diff --git a/CLAUDE.md b/CLAUDE.md
index 379c47a..3d7917d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -57,6 +57,10 @@ pm3py/
     replay.py              # Trace replay
     access_control/        # Wiegand, OSDP
   transponders/            # tag/transponder models (extracted from sim/)
+    bitfield.py            # BitField descriptor + Register base — self-describing config
+                           #   registers (T5577Config, Ntag21xConfig, UL EV1/C, MFC access
+                           #   bits). Fields are documented attrs -> shell completion + repr
+                           #   decode table. Layouts cross-checked vs the firmware submodule.
     hf/iso14443a/          # Tag14443A_3/4, MifareClassic, DESFire, NfcType2/4
                            #   nxp/type2,ntag21x,ultralight,ntag_i2c
                            #   (NTAG210-216, Ultralight/C/EV1, NTAG I2C plus)
diff --git a/pm3py/sim/__init__.py b/pm3py/sim/__init__.py
index a3b964f..678451b 100644
--- a/pm3py/sim/__init__.py
+++ b/pm3py/sim/__init__.py
@@ -20,16 +20,18 @@ from pm3py.transponders.hf.iso14443a.base import (
     Tag14443A, Tag14443A_3, Tag14443A_4, Reader14443A, State14443A,
 )
 from pm3py.transponders.hf.iso14443a.nxp.crypto1 import Crypto1
-from pm3py.transponders.hf.iso14443a.nxp.mifare_classic import MifareClassicTag, MifareClassicReader
+from pm3py.transponders.hf.iso14443a.nxp.mifare_classic import (
+    MifareClassicTag, MifareClassicReader, MifareAccessConditions,
+)
 from pm3py.transponders.hf.iso14443a.nxp.desfire import DesfireTag, DesfireReader
 from pm3py.transponders.hf.iso14443a.ndef import NfcType2Tag, NfcType4Tag
-from pm3py.transponders.hf.iso14443a.nxp.type2 import NxpType2Tag
+from pm3py.transponders.hf.iso14443a.nxp.type2 import NxpType2Tag, Type2Config
 from pm3py.transponders.hf.iso14443a.nxp.ntag21x import (
-    Ntag21x, NTAG210, NTAG212, NTAG213, NTAG215, NTAG216,
+    Ntag21x, NTAG210, NTAG212, NTAG213, NTAG215, NTAG216, Ntag21xConfig,
 )
 from pm3py.transponders.hf.iso14443a.nxp.ultralight import (
     MifareUltralight, MifareUltralightC, MifareUltralightEV1, MF0UL11, MF0UL21,
-    UL_C_DEFAULT_KEY,
+    UL_C_DEFAULT_KEY, UltralightEV1Config, UltralightCConfig,
 )
 from pm3py.transponders.hf.iso14443a.nxp.ntag_i2c import NtagI2C, NT3H2111, NT3H2211
 from pm3py.transponders.hf.iso14443a.st.st25tn import ST25TN, ST25TN512, ST25TN01K
@@ -63,9 +65,10 @@ from pm3py.transponders.hf.iso15693.ti.rf430frl import (
     RF430FRL15xH, RF430FRL152H, RF430FRL153H, RF430FRL154H,
 )
 from pm3py.transponders.lf.base import TagLF, ReaderLF, Modulation
-from pm3py.transponders.lf.em.em4100 import EM4100Tag, EM4100Reader
+from pm3py.transponders.lf.em.em4100 import EM4100Tag, EM4100Reader, EM4100Code
 from pm3py.transponders.lf.hid.hid import HIDProxTag, HIDReader
-from pm3py.transponders.lf.atmel.t5577 import T5577Tag, T5577Reader
+from pm3py.transponders.bitfield import BitField, Register
+from pm3py.transponders.lf.atmel.t5577 import T5577Tag, T5577Reader, T5577Config
 from pm3py.transponders.implants import xEM, xNT, xM1, FlexDF, NExT, MagicMifareClassicTag
 
 __all__ = [
@@ -76,17 +79,19 @@ __all__ = [
     "Reader", "ScriptedReader", "InteractiveReader", "ReaderStep", "StepResult",
     "Tag14443A", "Tag14443A_3", "Tag14443A_4", "Reader14443A", "State14443A",
     "Crypto1",
-    "MifareClassicTag", "MifareClassicReader",
+    "MifareClassicTag", "MifareClassicReader", "MifareAccessConditions",
     "Tag15693", "Reader15693", "State15693",
     "TagLF", "ReaderLF", "Modulation",
-    "EM4100Tag", "EM4100Reader",
+    "EM4100Tag", "EM4100Reader", "EM4100Code",
     "HIDProxTag", "HIDReader",
-    "T5577Tag", "T5577Reader",
+    "T5577Tag", "T5577Reader", "T5577Config",
+    "BitField", "Register",
     "NfcType2Tag", "NfcType4Tag", "NfcType5Tag",
-    "NxpType2Tag",
-    "Ntag21x", "NTAG210", "NTAG212", "NTAG213", "NTAG215", "NTAG216",
+    "NxpType2Tag", "Type2Config",
+    "Ntag21x", "NTAG210", "NTAG212", "NTAG213", "NTAG215", "NTAG216", "Ntag21xConfig",
     "MifareUltralight", "MifareUltralightC", "MifareUltralightEV1",
     "MF0UL11", "MF0UL21", "UL_C_DEFAULT_KEY",
+    "UltralightEV1Config", "UltralightCConfig",
     "NtagI2C", "NT3H2111", "NT3H2211",
     "ST25TN", "ST25TN512", "ST25TN01K",
     "ST25TA", "ST25TA512B", "ST25TA02KB", "ST25TA16K", "ST25TA64K",
diff --git a/pm3py/transponders/bitfield.py b/pm3py/transponders/bitfield.py
new file mode 100644
index 0000000..be2f6dd
--- /dev/null
+++ b/pm3py/transponders/bitfield.py
@@ -0,0 +1,166 @@
+"""Self-describing bit-field registers.
+
+A ``Register`` is a fixed-width integer — a config word, a status byte — whose bits carry
+named, documented meaning. Declare the layout once with ``BitField`` descriptors and you get,
+for free:
+
+  * read / edit by field name       ``cfg.modulation``, ``cfg.max_block = 3``
+  * ``<tab>`` completion of the field names in the shell, each field's one-line ``doc`` as the
+    autocomplete tooltip (bpython/ipython read the descriptor docstring)
+  * a decoded table when you print it — the "always-on hover"
+
+The point is that a bare ``0x00148040`` tells you nothing, while the register spells it out.
+Nobody hand-decodes these words from a datasheet if the object does it for them.
+"""
+from __future__ import annotations
+
+import sys
+from typing import Any
+
+# House-style ANSI (mirrors pm3py/trace/format.py); TTY-gated so tests/pipes stay plain text.
+_DIM = "\033[2m"
+_CYAN = "\033[36m"
+_RESET = "\033[0m"
+
+
+def _color(code: str, text: str) -> str:
+    if not code or not sys.stdout.isatty():
+        return text
+    return f"{code}{text}{_RESET}"
+
+
+class BitField:
+    """A named slice of a register's bits.
+
+    ``BitField(hi, lo)`` covers bits ``[lo, hi]`` inclusive (``lo`` defaults to ``hi`` for a
+    single bit). ``doc`` is the one-line meaning shown in completion tooltips and the decode
+    table. ``values`` is an optional ``{raw_code: label}`` map — when given, reads return the
+    label and writes accept either the label or the raw code (single-bit fields with no map
+    read/write as ``bool``).
+    """
+
+    def __init__(self, hi: int, lo: int | None = None, *, doc: str = "",
+                 values: dict[int, Any] | None = None):
+        if lo is None:
+            lo = hi
+        self.hi, self.lo = max(hi, lo), min(hi, lo)
+        self.width = self.hi - self.lo + 1
+        self.doc = doc
+        self.values = values
+        self._reverse = {v: k for k, v in values.items()} if values else {}
+        self.is_bool = self.width == 1 and values is None
+        self.name = "?"
+        self.__doc__ = doc  # so `cfg.field?` / bpython introspection shows the meaning
+
+    def __set_name__(self, owner: type, name: str) -> None:
+        self.name = name
+
+    @property
+    def _mask(self) -> int:
+        return ((1 << self.width) - 1) << self.lo
+
+    def raw(self, reg: "Register") -> int:
+        """The raw code stored in these bits, before any ``values`` mapping."""
+        return (reg.value >> self.lo) & ((1 << self.width) - 1)
+
+    def __get__(self, reg: "Register | None", owner: type | None = None):
+        if reg is None:
+            return self
+        code = self.raw(reg)
+        if self.values is not None:
+            return self.values.get(code, code)
+        return bool(code) if self.is_bool else code
+
+    def __set__(self, reg: "Register", value: Any) -> None:
+        code = self._encode(value)
+        if code < 0 or code >> self.width:
+            raise ValueError(f"{self.name}: {value!r} does not fit in {self.width} bit(s)")
+        reg.value = (reg.value & ~self._mask) | (code << self.lo)
+
+    def _encode(self, value: Any) -> int:
+        if self._reverse and value in self._reverse:
+            return self._reverse[value]
+        if isinstance(value, bool):
+            return int(value)
+        if isinstance(value, int):
+            return value
+        raise ValueError(f"{self.name}: cannot set to {value!r}")
+
+    def display(self, reg: "Register") -> str:
+        if self.is_bool:
+            return "on" if self.raw(reg) else "off"
+        return str(self.__get__(reg))
+
+    @property
+    def bits(self) -> str:
+        return f"[{self.hi}]" if self.width == 1 else f"[{self.hi}:{self.lo}]"
+
+
+class Register:
+    """Base for a fixed-width register. Subclass, set ``_width_bits`` / ``_label``, and declare
+    ``BitField`` class attributes. Construct from a raw value (``Reg(0x00148040)``) or empty."""
+
+    _width_bits = 32
+    _label = "register"
+
+    def __init__(self, value: "int | Register" = 0):
+        self.value = int(value) & ((1 << self._width_bits) - 1)
+
+    def __int__(self) -> int:
+        return self.value
+
+    def __index__(self) -> int:
+        return self.value
+
+    def __bytes__(self) -> bytes:
+        return self.value.to_bytes(self._width_bits // 8, "big")
+
+    def __eq__(self, other: object) -> bool:
+        if isinstance(other, Register):
+            return type(self) is type(other) and self.value == other.value
+        if isinstance(other, int):
+            return self.value == other
+        return NotImplemented
+
+    def __hash__(self) -> int:
+        return hash((type(self), self.value))
+
+    @classmethod
+    def fields(cls) -> list[tuple[str, BitField]]:
+        """All declared ``BitField``s, highest bit first (base classes included)."""
+        seen: dict[str, BitField] = {}
+        for klass in cls.__mro__:
+            for name, obj in vars(klass).items():
+                if isinstance(obj, BitField) and name not in seen:
+                    seen[name] = obj
+        return sorted(seen.items(), key=lambda kv: kv[1].lo, reverse=True)
+
+    def _rows(self) -> list[dict]:
+        """Decoded rows (dicts: name/value/bits/doc/lo), high bit first. Subclasses may add
+        computed rows via ``_extra_rows`` (e.g. a mode-dependent field)."""
+        rows = [
+            {"name": name, "value": bf.display(self), "bits": bf.bits, "doc": bf.doc, "lo": bf.lo}
+            for name, bf in self.fields()
+        ]
+        rows.extend(self._extra_rows())
+        rows.sort(key=lambda r: r["lo"], reverse=True)
+        return rows
+
+    def _extra_rows(self) -> list[dict]:
+        return []
+
+    def decode(self) -> dict[str, Any]:
+        """Plain ``{field: value}`` mapping — no ANSI, handy for tests and programmatic use."""
+        return {r["name"]: r["value"] for r in self._rows()}
+
+    def __repr__(self) -> str:
+        rows = self._rows()
+        hexw = self._width_bits // 4
+        head = f"{self._label} = 0x{self.value:0{hexw}X}"
+        name_w = max((len(r["name"]) for r in rows), default=0)
+        val_w = max((len(r["value"]) for r in rows), default=0)
+        lines = [head]
+        for r in rows:
+            line = f"  {r['name']:<{name_w}}  {r['value']:<{val_w}}  {_color(_DIM, r['bits'])}"
+            lines.append(line.rstrip())
+        return _color(_CYAN, head) + "\n" + "\n".join(lines[1:])
diff --git a/pm3py/transponders/hf/iso14443a/nxp/mifare_classic.py b/pm3py/transponders/hf/iso14443a/nxp/mifare_classic.py
index 6217fdd..8d23704 100644
--- a/pm3py/transponders/hf/iso14443a/nxp/mifare_classic.py
+++ b/pm3py/transponders/hf/iso14443a/nxp/mifare_classic.py
@@ -5,6 +5,7 @@ import struct
 
 from pm3py.sim.frame import RFFrame
 from pm3py.sim.medium import Medium
+from pm3py.transponders.bitfield import BitField, Register
 from ..base import Tag14443A_3, Reader14443A, _compute_bcc
 from .crypto1 import Crypto1
 
@@ -18,6 +19,118 @@ NACK = 0x00
 
 BLOCK_SIZE = 16
 
+# Per-block access-condition (C1 C2 C3) meanings. Data-block table matches Proxmark3
+# mifare4.c; the trailer table is written to the NXP MF1S50 datasheet (r=read, w=write,
+# A/B = which key, AB = either key, - = never).
+_MFAC_DATA = {
+    0: "r=AB w=AB incr=AB dec/tr/rst=AB  (transport)",
+    1: "r=AB dec/tr/rst=AB  (value block)",
+    2: "r=AB  (read-only)",
+    3: "r=B w=B",
+    4: "r=AB w=B",
+    5: "r=B  (read-only, key B)",
+    6: "r=AB w=B incr=B dec/tr/rst=AB  (value block)",
+    7: "no access",
+}
+_MFAC_TRAILER = {
+    0: "KeyA w=A; AC r=A; KeyB r=A w=A  (KeyB is readable data)",
+    1: "KeyA w=A; AC r=A w=A; KeyB r=A w=A  (transport/default)",
+    2: "KeyA w=-; AC r=A; KeyB r=A w=-",
+    3: "KeyA w=B; AC r=AB w=B; KeyB w=B",
+    4: "KeyA w=B; AC r=AB; KeyB w=B",
+    5: "AC r=AB w=B  (keys locked)",
+    6: "KeyA w=-; AC r=AB  (keys locked)",
+    7: "AC r=AB  (all keys locked)",
+}
+
+
+class MifareAccessConditions(Register):
+    """MIFARE Classic sector-trailer access bits — trailer bytes 6-9 (the three access bytes
+    plus the general-purpose byte), decoded as a 32-bit word.
+
+    Each of the sector's four blocks (three data blocks + the trailer) has a 3-bit access
+    condition ``C1 C2 C3``. The three access bytes store every condition *twice* — once true,
+    once inverted — so one flipped bit permanently bricks the sector. This spells out each
+    block in plain English, flags an inconsistent inverted copy (``.valid``), and, when you
+    :meth:`build` one, computes the inverted copy for you so it is always self-consistent::
+
+        MifareAccessConditions(0xFF078069)             # decode the factory-default trailer
+        ac = MifareAccessConditions.build(trailer=3)   # KeyB-controlled trailer
+        bytes(ac)                                       # -> the 4 trailer bytes 6..9 to write
+    """
+
+    _width_bits = 32
+    _label = "MIFARE access bits (trailer 6-9)"
+
+    gpb = BitField(7, 0, doc="general-purpose byte — free user data (e.g. the MAD GPB)")
+
+    _BLOCKS = ("block0", "block1", "block2", "trailer")
+
+    def _cond(self, blockn: int) -> int:
+        """The 3-bit access condition (C1<<2 | C2<<1 | C3) for one block of the sector."""
+        v = self.value
+        c1 = (v >> (20 + blockn)) & 1   # C1 = byte7 high nibble
+        c2 = (v >> (8 + blockn)) & 1    # C2 = byte8 low nibble
+        c3 = (v >> (12 + blockn)) & 1   # C3 = byte8 high nibble
+        return (c1 << 2) | (c2 << 1) | c3
+
+    @property
+    def block0(self) -> int:
+        return self._cond(0)
+
+    @property
+    def block1(self) -> int:
+        return self._cond(1)
+
+    @property
+    def block2(self) -> int:
+        return self._cond(2)
+
+    @property
+    def trailer(self) -> int:
+        return self._cond(3)
+
+    def describe(self, blockn: int) -> str:
+        table = _MFAC_TRAILER if blockn == 3 else _MFAC_DATA
+        return table.get(self._cond(blockn), "?")
+
+    @property
+    def valid(self) -> bool:
+        """True iff the redundant inverted copy is the nibble-complement of the true copy."""
+        v = self.value
+        return (((v >> 24) & 0x0F) == (((v >> 20) & 0x0F) ^ 0x0F) and   # ~C1 vs C1
+                ((v >> 28) & 0x0F) == (((v >> 8) & 0x0F) ^ 0x0F) and    # ~C2 vs C2
+                ((v >> 16) & 0x0F) == (((v >> 12) & 0x0F) ^ 0x0F))      # ~C3 vs C3
+
+    @classmethod
+    def build(cls, block0: int = 0, block1: int = 0, block2: int = 0,
+              trailer: int = 1, gpb: int = 0x00) -> "MifareAccessConditions":
+        """Compose access bytes from the four per-block conditions (0..7), filling in the
+        inverted copy so the result always passes ``.valid``. ``trailer`` defaults to 1
+        (the factory/transport trailer). ``gpb`` is the general-purpose byte."""
+        c1 = c2 = c3 = 0
+        for n, cond in enumerate((block0, block1, block2, trailer)):
+            if not 0 <= cond <= 7:
+                raise ValueError(f"block {n} access condition must be 0..7, got {cond}")
+            c1 |= ((cond >> 2) & 1) << n
+            c2 |= ((cond >> 1) & 1) << n
+            c3 |= (cond & 1) << n
+        byte6 = ((c2 ^ 0x0F) << 4) | (c1 ^ 0x0F)
+        byte7 = (c1 << 4) | (c3 ^ 0x0F)
+        byte8 = (c3 << 4) | c2
+        return cls((byte6 << 24) | (byte7 << 16) | (byte8 << 8) | (gpb & 0xFF))
+
+    def _rows(self) -> list[dict]:
+        rows = [
+            {"name": name, "value": f"{self._cond(n)}  {self.describe(n)}", "bits": "", "doc": ""}
+            for n, name in enumerate(self._BLOCKS)
+        ]
+        rows.append({"name": "gpb", "value": f"0x{self.gpb:02X}", "bits": "[7:0]", "doc": ""})
+        if not self.valid:
+            rows.append({"name": "(!)", "value": "inverted copy inconsistent — INVALID",
+                         "bits": "", "doc": ""})
+        return rows
+
 
 class MifareClassicTag(Tag14443A_3):
     """MIFARE Classic 1K/4K transponder with Crypto-1 authentication."""
@@ -63,6 +176,12 @@ class MifareClassicTag(Tag14443A_3):
         else:
             return 128 + (sector - 32) * 16 + 15
 
+    def access(self, sector: int) -> MifareAccessConditions:
+        """Decode the sector's trailer access bits (trailer bytes 6-9) as a self-describing
+        :class:`MifareAccessConditions` register."""
+        trailer = self.read_block_raw(self._sector_trailer_block(sector))
+        return MifareAccessConditions(int.from_bytes(trailer[6:10], "big"))
+
     def read_block_raw(self, block: int) -> bytes:
         """Direct read of block data (bypassing auth)."""
         offset = block * BLOCK_SIZE
diff --git a/pm3py/transponders/hf/iso14443a/nxp/ntag21x.py b/pm3py/transponders/hf/iso14443a/nxp/ntag21x.py
index 607b8d8..4687f09 100644
--- a/pm3py/transponders/hf/iso14443a/nxp/ntag21x.py
+++ b/pm3py/transponders/hf/iso14443a/nxp/ntag21x.py
@@ -14,7 +14,8 @@ Feature summary:
 from __future__ import annotations
 
 from pm3py.sim.frame import RFFrame
-from .type2 import NxpType2Tag, READ_CNT, FAST_READ, READ_CMD, _nak, NAK_ARG
+from pm3py.transponders.bitfield import BitField
+from .type2 import NxpType2Tag, Type2Config, READ_CNT, FAST_READ, READ_CMD, _nak, NAK_ARG
 
 # ACCESS byte bits (CFG1 byte 0)
 ACCESS_PROT = 0x80
@@ -23,11 +24,26 @@ ACCESS_NFC_CNT_EN = 0x10
 ACCESS_NFC_CNT_PWD_PROT = 0x08
 
 
+class Ntag21xConfig(Type2Config):
+    """NTAG21x CFG0/CFG1 config word (see :class:`Type2Config`), adding the NTAG-only ASCII
+    mirror and NFC-counter fields. Decode ``tag.config`` or build one and assign it back."""
+
+    _label = "NTAG21x config (CFG0/CFG1)"
+
+    mirror_conf = BitField(63, 62, doc="ASCII mirror mode",
+                           values={0: "off", 1: "UID", 2: "NFC counter", 3: "UID+counter"})
+    mirror_byte = BitField(61, 60, doc="byte offset within mirror_page where the mirror starts")
+    mirror_page = BitField(47, 40, doc="page where ASCII mirroring begins (0 = disabled)")
+    nfc_cnt_en = BitField(28, doc="enable the 24-bit NFC read counter (213/215/216)")
+    nfc_cnt_pwd_prot = BitField(27, doc="NFC counter readable only after PWD_AUTH")
+
+
 class Ntag21x(NxpType2Tag):
     """Base for the NTAG21x family. Concrete ICs set the class attributes."""
 
     _has_nfc_counter: bool = False   # True for 213/215/216
     _cc_default: bytes = b"\xE1\x10\x00\x00"
+    _config_cls = Ntag21xConfig
 
     def __init__(self, uid: bytes | None = None, ndef_message: bytes = b"",
                  password: bytes = b"\xFF\xFF\xFF\xFF",
diff --git a/pm3py/transponders/hf/iso14443a/nxp/type2.py b/pm3py/transponders/hf/iso14443a/nxp/type2.py
index 1f46cd1..1977fa8 100644
--- a/pm3py/transponders/hf/iso14443a/nxp/type2.py
+++ b/pm3py/transponders/hf/iso14443a/nxp/type2.py
@@ -13,6 +13,7 @@ from __future__ import annotations
 
 from pm3py.sim.frame import RFFrame
 from pm3py.trace.decode_iso14a import decode_14443a_type2
+from pm3py.transponders.bitfield import BitField, Register
 from ..ndef import NfcType2Tag, PAGE_SIZE, READ_CMD, WRITE_CMD
 
 # ---- Command codes (NFC Forum Type 2 / Ultralight / NTAG) ----
@@ -42,6 +43,25 @@ def _nak(code: int = NAK_ARG) -> RFFrame:
     return RFFrame.from_bytes(bytes([code]))
 
 
+class Type2Config(Register):
+    """The two NXP Type 2 configuration pages (CFG0 + CFG1) decoded as one 64-bit word:
+    the 4 bytes of CFG0 (MSB) followed by the 4 bytes of CFG1.
+
+    Common to Ultralight EV1 and NTAG21x. Only the fields every part shares live here —
+    NTAG adds its mirror / NFC-counter fields in :class:`Ntag21xConfig`. Bits are numbered on
+    the concatenated word, so CFG0 byte 0 is bits 63:56 and CFG1 byte 0 (ACCESS) is bits 31:24.
+    """
+
+    _width_bits = 64
+    _label = "Type 2 config (CFG0/CFG1)"
+
+    strg_mod_en = BitField(58, doc="strong load modulation enable (CFG0 MIRROR bit 2)")
+    auth0 = BitField(39, 32, doc="first page needing PWD_AUTH; >= last page ⇒ protection off")
+    prot = BitField(31, doc="0 = write protect from AUTH0; 1 = read+write protect")
+    cfglck = BitField(30, doc="lock CFG0/CFG1 permanently — irreversible")
+    authlim = BitField(26, 24, doc="failed-PWD_AUTH attempts before lockout (0 = unlimited)")
+
+
 class NxpType2Tag(NfcType2Tag):
     """NXP Type 2 platform: Ultralight EV1 / NTAG21x / NTAG I2C command core.
 
@@ -65,6 +85,26 @@ class NxpType2Tag(NfcType2Tag):
     _pwd_page: int | None = None    # 4-byte password (reads back as 00)
     _pack_page: int | None = None   # 2-byte PACK + RFUI (reads back as 00)
 
+    #: register class decoding the CFG0/CFG1 pages (subclasses with config pages set this)
+    _config_cls: type[Register] | None = None
+
+    @property
+    def config(self) -> Register:
+        """CFG0/CFG1 decoded as a self-describing register (see the part's ``_config_cls``).
+        Assign one back (``tag.config = cfg``) to program both config pages."""
+        if self._config_cls is None or self._cfg0_page is None or self._cfg1_page is None:
+            raise AttributeError(f"{type(self).__name__} has no CFG0/CFG1 config register")
+        raw = self._get_page(self._cfg0_page) + self._get_page(self._cfg1_page)
+        return self._config_cls(int.from_bytes(raw, "big"))
+
+    @config.setter
+    def config(self, value: "Register | int") -> None:
+        if self._cfg0_page is None or self._cfg1_page is None:
+            raise AttributeError(f"{type(self).__name__} has no CFG0/CFG1 config register")
+        raw = int(value).to_bytes(8, "big")
+        self._set_page(self._cfg0_page, raw[0:4])
+        self._set_page(self._cfg1_page, raw[4:8])
+
     def __init__(self, uid: bytes, *,
                  password: bytes = b"\xFF\xFF\xFF\xFF",
                  pack: bytes = b"\x00\x00",
diff --git a/pm3py/transponders/hf/iso14443a/nxp/ultralight.py b/pm3py/transponders/hf/iso14443a/nxp/ultralight.py
index ffe9d3b..f9120c5 100644
--- a/pm3py/transponders/hf/iso14443a/nxp/ultralight.py
+++ b/pm3py/transponders/hf/iso14443a/nxp/ultralight.py
@@ -14,14 +14,35 @@ from __future__ import annotations
 import os
 
 from pm3py.sim.frame import RFFrame
+from pm3py.transponders.bitfield import BitField, Register
 from .type2 import (
-    NxpType2Tag, _ack, _nak, NAK_ARG, NAK_AUTH,
+    NxpType2Tag, Type2Config, _ack, _nak, NAK_ARG, NAK_AUTH,
     READ_CNT, INCR_CNT, CHECK_TEARING, READ_CMD, WRITE_CMD,
 )
 
 AUTHENTICATE = 0x1A   # Ultralight C 3DES authenticate
 AUTH_CONT = 0xAF      # continuation byte (part 2)
 
+
+class UltralightEV1Config(Type2Config):
+    """Ultralight EV1 CFG0/CFG1 — the shared :class:`Type2Config` fields (auth0, prot, cfglck,
+    authlim, strg_mod_en). EV1 has no ASCII mirror or NFC counter, so nothing is added."""
+
+    _label = "Ultralight EV1 config (CFG0/CFG1)"
+
+
+class UltralightCConfig(Register):
+    """Ultralight C protection config: AUTH0 (page 2Ah byte 0) + the AUTH1 protection bit
+    (page 2Bh byte 0), packed as ``auth0 << 8 | auth1``. Protection from AUTH0 onward is
+    enforced by 3DES AUTHENTICATE rather than a password."""
+
+    _width_bits = 16
+    _label = "Ultralight C auth config"
+
+    auth0 = BitField(15, 8, doc="first page (>= 3) protected; 0x30+ ⇒ protection disabled")
+    auth1_prot = BitField(0, values={0: "read+write", 1: "write-only"},
+                          doc="protection scope from AUTH0 onward")
+
 # NXP factory default Ultralight C 3DES key (K1||K2, 16 bytes)
 UL_C_DEFAULT_KEY = bytes.fromhex("49454D4B41455242214E4143554F5946")
 
@@ -109,6 +130,20 @@ class MifareUltralightC(NxpType2Tag):
     def _auth1_read_protected(self) -> bool:
         return (self._get_page(self.AUTH1_PAGE)[0] & 0x01) == 0x00
 
+    @property
+    def config(self) -> UltralightCConfig:
+        """AUTH0 + AUTH1 protection decoded as a self-describing register. Assign one back
+        (``tag.config = cfg``) to program the AUTH0 / AUTH1 pages."""
+        return UltralightCConfig(
+            (self._get_page(self.AUTH0_PAGE)[0] << 8) | self._get_page(self.AUTH1_PAGE)[0]
+        )
+
+    @config.setter
+    def config(self, value: "UltralightCConfig | int") -> None:
+        v = int(value)
+        self._set_page(self.AUTH0_PAGE, bytes([(v >> 8) & 0xFF, 0, 0, 0]))
+        self._set_page(self.AUTH1_PAGE, bytes([v & 0x01, 0, 0, 0]))
+
     async def power_on(self) -> None:
         await super().power_on()
         self._auth_state = "NONE"
@@ -196,6 +231,7 @@ class MifareUltralightEV1(NxpType2Tag):
     CHECK_TEARING_EVENT). Concrete ICs: MF0UL11, MF0UL21."""
 
     _has_cc = False   # page 3 is OTP; format for NDEF to add a CC
+    _config_cls = UltralightEV1Config
 
     def __init__(self, uid: bytes | None = None, ndef_message: bytes = b"",
                  password: bytes = b"\xFF\xFF\xFF\xFF", pack: bytes = b"\x00\x00",
diff --git a/pm3py/transponders/lf/atmel/t5577.py b/pm3py/transponders/lf/atmel/t5577.py
index 5458243..b7648d5 100644
--- a/pm3py/transponders/lf/atmel/t5577.py
+++ b/pm3py/transponders/lf/atmel/t5577.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 import struct
 
 from pm3py.sim.frame import RFFrame
+from pm3py.transponders.bitfield import BitField, Register
 from ..base import TagLF, ReaderLF, Modulation
 from ..em.em4100 import EM4100Tag
 
@@ -16,6 +17,108 @@ CMD_PWD_WRITE = 0x04
 # T5577 block 0 config bits for common modes
 CONFIG_EM4100 = 0x00148040  # ASK/Manchester, RF/64, 64 bits, maxblk=2
 
+# modulation codes (config bits 16:12) — datasheet / ATA5577C
+_MODULATION = {
+    0x00: "direct", 0x01: "PSK1", 0x02: "PSK2", 0x03: "PSK3",
+    0x04: "FSK1", 0x05: "FSK2", 0x06: "FSK1a", 0x07: "FSK2a",
+    0x08: "ASK", 0x10: "biphase", 0x18: "biphase'",
+}
+# PSK carrier frequency (config bits 11:10), PSK modes only
+_PSK_CF = {0x00: "RF/2", 0x01: "RF/4", 0x02: "RF/8", 0x03: "reserved"}
+# master key nibble (config bits 31:28)
+_MASTER_KEY = {0x0: "0 (normal)", 0x6: "6 (extended)", 0x9: "9 (extended)"}
+# standard-mode data bit rate: 3-bit code (bits 20:18) <-> RF/n divider
+_RF_BY_CODE = {0: 8, 1: 16, 2: 32, 3: 40, 4: 50, 5: 64, 6: 100, 7: 128}
+_CODE_BY_RF = {v: k for k, v in _RF_BY_CODE.items()}
+
+
+class T5577Config(Register):
+    """T5577 / ATA5577C configuration word — page 0, block 0.
+
+    Decode a value you read, or build one to write::
+
+        T5577Config(0x00148040)            # -> decoded table (EM4100/unique)
+        cfg = T5577Config.preset("em4100")
+        cfg.max_block = 3
+        cfg.st = True
+        lf.t55.writebl(0, int(cfg))        # int(cfg) is the raw word to program
+
+    Fields (high->low): master_key, data_bit_rate, x_mode, modulation, psk_cf, aor, otp,
+    max_block, pwd, st, fast_write, inverse, por_delay. ``data_bit_rate`` is mode-dependent
+    (see its docstring), so it is a computed property rather than a plain ``BitField``.
+    """
+
+    _width_bits = 32
+    _label = "T5577 config (block0)"
+
+    master_key = BitField(31, 28, values=_MASTER_KEY,
+                          doc="lock/extended-mode key: 0=normal, 6/9 enable extended (x-mode)")
+    x_mode = BitField(17, doc="extended mode: 6-bit data rate + fast-write / AOR / OTP semantics")
+    modulation = BitField(16, 12, values=_MODULATION, doc="modulation scheme (ASK, FSK, PSK, ...)")
+    psk_cf = BitField(11, 10, values=_PSK_CF, doc="PSK sub-carrier frequency (PSK modes only)")
+    aor = BitField(9, doc="answer-on-request: stay silent until woken by a read command")
+    otp = BitField(8, doc="one-time-programmable: block further writes once set (irreversible)")
+    max_block = BitField(7, 5, doc="number of data blocks the tag streams back (1..7)")
+    pwd = BitField(4, doc="password enabled: reads/writes require the block-7 password")
+    st = BitField(3, doc="sequence terminator: marks end of data for reader auto-detect")
+    fast_write = BitField(2, doc="fast downlink write timing (x-mode)")
+    inverse = BitField(1, doc="invert the data output")
+    por_delay = BitField(0, doc="power-on-reset delay (ride through brief field dropouts)")
+
+    #: known block-0 words (from the Proxmark3 config-block table) for common tag types
+    _PRESETS = {
+        "em4100": 0x00148040,   # ASK/Manchester, RF/64, 2 blocks
+        "default": 0x000880E8,  # ASK/Manchester, RF/32, ST, 7 blocks
+        "hid": 0x00107060,      # FSK2a, RF/50, 3 blocks (HID 26-bit)
+        "indala": 0x00081040,   # PSK1, RF/32, 2 blocks
+        "fdxb": 0x903F0082,     # biphase, x-mode, RF/32, 4 blocks
+        "viking": 0x00088040,   # ASK/Manchester, RF/32, 2 blocks
+    }
+
+    @classmethod
+    def preset(cls, name: str) -> "T5577Config":
+        try:
+            return cls(cls._PRESETS[name])
+        except KeyError:
+            raise ValueError(
+                f"unknown preset {name!r}; known: {sorted(cls._PRESETS)}"
+            ) from None
+
+    @property
+    def data_bit_rate(self) -> int:
+        """Data bit rate as ``RF/n`` (the carrier is divided by this before sampling).
+
+        Standard mode uses a 3-bit code -> RF/8, 16, 32, 40, 50, 64, 100 or 128. When
+        ``x_mode`` is set the field is 6 bits and the rate is ``2*code + 2`` (any even RF/n)."""
+        raw6 = (self.value >> 18) & 0x3F
+        if self.x_mode:
+            return 2 * raw6 + 2
+        return _RF_BY_CODE.get(raw6 & 0x07, raw6 & 0x07)
+
+    @data_bit_rate.setter
+    def data_bit_rate(self, rf: int) -> None:
+        if self.x_mode:
+            if rf < 2 or rf % 2:
+                raise ValueError("x-mode data rate must be an even RF/n >= 2")
+            code = (rf - 2) // 2
+            if code >> 6:
+                raise ValueError(f"RF/{rf} too large for x-mode (6-bit field)")
+            self.value = (self.value & ~(0x3F << 18)) | (code << 18)
+        else:
+            if rf not in _CODE_BY_RF:
+                raise ValueError(
+                    f"standard data rate must be RF/n with n in {sorted(_CODE_BY_RF)}; got RF/{rf}"
+                )
+            self.value = (self.value & ~(0x07 << 18)) | (_CODE_BY_RF[rf] << 18)
+
+    def _extra_rows(self) -> list[dict]:
+        hi = 23 if self.x_mode else 20
+        return [{
+            "name": "data_bit_rate", "value": f"RF/{self.data_bit_rate}",
+            "bits": f"[{hi}:18]", "lo": 18,
+            "doc": "carrier divider RF/n (see data_bit_rate)",
+        }]
+
 
 class T5577Tag(TagLF):
     """T5577 programmable LF transponder.
@@ -35,6 +138,16 @@ class T5577Tag(TagLF):
     def blocks(self) -> list[int]:
         return self._blocks
 
+    @property
+    def config(self) -> T5577Config:
+        """Block 0 decoded as a self-describing :class:`T5577Config`. Assign one back
+        (``tag.config = cfg``) to program block 0."""
+        return T5577Config(self._blocks[0])
+
+    @config.setter
+    def config(self, value: "T5577Config | int") -> None:
+        self._blocks[0] = int(value)
+
     async def handle_frame(self, frame: RFFrame) -> RFFrame | None:
         if not self._powered or not frame.data:
             return None
diff --git a/pm3py/transponders/lf/em/em4100.py b/pm3py/transponders/lf/em/em4100.py
index 4f6e380..40a57fb 100644
--- a/pm3py/transponders/lf/em/em4100.py
+++ b/pm3py/transponders/lf/em/em4100.py
@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 from pm3py.sim.frame import RFFrame
+from pm3py.transponders.bitfield import Register
 from ..base import TagLF, ReaderLF, Modulation
 
 
@@ -34,6 +35,14 @@ class EM4100Tag(TagLF):
         """Return 64-bit encoded data as list of 0/1 values."""
         return list(self._encoded)
 
+    @property
+    def code(self) -> "EM4100Code":
+        """The tag's 64-bit frame decoded as a self-describing :class:`EM4100Code`."""
+        value = 0
+        for bit in self._encoded:
+            value = (value << 1) | bit
+        return EM4100Code(value)
+
     def _get_response(self) -> RFFrame:
         """Return encoded data as RFFrame."""
         from bitarray import bitarray
@@ -85,6 +94,109 @@ class EM4100Tag(TagLF):
         return tag_id
 
 
+class EM4100Code(Register):
+    """The 64-bit EM4100 code, decoded — header, the ten 4-bit data rows with their even
+    row parities, the four column parities, and the stop bit. Covers EM4102 and the EM4200's
+    64-bit mode (same frame). Build from a 40-bit ID (parities computed for you) or decode a
+    captured frame and check it::
+
+        EM4100Code.build(0x0102030405)     # -> frame with correct parities
+        EM4100Code(0xFF00...).valid         # header + every parity + stop bit consistent?
+        code.tag_id                         # 40-bit ID (customer_id byte + card_number)
+
+    The redundant parities are the classic EM footgun — this validates all of them and, when
+    you build a code, gets them right so you never program a tag a reader won't accept.
+    """
+
+    _width_bits = 64
+    _label = "EM4100 64-bit code"
+
+    def _bits(self) -> list[int]:
+        """The frame as 64 bits, MSB (first transmitted) first — matches ``_encode`` order."""
+        return [(self.value >> (63 - i)) & 1 for i in range(64)]
+
+    @property
+    def header(self) -> int:
+        return (self.value >> 55) & 0x1FF
+
+    @property
+    def header_ok(self) -> bool:
+        return self.header == 0x1FF
+
+    @property
+    def tag_id(self) -> int:
+        """The 40-bit ID carried in the data rows (parities stripped)."""
+        return EM4100Tag.decode_data(self._bits())
+
+    @property
+    def customer_id(self) -> int:
+        """First data byte — the EM 'customer'/version field."""
+        return (self.tag_id >> 32) & 0xFF
+
+    @property
+    def card_number(self) -> int:
+        """Low 32 bits of the ID — the card number readers usually display."""
+        return self.tag_id & 0xFFFFFFFF
+
+    def _bad_rows(self) -> list[int]:
+        b = self._bits()
+        bad = []
+        for row in range(10):
+            start = 9 + row * 5
+            if b[start + 4] != sum(b[start:start + 4]) % 2:
+                bad.append(row)
+        return bad
+
+    def _bad_cols(self) -> list[int]:
+        b = self._bits()
+        return [c for c in range(4)
+                if b[59 + c] != sum(b[9 + r * 5 + c] for r in range(10)) % 2]
+
+    @property
+    def rows_ok(self) -> bool:
+        return not self._bad_rows()
+
+    @property
+    def cols_ok(self) -> bool:
+        return not self._bad_cols()
+
+    @property
+    def stop_ok(self) -> bool:
+        return (self.value & 1) == 0
+
+    @property
+    def valid(self) -> bool:
+        return self.header_ok and self.rows_ok and self.cols_ok and self.stop_ok
+
+    @classmethod
+    def build(cls, tag_id: int) -> "EM4100Code":
+        """Encode a 40-bit ID into a full, correctly-parity'd 64-bit code."""
+        value = 0
+        for bit in EM4100Tag._encode(tag_id):
+            value = (value << 1) | bit
+        return cls(value)
+
+    def _rows(self) -> list[dict]:
+        _bad_rows = self._bad_rows()
+        _bad_cols = self._bad_cols()
+        return [
+            {"name": "header", "bits": "[63:55]", "doc": "",
+             "value": f"0x{self.header:03X}" + ("" if self.header_ok else "  BAD (expect 1FF)")},
+            {"name": "tag_id", "bits": "40-bit", "doc": "", "value": f"{self.tag_id:010X}"},
+            {"name": "customer_id", "bits": "", "doc": "", "value": f"0x{self.customer_id:02X}"},
+            {"name": "card_number", "bits": "", "doc": "",
+             "value": f"0x{self.card_number:08X} ({self.card_number})"},
+            {"name": "row_parity", "bits": "", "doc": "",
+             "value": "ok" if self.rows_ok else f"BAD rows {_bad_rows}"},
+            {"name": "col_parity", "bits": "[4:1]", "doc": "",
+             "value": "ok" if self.cols_ok else f"BAD cols {_bad_cols}"},
+            {"name": "stop", "bits": "[0]", "doc": "",
+             "value": f"{self.value & 1}" + ("" if self.stop_ok else "  BAD (expect 0)")},
+            {"name": "valid", "bits": "", "doc": "",
+             "value": "yes" if self.valid else "NO — header/parity/stop error"},
+        ]
+
+
 class EM4100Reader(ReaderLF):
     """EM4100 reader — decode Manchester ASK response."""
 
diff --git a/tests/test_bitfield.py b/tests/test_bitfield.py
new file mode 100644
index 0000000..4e435d5
--- /dev/null
+++ b/tests/test_bitfield.py
@@ -0,0 +1,301 @@
+"""Bit-field register infrastructure + the T5577 config register. Hardware-free."""
+import pytest
+
+from pm3py.transponders.bitfield import BitField, Register
+from pm3py.sim import (
+    T5577Config, T5577Tag,
+    Ntag21xConfig, NTAG213, MF0UL21, MifareUltralightC,
+    MifareAccessConditions, MifareClassicTag,
+    EM4100Code, EM4100Tag,
+)
+
+
+class Demo(Register):
+    _width_bits = 16
+    _label = "demo"
+    top = BitField(15, 12, doc="top nibble")
+    flag = BitField(3, doc="a flag")
+    mode = BitField(1, 0, values={0: "a", 1: "b", 2: "c"}, doc="a labelled mode")
+
+
+class TestBitFieldCore:
+    def test_read_fields(self):
+        d = Demo(0xF008)  # top=0xF, flag(bit3)=1, mode=0
+        assert d.top == 0xF
+        assert d.flag is True
+        assert d.mode == "a"
+
+    def test_single_bit_is_bool(self):
+        assert Demo(0).flag is False
+        assert Demo(0x8).flag is True
+
+    def test_write_by_field_is_isolated(self):
+        d = Demo(0)
+        d.top = 0xA
+        assert d.value == 0xA000
+        d.flag = True
+        assert d.value == 0xA008
+        d.top = 0x5  # must not disturb the flag bit
+        assert d.value == 0x5008
+
+    def test_values_map_both_directions(self):
+        d = Demo(0)
+        d.mode = "c"
+        assert d.mode == "c" and d.value == 0x0002
+        d.mode = 1  # raw code still accepted
+        assert d.mode == "b"
+
+    def test_overflow_rejected(self):
+        with pytest.raises(ValueError):
+            Demo(0).top = 0x1F  # 5 bits into a 4-bit field
+
+    def test_int_and_index(self):
+        d = Demo(0x1234)
+        assert int(d) == 0x1234
+        assert hex(d) == "0x1234"
+
+    def test_fields_ordered_high_to_low(self):
+        assert [n for n, _ in Demo.fields()] == ["top", "flag", "mode"]
+
+    def test_descriptor_doc_visible(self):
+        # the completion tooltip / `field?` help comes from the descriptor docstring
+        assert Demo.top.__doc__ == "top nibble"
+
+
+# The three anchor constants below are lifted from the Proxmark3 config-block table
+# (client/src/cmdlft55xx.h) with their documented meanings — they pin the bit layout.
+class TestT5577Decode:
+    def test_em4100(self):
+        cfg = T5577Config(0x00148040)
+        assert cfg.modulation == "ASK"
+        assert cfg.data_bit_rate == 64
+        assert cfg.max_block == 2
+        assert cfg.x_mode is False
+        assert cfg.st is False
+        assert cfg.master_key == "0 (normal)"
+
+    def test_default_ask_manchester(self):
+        cfg = T5577Config(0x000880E8)  # ASK, RF/32, STT, 7 data blocks
+        assert cfg.modulation == "ASK"
+        assert cfg.data_bit_rate == 32
+        assert cfg.max_block == 7
+        assert cfg.st is True
+
+    def test_pac_is_nrz(self):
+        cfg = T5577Config(0x00080080)  # NRZ, RF/32, 4 data blocks
+        assert cfg.modulation == "direct"
+        assert cfg.max_block == 4
+
+    def test_hid_fsk(self):
+        cfg = T5577Config(0x00107060)  # FSK2a, RF/50, 3 blocks
+        assert cfg.modulation == "FSK2a"
+        assert cfg.data_bit_rate == 50
+        assert cfg.max_block == 3
+
+    def test_xmode_data_rate(self):
+        cfg = T5577Config(0x903F0082)  # fdx-b, x-mode
+        assert cfg.x_mode is True
+        assert cfg.modulation == "biphase"
+        assert cfg.data_bit_rate == 32  # 2*code+2 over the 6-bit field
+
+
+class TestT5577Build:
+    def test_preset_roundtrip(self):
+        assert int(T5577Config.preset("em4100")) == 0x00148040
+
+    def test_unknown_preset(self):
+        with pytest.raises(ValueError):
+            T5577Config.preset("nope")
+
+    def test_edit_by_field(self):
+        cfg = T5577Config.preset("em4100")
+        cfg.max_block = 3
+        cfg.st = True
+        # max_block bits 7:5 -> 3<<5 = 0x60; st bit3 -> 0x08; base had maxblk 2 (0x40)
+        assert cfg.max_block == 3 and cfg.st is True
+        assert int(cfg) == (0x00148040 & ~(0x07 << 5)) | (3 << 5) | (1 << 3)
+
+    def test_set_data_rate_standard(self):
+        cfg = T5577Config(0x00148040)
+        cfg.data_bit_rate = 32
+        assert cfg.data_bit_rate == 32
+        with pytest.raises(ValueError):
+            cfg.data_bit_rate = 33  # not a legal standard divider
+
+    def test_set_modulation_by_name(self):
+        cfg = T5577Config(0)
+        cfg.modulation = "FSK2"
+        assert cfg.modulation == "FSK2"
+        assert cfg.modulation != "ASK"
+
+    def test_decode_mapping_and_repr(self):
+        cfg = T5577Config(0x00148040)
+        d = cfg.decode()
+        assert d["modulation"] == "ASK"
+        assert d["data_bit_rate"] == "RF/64"
+        assert d["max_block"] == "2"
+        text = repr(cfg)  # not a tty in tests -> plain text
+        assert "T5577 config (block0) = 0x00148040" in text
+        assert "modulation" in text and "ASK" in text
+        assert "data_bit_rate" in text and "RF/64" in text
+
+
+class TestT5577TagBinding:
+    def test_config_reads_block0(self):
+        tag = T5577Tag()
+        tag.blocks[0] = 0x00148040
+        assert tag.config.modulation == "ASK"
+        assert tag.config.max_block == 2
+
+    def test_assign_config_programs_block0(self):
+        tag = T5577Tag()
+        cfg = T5577Config.preset("hid")
+        tag.config = cfg
+        assert tag.blocks[0] == 0x00107060
+        # accepts a raw int too
+        tag.config = 0x00148040
+        assert tag.config.modulation == "ASK"
+
+
+class TestNtag21xConfig:
+    def test_fields_span_both_pages(self):
+        cfg = Ntag21xConfig(0)
+        cfg.auth0 = 0x04       # CFG0 byte 3
+        cfg.prot = True        # CFG1 byte 0 bit 7 (ACCESS.PROT)
+        # CFG0 = 00 00 00 04, CFG1 = 80 00 00 00
+        assert bytes(cfg) == b"\x00\x00\x00\x04\x80\x00\x00\x00"
+        assert cfg.auth0 == 0x04 and cfg.prot is True
+
+    def test_mirror_and_counter_fields(self):
+        cfg = Ntag21xConfig(0)
+        cfg.mirror_conf = "UID+counter"
+        cfg.mirror_page = 0x10
+        cfg.nfc_cnt_en = True
+        cfg.authlim = 5
+        assert cfg.mirror_conf == "UID+counter"
+        assert cfg.mirror_page == 0x10
+        assert cfg.nfc_cnt_en is True and cfg.authlim == 5
+        # re-decoding the raw word must reproduce everything
+        assert Ntag21xConfig(int(cfg)).decode() == cfg.decode()
+
+    def test_tag_binding_roundtrip(self):
+        tag = NTAG213()                    # fresh: auth0=0xFF, prot off
+        assert tag.config.auth0 == 0xFF
+        assert tag.config.prot is False
+        cfg = tag.config
+        cfg.auth0 = 0x10
+        cfg.prot = True
+        tag.config = cfg                   # program both config pages
+        assert tag.config.auth0 == 0x10 and tag.config.prot is True
+
+
+class TestUltralightConfig:
+    def test_ev1_uses_shared_fields(self):
+        tag = MF0UL21()
+        cfg = tag.config
+        cfg.auth0 = 0x08
+        cfg.cfglck = True
+        tag.config = cfg
+        assert tag.config.auth0 == 0x08 and tag.config.cfglck is True
+        assert "NTAG21x" not in repr(cfg)  # EV1 has no mirror/counter fields
+
+    def test_ulc_auth_config(self):
+        tag = MifareUltralightC(auth0=0x30, auth1_read_protect=True)
+        cfg = tag.config
+        assert cfg.auth0 == 0x30
+        assert cfg.auth1_prot == "read+write"   # AUTH1 bit0 == 0
+        cfg.auth1_prot = "write-only"
+        cfg.auth0 = 0x2A
+        tag.config = cfg
+        assert tag.config.auth0 == 0x2A
+        assert tag.config.auth1_prot == "write-only"
+
+
+class TestMifareAccessConditions:
+    def test_factory_default_trailer(self):
+        # FF 07 80 access bytes + GPB 0x69 = the transport/default trailer
+        ac = MifareAccessConditions(0xFF078069)
+        assert ac.valid is True
+        assert ac.block0 == 0 and ac.block1 == 0 and ac.block2 == 0
+        assert ac.trailer == 1
+        assert ac.gpb == 0x69
+
+    def test_build_matches_default(self):
+        ac = MifareAccessConditions.build(0, 0, 0, trailer=1, gpb=0x69)
+        assert int(ac) == 0xFF078069
+        assert bytes(ac) == b"\xFF\x07\x80\x69"   # trailer bytes 6..9
+
+    def test_build_is_always_valid(self):
+        for t in range(8):
+            for d in range(8):
+                assert MifareAccessConditions.build(d, d, d, trailer=t).valid
+
+    def test_all_locked(self):
+        ac = MifareAccessConditions.build(7, 7, 7, trailer=7)
+        assert ac.trailer == 7 and ac.block0 == 7
+        assert "no access" in ac.describe(0)
+        assert "locked" in ac.describe(3)
+
+    def test_inconsistent_copy_flagged(self):
+        ac = MifareAccessConditions(0x00000000)  # C == ~C == 0, impossible
+        assert ac.valid is False
+        assert "INVALID" in repr(ac)
+
+    def test_bad_condition_rejected(self):
+        with pytest.raises(ValueError):
+            MifareAccessConditions.build(trailer=8)
+
+    def test_describe_and_decode(self):
+        ac = MifareAccessConditions(0xFF078069)
+        assert "transport" in ac.describe(0)
+        assert "transport/default" in ac.describe(3)
+        d = ac.decode()
+        assert d["block0"].startswith("0")
+        assert d["trailer"].startswith("1")
+
+    def test_tag_access_reads_trailer(self):
+        tag = MifareClassicTag(uid=b"\x01\x02\x03\x04")
+        # write a full trailer for sector 1: KeyA(6) + FF0780 + GPB69 + KeyB(6)
+        trailer_blk = tag._sector_trailer_block(1)
+        trailer = b"\xA0\xA1\xA2\xA3\xA4\xA5" + b"\xFF\x07\x80\x69" + b"\xB0\xB1\xB2\xB3\xB4\xB5"
+        tag.write_block_raw(trailer_blk, trailer)
+        ac = tag.access(1)
+        assert ac.trailer == 1 and ac.gpb == 0x69 and ac.valid
+
+
+class TestEM4100Code:
+    def test_build_roundtrip(self):
+        code = EM4100Code.build(0x0102030405)
+        assert code.tag_id == 0x0102030405
+        assert code.customer_id == 0x01
+        assert code.card_number == 0x02030405
+        assert code.valid
+
+    def test_build_is_always_valid(self):
+        for tid in (0x0, 0xFFFFFFFFFF, 0x1234567890, 0xDEADBEEF00, 0xA5A5A5A5A5):
+            assert EM4100Code.build(tid).valid
+
+    def test_matches_model_encoding(self):
+        # the register must agree bit-for-bit with the tag's own encoder
+        tag = EM4100Tag(0xCAFEBABE99)
+        assert tag.code.valid
+        assert tag.code.tag_id == 0xCAFEBABE99
+        assert int(tag.code) == int(EM4100Code.build(0xCAFEBABE99))
+
+    def test_flipped_data_bit_breaks_row_parity(self):
+        code = EM4100Code.build(0x0102030405)
+        broken = EM4100Code(int(code) ^ (1 << 40))  # flip a data bit in the rows region
+        assert broken.valid is False
+        assert broken._bad_rows()          # at least one row parity fails
+
+    def test_bad_header_and_stop_detected(self):
+        good = EM4100Code.build(0)
+        assert good.valid
+        assert EM4100Code(int(good) | 1).valid is False          # stop bit set
+        assert EM4100Code(int(good) & ~(0x1FF << 55)).valid is False  # header zeroed
+
+    def test_repr_shows_id_and_validity(self):
+        text = repr(EM4100Code.build(0x0102030405))
+        assert "EM4100 64-bit code" in text
+        assert "0102030405" in text
+        assert "valid" in text and "yes" in text