in cands for b in (0, 1)]
    if kind == 'mux':
        vals = sorted({data[c] for c in cands})
        return vals[0] if len(vals) == 1 and vals[0] in '01' else 'x'
    en = data[0]
    out = []
    for i in range(2 ** len(sel)):
        if en == '0':
            out.append('0')
        elif len(cands) > 1 and i in cands or en == 'x':
            out.append('x')
        elif i in cands:
            out.append('1')
        else:
            out.append('0')
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x0', ['1', '0', '1', '0']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '01', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '00', ['x']], ['x', '0', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x0', ['1', '0', '1', '0']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '01', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '00', ['x']], ['x', '0', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])], [('mux unknown select differing data', ['mux', 'x', ['0', '1']], 'x'), ('mux unknown select agreeing data', ['mux', 'x', ['1', '1']], '1'), ('mux4 unknown low select bit', ['mux', '1x', ['0', '1', '1', '1']], '1'), ('mux4 unknown high select bit', ['mux', 'x1', ['0', '1', '0', '1']], '1'), ('mux unknown select floating data', ['mux', 'z', ['z', 'z']], 'x'), ('mux4 known select', ['mux', '10', ['0', '1', '1', '0']], '1'), ('decoder unknown select bit', ['dec', '1x', ['1']], ['0', '0', 'x', 'x']), ('decoder unknown enable', ['dec', '01', ['x']], ['0', 'x', '0', '0']), ('decoder enabled', ['dec', '10', ['1']], ['0', '0', '1', '0'])]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
mux unknown select differing dataxxPassed
mux unknown select agreeing data11Passed
mux4 unknown low select bit11Passed
mux4 unknown high select bit11Passed
mux unknown select floating dataxxPassed
mux4 known select11Passed
decoder unknown select bit['0', '0', 'x', 'x']['0', '0', 'x', 'x']Passed
decoder unknown enable['x', 'x', 'x', 'x']['0', 'x', '0', '0']Failed
decoder enabled['0', '0', '1', '0']['0', '0', '1', '0']Passed

SHA-256 / e212ae6f8599edbe9e6dd56863f4f3843e38a5bb93e68c03c3e1a9dae6b862d5

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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Verification & scope

A deterministic bounded teaching model of one simulator rule set; the contract is stipulated and is not a claim of conformance to any HDL standard or commercial simulator. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:51:15.551955+00:00.

Case digest / eb9bea72a3e1766142e9d6df5ab8e3c5c8c88c4e7e409b1356f1bb52d750743b