diff --git a/install.ps1 b/install.ps1
index 51ed254d..f67d62ec 100644
--- a/install.ps1
+++ b/install.ps1
@@ -3,16 +3,15 @@
 # Everything runs in a script block of its own: piped into iex, the script
 # would otherwise leave its variables and $ErrorActionPreference = "Stop"
 # in the user's session, where every later error would stop their commands.
+# Neither the script nor the block declares a parameter: one [Parameter()]
+# attribute makes it an advanced one, whose common parameters took -v
+# (-Verbose), -vb, -d (-Debug) and -ea before install.py could see them.
 & {
-    [CmdletBinding()]
-    param(
-        [Parameter(ValueFromRemainingArguments = $true)]
-        [string[]]$InstallerArguments
-    )
+    $InstallerArguments = @($args)
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "496e5a3aaf817301c40f45f184b97fc5607147184195d827a46b92c28fd3d2b0"
+    $defaultInstallSha256 = "1cf3bd0d3985e3a728513be75cb80682fe1e7a9224da6437e8f1cd9b2a3eedb1"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
diff --git a/install.py b/install.py
index aa988572..4e3581a9 100644
--- a/install.py
+++ b/install.py
@@ -26,6 +26,7 @@ import platform
 import re
 import shlex
 import shutil
+import socket
 import stat
 import subprocess
 import sys
@@ -146,6 +147,8 @@ _REGION_PARENT = {
     member: parent for parent, members in REGION_TREE.items() for member in members
 }
 REGIONS = frozenset({WORLD_REGION} | set(REGION_TREE) | set(_REGION_PARENT))
+# Marks a region group named after one file that no system names.
+FILE_GROUP = ":file:"
 MAX_SELECTION_TAGS = 1000
 SELECTION_FIELDS = ("systems", "regions", "region_groups", "region_system_groups")
 # The files cores load for systems the platform does not list (game engines,
@@ -207,6 +210,9 @@ def _os_default_dests(os_type: str) -> dict[str, Path]:
             "retroarch": _host_path("C:/RetroArch-Win64/system"),
             # RetroBat's installer defaults to C:\RetroBat (installer.iss).
             "retrobat": _host_path("C:/RetroBat/bios"),
+            # EmuDeck's Windows edition: C:\Emulation when its settings name
+            # no folder (emudeck-we functions/all.ps1:12-14).
+            "emudeck": _host_path("C:/Emulation/bios"),
         }
     if os_type == "wsl":
         # RetroArch may be the Linux one inside the distribution, while RetroBat
@@ -715,6 +721,28 @@ _BOARD_SOURCES: dict[str, tuple[str, str]] = {
 }
 
 
+# A derivative installed under a platform's name, with its own board file.
+# Knulli moves /usr/share/batocera to /usr/share/knulli (knulli-linux
+# 57afa662, board/scripts/post-build-script.sh:264-278) and writes its board
+# to knulli.arch (package/system/knulli-system/knulli-system.mk:98-99). Its
+# packages are its own, so Batocera's per-board core lists do not apply.
+_DERIVATIVE_BOARDS: dict[str, tuple[str, str]] = {
+    "batocera": ("Knulli", "/usr/share/knulli/knulli.arch"),
+}
+
+
+def derivative_board(platform: str) -> tuple[str, str] | None:
+    """(derivative, board) when a derivative of the platform names its board."""
+    name, where = _DERIVATIVE_BOARDS.get(platform, ("", ""))
+    if not where:
+        return None
+    try:
+        board = Path(where).read_text(encoding="utf-8", errors="replace").strip().lower()
+    except OSError:
+        return None
+    return (name, board) if board else None
+
+
 def detect_board(platform: str) -> str | None:
     """The board this system was built for, or None where it names none."""
     kind, where = _BOARD_SOURCES.get(platform, ("", ""))
@@ -750,7 +778,11 @@ def _parse_ps1_var(path: Path, key: str) -> str | None:
         return None
     normalized = key.lstrip("$")
     try:
-        for line in path.read_text(encoding="utf-8", errors="replace").splitlines():
+        raw = path.read_bytes()
+        # Windows PowerShell's Out-File writes UTF-16, Set-Content -Encoding
+        # UTF8 a byte order mark: EmuDeck's settings.ps1 passes through both.
+        encoding = "utf-16" if raw[:2] in (b"\xff\xfe", b"\xfe\xff") else "utf-8-sig"
+        for line in raw.decode(encoding, errors="replace").splitlines():
             line = line.strip()
             check = line.lstrip("$")
             if check.startswith(f"{normalized}="):
@@ -761,19 +793,25 @@ def _parse_ps1_var(path: Path, key: str) -> str | None:
     return None
 
 
-# Embedded systems, first match wins: Knulli also carries the Batocera
-# markers. The registry publishes the same rules as each manifest's
-# `detect`, and a test holds the two equal. MiSTer is a path rule below.
+# Embedded systems, first match wins. The registry publishes the same rules
+# as each manifest's `detect`, and a test holds the two equal. MiSTer is a
+# path rule below.
 _EMBEDDED_RULES: tuple[tuple[str, str, tuple[str, ...], str], ...] = (
     # ROCKNIX writes OS_NAME and no ID since 2025 (scripts/image). Older
     # images wrote ID=rocknix.
     ("rocknix", "os_release", ("OS_NAME=rocknix", "ID=rocknix"), "/storage/roms/bios"),
-    ("batocera", "file_exists", ("/etc/knulli-release",), "/userdata/bios"),
+    # Knulli (knulli-linux 57afa662): OS_NAME in os-release
+    # (board/scripts/post-build-script.sh:233-236) and knulli.version
+    # (package/system/knulli-system/knulli-system.mk:98-102).
+    ("batocera", "os_release", ("OS_NAME=knulli",), "/userdata/bios"),
+    ("batocera", "file_exists", ("/usr/share/knulli/knulli.version",), "/userdata/bios"),
     ("lakka", "os_release", ("ID=lakka",), "/storage/system"),
-    ("batocera", "file_exists", ("/etc/batocera-version",), "/userdata/bios"),
+    # batocera.linux 38845725, batocera-system.mk:21-22 (the same lines in
+    # batocera-30): batocera.arch and batocera.version.
+    ("batocera", "file_exists", ("/usr/share/batocera/batocera.version",), "/userdata/bios"),
     ("recalbox", "file_exists", ("/recalbox/recalbox.version",), "/recalbox/share/bios"),
     ("recalbox", "file_exists", ("/usr/bin/recalbox-settings",), "/recalbox/share/bios"),
-    ("retroarch", "file_exists", ("/opt/muos",), "/mnt/mmc/MUOS/bios"),
+    ("retroarch", "file_exists", ("/opt/muos",), "/run/muos/storage/bios"),
     ("retroarch", "file_exists", ("/mnt/mmc/MUOS",), "/mnt/mmc/MUOS/bios"),
     ("retroarch", "file_exists", ("/home/ark", "/opt/system"), "/roms/bios"),
     ("retroarch", "file_exists", ("/mnt/vendor/bin/dmenu.bin",), "/mnt/mmc/bios"),
@@ -797,6 +835,30 @@ def _detect_embedded() -> list[tuple[str, Path]]:
     return []
 
 
+# muOS runs RetroArch on the config it copies from its default on first launch
+# (internal f41f4a89, script/control/retroarch.sh:8-20). The default names
+# /run/muos/storage/bios, a bind mount of MUOS/bios on the second card when
+# that card holds one, else on the first (script/device/bind.sh:4,116-150),
+# and loads cores from /opt/muos/share/core (retroarch.default.cfg:2806,3151).
+MUOS_ROOT = Path("/opt/muos")
+MUOS_RETROARCH_CONFIGS = (
+    MUOS_ROOT / "share" / "info" / "config" / "retroarch.cfg",
+    MUOS_ROOT / "share" / "emulator" / "retroarch" / "retroarch.default.cfg",
+)
+
+
+def _muos_system_dir() -> Path | None:
+    """The system directory of muOS's RetroArch, read from its config, which
+    also records the config for finding the installed cores."""
+    if not MUOS_ROOT.is_dir():
+        return None
+    for cfg in MUOS_RETROARCH_CONFIGS:
+        system_dir = _parse_retroarch_system_dir(cfg)
+        if system_dir is not None:
+            return system_dir
+    return None
+
+
 def _android_storage_root() -> Path:
     """Shared storage root, the parent RetroArch builds its defaults from."""
     value = os.environ.get("EXTERNAL_STORAGE", "").strip()
@@ -1042,6 +1104,9 @@ _PATH_RULES: tuple[tuple[str, str, str, str | None], ...] = (
     ("linux", "emudeck", "$HOME/Emulation/bios", None),
     ("linux", "retrodeck", "$HOME/.var/app/net.retrodeck.retrodeck", "$HOME/retrodeck"),
     ("linux", "romm", "/romm/library/bios", None),
+    # A library laid out as {platform}/bios has no library/bios: RomM's
+    # config folder is the evidence (config_manager.py:264-265).
+    ("linux", "romm", "/romm/config/config.yml", "/romm/library/bios"),
     ("linux", "retropie", "$HOME/RetroPie/BIOS", None),
     ("windows", "bizhawk", "%USERPROFILE%\\BizHawk\\Firmware", None),
     ("linux", "misterfpga", "/media/fat/MiSTer", "/media/fat/games"),
@@ -1093,6 +1158,64 @@ def _detect_by_executable(os_type: str, found: list[tuple[str, Path]]) -> None:
             found.append((platform_name, newest.parent / subdir))
 
 
+# The registry's registry rules: (os, platform, key, value, BIOS folder under
+# the folder the value names). RetroBat.exe writes its own folder there on
+# every start (retrobat-executable 702da66a, RetroBat/RetroBat/Program.cs:57
+# and 99, RegistryTools.cs:15-50), wherever it was installed. The path rules
+# only knew C:\RetroBat and the profile folder.
+_REGISTRY_RULES: tuple[tuple[str, str, str, str, str], ...] = (
+    ("windows", "retrobat", "HKCU\\SOFTWARE\\RetroBat", "LatestKnownInstallPath", "bios"),
+)
+
+
+def _read_registry(key: str, value: str) -> str | None:
+    """A string value of the Windows registry, read natively or, under WSL,
+    through reg.exe. None when the key, the value or the means is missing."""
+    if os.name == "nt":
+        import winreg  # noqa: PLC0415
+
+        hive, _, subkey = key.partition("\\")
+        roots = {"HKCU": winreg.HKEY_CURRENT_USER, "HKLM": winreg.HKEY_LOCAL_MACHINE}
+        try:
+            with winreg.OpenKey(roots[hive], subkey) as handle:
+                data, _kind = winreg.QueryValueEx(handle, value)
+        except (OSError, KeyError):
+            return None
+        return data.strip() if isinstance(data, str) and data.strip() else None
+    if not _WSL["active"]:
+        return None
+    try:
+        result = subprocess.run(
+            ["reg.exe", "query", key, "/v", value],
+            capture_output=True, text=True, errors="replace", timeout=10,
+            stdin=subprocess.DEVNULL, cwd="/mnt/c" if Path("/mnt/c").is_dir() else None,
+        )
+    except (OSError, subprocess.SubprocessError):
+        return None
+    match = re.search(
+        rf"^\s*{re.escape(value)}\s+REG_(?:EXPAND_)?SZ\s+(.+?)\s*$", result.stdout, re.M
+    )
+    return match.group(1) if match else None
+
+
+def _detect_by_registry(os_type: str, found: list[tuple[str, Path]]) -> None:
+    """Add a platform whose own program names its folder in the registry.
+
+    A folder since moved or removed is passed over, so the path rules still
+    get their turn.
+    """
+    systems = ("windows",) if os_type in ("windows", "wsl") else ()
+    for rule_os, platform_name, key, value, subdir in _REGISTRY_RULES:
+        if rule_os not in systems or any(name == platform_name for name, _ in found):
+            continue
+        folder = _read_registry(key, value)
+        if not folder:
+            continue
+        base = _host_path(folder.rstrip("\\/") or folder)
+        if base.is_dir():
+            found.append((platform_name, base / subdir))
+
+
 def _detect_by_path(os_type: str, found: list[tuple[str, Path]]) -> None:
     """Add what a registry path rule finds, for a platform not found yet."""
     systems = ("linux", "windows") if os_type == "wsl" else (os_type,)
@@ -1111,7 +1234,9 @@ def detect_platforms(os_type: str) -> list[tuple[str, Path]]:
         found.extend(_detect_android())
 
     if os_type in ("linux", "wsl"):
-        found.extend(_detect_embedded())
+        embedded = _detect_embedded()
+        muos = _muos_system_dir() if embedded and embedded[0][0] == "retroarch" else None
+        found.extend([("retroarch", muos)] if muos else embedded)
 
         # EmuDeck (Linux/SteamOS)
         home = Path.home()
@@ -1174,13 +1299,9 @@ def detect_platforms(os_type: str) -> list[tuple[str, Path]]:
                 break
 
     if os_type in ("windows", "wsl"):
-        # EmuDeck Windows
-        home = Path.home()
-        emudeck_ps1 = Path(os.environ.get("APPDATA", "")) / "EmuDeck" / "settings.ps1"
-        if emudeck_ps1.exists():
-            emu_path = _parse_ps1_var(emudeck_ps1, "$emulationPath")
-            if emu_path:
-                found.append(("emudeck", Path(emu_path) / "bios"))
+        emudeck = _emudeck_windows_bios()
+        if emudeck is not None:
+            found.append(("emudeck", emudeck))
 
         # RetroArch Windows: open_default_config_file reads retroarch.cfg
         # beside the exe, then under %APPDATA%\RetroArch, and a value left at
@@ -1240,10 +1361,32 @@ def detect_platforms(os_type: str) -> list[tuple[str, Path]]:
             found.append(("bizhawk", bizhawk))
 
     _detect_by_executable(os_type, found)
+    _detect_by_registry(os_type, found)
     _detect_by_path(os_type, found)
     return found
 
 
+def _emudeck_windows_bios() -> Path | None:
+    """The BIOS folder of EmuDeck's Windows edition, as its scripts find it.
+
+    functions/all.ps1:4-14 (emudeck-we 037460ef) loads
+    %APPDATA%\\EmuDeck\\settings.ps1, else %USERPROFILE%\\EmuDeck\\settings.ps1,
+    and takes C:\\Emulation when neither sets $emulationPath. A network
+    install sets it through a drive mapped to $VolName in that session
+    (JSONtoPS1.ps1:166-172), which is the share itself here.
+    """
+    candidates = [Path(os.environ[var]) / "EmuDeck" / "settings.ps1"
+                  for var in ("APPDATA", "USERPROFILE") if os.environ.get(var)]
+    settings = next((path for path in candidates if path.is_file()), None)
+    if settings is None:
+        return None
+    emu_path = _parse_ps1_var(settings, "$emulationPath")
+    if emu_path and ("$" in emu_path or "+" in emu_path):
+        share = _parse_ps1_var(settings, "$VolName")
+        emu_path = f"{share.rstrip(chr(92))}\\Emulation" if share else None
+    return _host_path(emu_path or "C:\\Emulation") / "bios"
+
+
 # Identity Name in pkg/msvc-uwp/RetroArch-msvcUWP/Package.appxmanifest.
 RETROARCH_UWP_ID = "1e4cf179-f3c2-404f-b9f3-cb2070a5aad8"
 
@@ -2230,7 +2373,10 @@ def _region_rank(file_regions: set[str], requested: list[str]) -> int:
 
 
 def region_drops(
-    entries: list[dict], requested: list[str], systems: list[str] | None = None
+    entries: list[dict],
+    requested: list[str],
+    systems: list[str] | None = None,
+    held: set[str] | None = None,
 ) -> set[str]:
     """Destinations a region priority list withdraws.
 
@@ -2238,10 +2384,16 @@ def region_drops(
     groups the manifest records. Per group, the best-ranked regional file
     stays. A world file beats unmatched regional ones the code does not search
     before it (`priority`, lower first). When nothing matches and no world
-    file exists, every regional file stays, so a group is never emptied. A file with no region never competes and always stays.
+    file exists, every regional file stays, so a group is never emptied. A
+    file with no region never competes and always stays.
 
     A core extra competes whatever systems are chosen, while a platform's own
     declaration competes only when its system is kept, as in the pack.
+
+    ``held`` names the destinations the manifest can install. A match it
+    lacks never displaces a held file: the American BIOS stays under
+    --region japan when the Japanese one is missing. A group named after a
+    single file (FILE_GROUP) competes with nothing and keeps nothing.
     """
     if not requested:
         return set()
@@ -2263,7 +2415,9 @@ def region_drops(
             groups.setdefault(group, []).append((set(regions), entry["dest"], priority))
     keep: set[str] = set()
     drop: set[str] = set()
-    for members in groups.values():
+    for group_id, members in groups.items():
+        if FILE_GROUP in group_id:
+            continue
         world = {dest for regions, dest, _p in members if WORLD_REGION in regions}
         ranks = {dest: priority for _regions, dest, priority in members}
         regional = [
@@ -2275,12 +2429,19 @@ def region_drops(
         if not regional:
             continue
         matched = [(rank, dest) for rank, dest in regional if rank < len(requested)]
-        if matched:
-            best = min(rank for rank, _dest in matched)
-            keep |= {dest for rank, dest in matched if rank == best}
-            drop |= {dest for _rank, dest in regional}
-        elif world:
-            world_ranks = [ranks[dest] for dest in world]
+        winners, rivals = matched, world
+        if held is not None:
+            winners = [(rank, dest) for rank, dest in matched if dest in held]
+            rivals = world & held
+        if winners:
+            best = min(rank for rank, _dest in winners)
+            keep |= {
+                dest for rank, dest in matched
+                if rank == best or (rank < best and dest not in (held or ()))
+            }
+            drop |= {dest for _rank, dest in regional if dest not in keep}
+        elif rivals:
+            world_ranks = [ranks[dest] for dest in rivals]
             best_world = min(world_ranks) if None not in world_ranks else None
             for _rank, dest in regional:
                 ours = ranks[dest]
@@ -2320,7 +2481,10 @@ def narrow(
         files = _filter_by_target(files, cores)
         omitted = _filter_by_target(omitted, cores)
     if regions:
-        dropped = region_drops(files + omitted, regions, systems)
+        # The builder's held set: what the collection serves. An omission
+        # answers the request but takes no installable file's place.
+        held = {f["dest"] for f in files}
+        dropped = region_drops(files + omitted, regions, systems, held)
         files = [f for f in files if f["dest"] not in dropped]
         omitted = [o for o in omitted if o["dest"] not in dropped]
     if systems:
@@ -2395,7 +2559,8 @@ def resolve_choices(values: list[str], known: list[str], label: str) -> list[str
     if unknown:
         if not known:
             raise ValueError(f"unknown {label}: {', '.join(unknown)}. This list records none.")
-        listing = "--list-systems" if label == "system" else "--list-cores"
+        listing = {"system": "--list-systems", "emulator": "--list-emulators"}.get(
+            label, "--list-cores")
         raise ValueError(
             f"unknown {label}: {', '.join(unknown)}. {did_you_mean(unknown, known, listing)}"
         )
@@ -2563,6 +2728,7 @@ def _download_one(
     resumable = expected_size >= RESUME_FROM_SIZE
     reason = "download failed"
     refused = False
+    full_partial = False
     for attempt in range(1, MAX_RETRIES + 1):
         tmp_path: Path | None = None
         counted = 0
@@ -2581,7 +2747,17 @@ def _download_one(
                     tmp_path = None
                 else:
                     offset = os.fstat(partial.fileno()).st_size
-                    if offset >= expected_size:
+                    if offset >= expected_size and _NO_RENAME_WHILE_OPEN:
+                        # The run that verified this copy releases its lock
+                        # to rename it: truncating it there destroyed that
+                        # copy and failed the rename. It is left alone and
+                        # this run writes a private file.
+                        partial.close()
+                        partial = None
+                        tmp_path = None
+                        offset = 0
+                        full_partial = True
+                    elif offset >= expected_size:
                         partial.truncate(0)
                         offset = 0
             request = urllib.request.Request(url)
@@ -2652,11 +2828,15 @@ def _download_one(
             # written with sudo has to stay readable by the emulator's user.
             os.chmod(tmp_path, (0o777 if f.get("executable") else 0o666) & ~_UMASK)
             _clear_read_only(dest)
-            if partial is not None and os.name == "nt":
+            if partial is not None and _NO_RENAME_WHILE_OPEN:
                 # Windows renames no open file. Elsewhere the lock is held
                 # until the copy has its final name.
                 partial.close()
-            os.replace(tmp_path, dest)
+                _replace_released(tmp_path, dest, expected)
+            else:
+                os.replace(tmp_path, dest)
+            if full_partial:
+                _drop_full_partial(dest, expected_size)
             return f["dest"], True, ""
 
         except _Interrupted:
@@ -2682,6 +2862,8 @@ def _download_one(
             reason = f"HTTP {exc.code}"
             if exc.code in (403, 429):
                 reason += " (GitHub rate limit or access refused)"
+            if exc.code >= 500 or exc.code == 429:
+                reason = NetworkReason(reason)
             if verbose:
                 print(f"    Attempt {attempt} failed: {exc}", file=sys.stderr)
             if tmp_path is not None and partial is None and tmp_path.exists():
@@ -2702,6 +2884,8 @@ def _download_one(
                 # place is not a folder this user cannot write.
                 reason = READ_ONLY if _read_only_file(dest) else NOT_WRITABLE
                 refused = True
+            elif _transfer_failure(exc):
+                reason = NetworkReason(getattr(exc, "reason", "") or exc)
             else:
                 reason = str(getattr(exc, "reason", "") or exc)
             if verbose:
@@ -2726,6 +2910,71 @@ def _download_one(
     return f["dest"], False, reason
 
 
+# Windows renames no file another handle holds open, and Python opens files
+# without FILE_SHARE_DELETE: a locked partial copy is closed before its rename.
+_NO_RENAME_WHILE_OPEN = os.name == "nt"
+# Seconds a closed partial copy waits for a sharing violation to clear: a
+# second run closing it again, or a scanner reading the new file.
+RELEASE_WAIT = 30.0
+_SHARING_VIOLATIONS = (32, 33)
+
+
+def _replace_released(source: Path, dest: Path, expected: dict[str, str]) -> None:
+    """Rename a verified copy whose lock was just released.
+
+    Another run may open the copy in that moment, which refuses the rename
+    with a sharing violation until it lets go. When the copy is gone, that
+    run placed the same file and removed it: the destination is checked
+    against the manifest's hashes before it counts as installed.
+    """
+    deadline = time.monotonic() + RELEASE_WAIT
+    while True:
+        try:
+            os.replace(source, dest)
+            return
+        except FileNotFoundError:
+            if expected and dest.is_file() and _digest_file(dest, tuple(expected)) == expected:
+                return
+            raise
+        except PermissionError as exc:
+            if (getattr(exc, "winerror", None) not in _SHARING_VIOLATIONS
+                    or time.monotonic() >= deadline):
+                raise
+            time.sleep(0.2)
+
+
+def _drop_full_partial(dest: Path, expected_size: int) -> None:
+    """Remove a complete partial copy this run left alone.
+
+    A copy at full size that nothing holds is what a run stopped between its
+    check and its rename leaves behind. The destination now holds the
+    verified file, so the copy is only clutter. One another run holds is
+    left to it.
+    """
+    path = dest.parent / f".{dest.name}.part"
+    if not path.is_file():
+        return
+    try:
+        handle = _claim_partial(path)
+    except OSError:
+        return
+    if handle is None:
+        return
+    with handle:
+        complete = os.fstat(handle.fileno()).st_size >= expected_size
+    if complete:
+        try:
+            path.unlink()
+        except OSError:
+            pass  # another run opened it meanwhile and finishes with it
+    else:
+        try:
+            if path.stat().st_size == 0:
+                path.unlink()
+        except OSError:
+            pass  # created by the claim and taken since by another run
+
+
 def _partial_size(path: Path) -> int:
     """Bytes already held by a partial copy. Anything but a plain file is dropped."""
     try:
@@ -2810,6 +3059,10 @@ NOT_WRITABLE = "folder not writable"
 
 
 def _write_refused(exc: OSError) -> bool:
+    # A sharing violation is a file another process holds open, which a
+    # later attempt can find released: no refusal of the folder.
+    if getattr(exc, "winerror", None) in _SHARING_VIOLATIONS:
+        return False
     return isinstance(exc, PermissionError) or exc.errno == errno.EROFS
 
 
@@ -2973,6 +3226,34 @@ def _plural(count: int, word: str) -> str:
     return f"{count} {word}" if count == 1 else f"{count} {word}s"
 
 
+class NetworkReason(str):
+    """A failure the connection caused: cut, timed out, refused by a server
+    or a proxy. The summary names the network instead of the folder."""
+
+
+def _transfer_failure(exc: BaseException) -> bool:
+    """Whether a download error came from the connection, not the disk."""
+    # urlopen raises URLError for the name lookup, the connection and TLS.
+    # A cut read surfaces as a timeout (socket.timeout is no TimeoutError
+    # before 3.10), a reset, an SSL error or a size that does not add up.
+    ssl_error = getattr(sys.modules.get("ssl"), "SSLError", ())
+    if isinstance(exc, (urllib.error.URLError, http.client.HTTPException, ValueError,
+                        ConnectionError, TimeoutError, socket.timeout)):
+        return True
+    if ssl_error and isinstance(exc, ssl_error):
+        return True
+    return isinstance(exc, OSError) and exc.errno in (
+        errno.ECONNRESET, errno.ECONNREFUSED, errno.ECONNABORTED, errno.ETIMEDOUT,
+        errno.EHOSTUNREACH, errno.ENETUNREACH, errno.ENETDOWN, errno.EPIPE,
+    )
+
+
+NETWORK_FAILURE_ADVICE = (
+    "  The transfers were cut or refused on the way: check the connection, and "
+    "HTTPS_PROXY if this network needs a proxy.\n"
+    f"  More help: {TROUBLESHOOTING_PAGE}")
+
+
 def _report_outcome(
     paths: list[Path],
     downloaded: int,
@@ -3034,11 +3315,18 @@ def _report_outcome(
     if any(reason == READ_ONLY for _dest, reason in failures or []):
         advice.append("  Files marked read-only are in the way: clear that attribute "
                       "(or delete them), then run again.")
+    if any(isinstance(reason, NetworkReason) for _dest, reason in failures or []):
+        advice.append(NETWORK_FAILURE_ADVICE)
     paths = paths + [folder for folder in elsewhere if folder not in paths]
 
     if errors and not downloaded:
         print(f"\nNothing was installed. {_plural(errors, 'file')} failed.")
-        print(f"Location that was being written to: {_locations(paths)}")
+        if failures and all(isinstance(r, NetworkReason) for _d, r in failures):
+            # The folder was never the problem: naming it as the place being
+            # written to sent people looking at their disk.
+            print(f"Destination: {_locations(paths)}")
+        else:
+            print(f"Location that was being written to: {_locations(paths)}")
         for line in advice:
             print(line)
         print("Run the same command again once that is resolved. "
@@ -3441,7 +3729,9 @@ def do_standalone_copies(
 
     Returns (copied, skipped, failed): skipped is a target directory that
     does not exist, failed a copy the system refused (permission, disk
-    full, read-only mount), which used to be counted as "dir not found".
+    full, read-only mount) or a name leading out of the folder, which used
+    to be counted as "dir not found". A link already at a file's name is
+    replaced by the file, never written through.
     Each failure is also appended to failures, by absolute path, for the
     summary to name.
     """
@@ -3506,14 +3796,23 @@ def do_standalone_copies(
             resolved_dir = target_dir.resolve()
             for src in sources:
                 dest = target_dir / src.name
-                # A symlink already sitting at the destination would redirect
-                # the write outside the directory the user opted into, and a
-                # crafted source name would climb out of it.
-                if dest.is_symlink() or not _within(dest, resolved_dir):
-                    skipped += 1
+                linked = dest.is_symlink()
+                # A crafted source name would climb out of the directory.
+                if not linked and not _within(dest, resolved_dir):
+                    print(f"  ERROR: {dest}: {OUTSIDE_TARGET}")
+                    failed += 1
+                    if failures is not None:
+                        failures.append((str(dest), OUTSIDE_TARGET))
                     continue
                 try:
-                    shutil.copy2(src, dest)
+                    if linked:
+                        # Writing through a link already at the name would
+                        # leave the folder the user opted into. The link
+                        # itself gives way to the file.
+                        _replace_with_copy(src, dest)
+                        print(f"  {dest}: link replaced by the file")
+                    else:
+                        shutil.copy2(src, dest)
                     copied += 1
                 except OSError as exc:
                     print(f"  ERROR: {dest}: {exc.strerror or exc}")
@@ -3526,6 +3825,22 @@ def do_standalone_copies(
     return copied, skipped, failed
 
 
+OUTSIDE_TARGET = "name leads outside the emulator folder"
+
+
+def _replace_with_copy(src: Path, dest: Path) -> None:
+    """Put a copy of src at dest's name without following what is there."""
+    fd, scratch = tempfile.mkstemp(dir=dest.parent, prefix=f".{dest.name}.", suffix=".part")
+    os.close(fd)
+    try:
+        shutil.copy2(src, scratch)
+        os.replace(scratch, dest)
+    except OSError:
+        if os.path.lexists(scratch):
+            os.unlink(scratch)
+        raise
+
+
 def format_size(n: int) -> str:
     """Human-readable file size."""
     if n < 1024:
@@ -3604,8 +3919,7 @@ def _default_dest(os_type: str, platform_name: str) -> Path | None:
     if own is not None:
         return own
     if os_type == "windows":
-        # EmuDeck's Windows edition keeps the same folder under the profile.
-        return DEFAULT_DESTS["emudeck"] if platform_name == "emudeck" else None
+        return None
     if platform_name == "retrobat":
         return None
     return DEFAULT_DESTS.get(platform_name)
@@ -3799,6 +4113,138 @@ def _print_choices(
         print(f"  {name:<{width}}  {_sized(held)}")
 
 
+def emulator_names(manifest: dict, files: list[dict], omitted: list[dict]) -> dict[str, str]:
+    """Every name --emulator accepts, with the profile each one stands for.
+
+    A profile answers to its own name and to the names its core is built
+    under (core_names: beetle_psx and mednafen_psx). A core named by a file
+    and by no profile stands for itself.
+    """
+    names = {core: core for core in available_choices(files, omitted)["cores"]}
+    for profile, aliases in (manifest.get("core_names") or {}).items():
+        names[profile] = profile
+        for alias in aliases:
+            names.setdefault(alias, profile)
+    return names
+
+
+def _print_emulators(plat: str, manifest: dict, files: list[dict], omitted: list[dict]) -> None:
+    """What --emulator accepts for a platform, one name per line."""
+    names = emulator_names(manifest, files, omitted)
+    if not names:
+        print(f"  {plat} records no emulators")
+        return
+    width = min(max(len(name) for name in names), 40)
+    for name in sorted(names):
+        profile = names[name]
+        print(f"  {name:<{width}}  {'' if profile == name else 'same as ' + profile}".rstrip())
+
+
+# RomM 5.4 places each platform's firmware by `filesystem.structure.firmware`
+# in config.yml, "bios/{platform}" by default, relative to the library
+# (rommapp/romm 5.4.0 68f71259, backend/config/config_manager.py:58,101-109,
+# 205-225,564-568). The library and config.yml both sit under the RomM base
+# folder (config/__init__.py:46-48, config_manager.py:264-265). A library
+# laid out as {platform}/roms makes RomM stop until config.yml declares its
+# templates, and RomM suggests firmware: "{platform}/bios" there
+# (config_manager.py:1105-1124).
+
+
+def parse_romm_firmware(template: str) -> tuple[tuple[str, ...], tuple[str, ...]]:
+    """The literal folders before and after {platform}, as RomM parses them."""
+    sections = [section for section in template.strip().split("/") if section]
+    if not sections:
+        raise ValueError("the RomM firmware template is empty")
+
+    def macro(section: str) -> str | None:
+        if section.startswith("{") and section.endswith("}"):
+            return section[1:-1].strip()
+        return None
+
+    if any(section in (".", "..") for section in sections):
+        raise ValueError(f"the RomM firmware template {template!r} climbs out of the library")
+    platform_at = [i for i, section in enumerate(sections) if macro(section) == "platform"]
+    if len(platform_at) != 1:
+        raise ValueError(f"the RomM firmware template {template!r} needs one '{{platform}}'")
+    before, after = sections[:platform_at[0]], sections[platform_at[0] + 1:]
+    if any(macro(section) is not None for section in before + after):
+        raise ValueError(f"the RomM firmware template {template!r} takes literal folder "
+                         "names around '{platform}'")
+    return tuple(before), tuple(after)
+
+
+def _romm_configured_firmware(config: Path) -> str | None:
+    """filesystem.structure.firmware from RomM's config.yml, read without a
+    YAML library: only that nested scalar is needed."""
+    try:
+        lines = config.read_text(encoding="utf-8", errors="replace").splitlines()
+    except OSError:
+        return None
+    path: list[tuple[int, str]] = []
+    for line in lines:
+        stripped = line.split(" #", 1)[0].rstrip()
+        if not stripped.strip() or stripped.lstrip().startswith(("#", "-")):
+            continue
+        indent = len(stripped) - len(stripped.lstrip())
+        key, sep, value = stripped.strip().partition(":")
+        if not sep:
+            continue
+        while path and path[-1][0] >= indent:
+            path.pop()
+        keys = [name for _indent, name in path] + [key.strip().strip("'\"")]
+        if keys == ["filesystem", "structure", "firmware"] and value.strip():
+            return value.strip().strip("'\"")
+        path.append((indent, key.strip().strip("'\"")))
+    return None
+
+
+def romm_firmware_layout(
+    bios_path: Path, template: str | None
+) -> tuple[tuple[str, ...], tuple[str, ...]] | None:
+    """Where RomM reads each platform's firmware, relative to its library.
+
+    bios_path is the folder the default template names, so its parent is the
+    library. The template comes from --romm-firmware, else from config.yml
+    beside the library. None keeps the pack's own bios/<platform> layout.
+    """
+    library = bios_path.parent
+    if template is None:
+        template = _romm_configured_firmware(library.parent / "config" / "config.yml")
+    if template is not None:
+        return parse_romm_firmware(template)
+    platform_first = not (library / "roms").is_dir() and any(
+        path.is_dir() for path in library.glob("*/roms")
+    )
+    if platform_first:
+        raise ValueError(
+            f"{library} keeps its roms inside each platform folder, and RomM then "
+            "reads firmware where filesystem.structure.firmware in its config.yml "
+            "says, which was not found here. RomM suggests \"{platform}/bios\" for "
+            "this layout: run again with --romm-firmware \"{platform}/bios\", or "
+            "with the value config.yml holds."
+        )
+    return None
+
+
+def romm_relayout(
+    files: list[dict], bios_path: Path,
+    before: tuple[str, ...], after: tuple[str, ...],
+) -> tuple[list[dict], Path]:
+    """The manifest's bios/<platform>/<file> entries under RomM's template,
+    with the library as the root they are relative to."""
+    if (before, after) == (("bios",), ()):
+        return files, bios_path
+    moved = []
+    for entry in files:
+        slug, _, rest = str(entry["dest"]).partition("/")
+        if not rest:
+            # Firmware of no platform: RomM lists none, the default place stays.
+            moved.append({**entry, "dest": f"bios/{slug}"})
+            continue
+        moved.append({**entry, "dest": "/".join((*before, slug, *after, rest))})
+    return moved, bios_path.parent
+
+
 def _older_list_advice() -> str:
     """How to reach a file list that records what an option needs.
 
@@ -3828,11 +4274,7 @@ def _narrow_to_emulators(
               "which systems, which --emulator needs.", file=sys.stderr)
         print(_older_list_advice(), file=sys.stderr)
         sys.exit(1)
-    known = sorted({
-        name
-        for profile, aliases in (manifest.get("core_names") or {}).items()
-        for name in [profile, *aliases]
-    } | set(available_choices(files, omitted)["cores"]))
+    known = sorted(emulator_names(manifest, files, omitted))
     try:
         chosen = resolve_choices(names, known, "emulator")
     except ValueError as exc:
@@ -3969,7 +4411,7 @@ def main() -> None:
         action="append",
         metavar="NAME",
         help="install what these emulators read on the platform, as a pack "
-             "built for them carries (names: --list-cores)",
+             "built for them carries (names: --list-emulators)",
     )
     parser.add_argument(
         "--installed-cores",
@@ -4014,6 +4456,18 @@ def main() -> None:
         action="store_true",
         help="list the cores whose extra files a platform installs and exit",
     )
+    parser.add_argument(
+        "--romm-firmware",
+        metavar="TEMPLATE",
+        help="RomM: where each platform's firmware goes in the library, as "
+             "filesystem.structure.firmware in its config.yml (default: read "
+             "from config.yml, else bios/{platform})",
+    )
+    parser.add_argument(
+        "--list-emulators",
+        action="store_true",
+        help="list every name --emulator accepts for a platform and exit",
+    )
     parser.add_argument(
         "--no-input",
         action="store_true",
@@ -4059,7 +4513,7 @@ def main() -> None:
             parser.error(f"{option} needs at least one name")
     if args.quiet and (
         args.check or args.dry_run or args.list_platforms or args.list_targets
-        or args.list_systems or args.list_cores
+        or args.list_systems or args.list_cores or args.list_emulators
     ):
         parser.error("--quiet applies to an install, not to --check, --dry-run or a listing")
     if args.every_board and args.target:
@@ -4147,7 +4601,8 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
     """One run of the installer, once the options are read."""
     term = Terminal(real_stdout, quiet=args.quiet)
     interactive = sys.stdin.isatty() and not args.no_input and not args.quiet
-    listing = args.list_targets or args.list_systems or args.list_cores
+    listing = (args.list_targets or args.list_systems or args.list_cores
+               or args.list_emulators)
     print(f"{term.bold('RetroBIOS')} {term.dim(installer_id())}\n")
 
     os_type = detect_os()
@@ -4265,6 +4720,18 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
                           "--platform to install it alone.", file=sys.stderr)
                 sys.exit(1)
 
+    if args.romm_firmware is not None:
+        # Applied or refused, never ignored.
+        if not any(name == "romm" for name, _path in platforms):
+            print("Error: --romm-firmware applies to RomM, which this run does not "
+                  "install for.", file=sys.stderr)
+            sys.exit(1)
+        try:
+            parse_romm_firmware(args.romm_firmware)
+        except ValueError as exc:
+            print(f"Error: {exc}", file=sys.stderr)
+            sys.exit(1)
+
     # Every check --installed-cores makes before anything is written: a
     # refusal for the second platform must not follow an install of the first.
     cores_found: dict[tuple[str, Path], list[str]] = {}
@@ -4331,6 +4798,15 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
                 if board in known:
                     target = board
                     print(f"  Board {board} detected (--every-board installs all boards)")
+                else:
+                    print(f"  Board {board} has no {PLATFORM_NAMES.get(plat_name, plat_name)} "
+                          "core list: installing the files of every board")
+            else:
+                derived = derivative_board(plat_name)
+                if derived:
+                    print(f"  {derived[0]} board {derived[1]} detected. {derived[0]} "
+                          "builds its own set of cores and publishes no core list "
+                          "per board: installing the files of every board")
         if target:
             targets = target_lists.get(plat_name)
             if targets is None:
@@ -4376,6 +4852,10 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
             )
             print(f"  Narrowed {before} -> {len(files)} files to the installed cores")
 
+        if args.list_emulators:
+            _print_emulators(plat_name, manifest, files, omitted_files)
+            continue
+
         if args.emulator:
             files, omitted_files = _narrow_to_emulators(
                 args.emulator, manifest, plat_name, files, omitted_files
@@ -4442,6 +4922,20 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
             total_omitted += len(omitted_files)
             omitted_seen.extend(omitted_files)
 
+        if plat_name == "romm":
+            try:
+                layout = romm_firmware_layout(bios_path, args.romm_firmware)
+            except ValueError as exc:
+                print(f"Error: {exc}", file=sys.stderr)
+                sys.exit(1)
+            if layout is not None:
+                files, library = romm_relayout(files, bios_path, *layout)
+                if library != bios_path:
+                    installed_paths[installed_paths.index(bios_path)] = library
+                    print(f"  Firmware laid out as {'/'.join(layout[0] + ('{platform}',) + layout[1])} "
+                          f"under {library}")
+                    bios_path = library
+
         print("\nChecking existing files...")
         to_download, up_to_date, mismatched = check_local(
             files, bios_path, jobs=args.jobs, term=term
diff --git a/install.sh b/install.sh
index 566e5ee1..c5c3ec4a 100755
--- a/install.sh
+++ b/install.sh
@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="496e5a3aaf817301c40f45f184b97fc5607147184195d827a46b92c28fd3d2b0"
+DEFAULT_INSTALL_SHA256="1cf3bd0d3985e3a728513be75cb80682fe1e7a9224da6437e8f1cd9b2a3eedb1"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/platforms/_registry.yml b/platforms/_registry.yml
index d8ef431d..22c77a2d 100644
--- a/platforms/_registry.yml
+++ b/platforms/_registry.yml
@@ -269,10 +269,23 @@ platforms:
         parse_key: system_directory
       # Handheld firmwares that run RetroArch as their frontend: muOS,
       # ArkOS, and the stock Linux of Anbernic devices (dmenu).
+      # muOS copies its default config on first launch and RetroArch reads
+      # /run/muos/storage/bios, bound to MUOS/bios on the second card when
+      # it holds one, else the first (MustardOS/internal f41f4a89,
+      # script/control/retroarch.sh:8-20, script/device/bind.sh:4,116-150,
+      # share/emulator/retroarch/retroarch.default.cfg:2806,3151).
+      - os: linux
+        method: config_file
+        config: /opt/muos/share/info/config/retroarch.cfg
+        parse_key: system_directory
+      - os: linux
+        method: config_file
+        config: /opt/muos/share/emulator/retroarch/retroarch.default.cfg
+        parse_key: system_directory
       - os: linux
         method: file_exists
         file: /opt/muos
-        bios_path: /mnt/mmc/MUOS/bios
+        bios_path: /run/muos/storage/bios
       - os: linux
         method: file_exists
         file: /mnt/mmc/MUOS
@@ -300,14 +313,22 @@ platforms:
     target_source: https://github.com/batocera-linux/batocera.linux
     install:
       detect:
-      # Knulli is a Batocera derivative and carries its markers too.
+      # Knulli, a Batocera derivative (knulli-cfw/knulli-linux 57afa662):
+      # OS_NAME in os-release (board/scripts/post-build-script.sh:233-236),
+      # and knulli.version (package/system/knulli-system/knulli-system.mk:98-102).
       - os: linux
-        method: file_exists
-        file: /etc/knulli-release
+        method: os_release
+        match: [OS_NAME=knulli]
         bios_path: /userdata/bios
       - os: linux
         method: file_exists
-        file: /etc/batocera-version
+        file: /usr/share/knulli/knulli.version
+        bios_path: /userdata/bios
+      # batocera.linux 38845725, package/batocera/core/batocera-system/
+      # batocera-system.mk:21-22 writes batocera.arch and batocera.version.
+      - os: linux
+        method: file_exists
+        file: /usr/share/batocera/batocera.version
         bios_path: /userdata/bios
       standalone_copies: *standalone_copies
   recalbox:
@@ -351,6 +372,14 @@ platforms:
     target_source: null
     install:
       detect:
+      # RetroBat.exe writes its own folder there on every start, wherever it
+      # was installed (retrobat-executable 702da66a,
+      # RetroBat/RetroBat/Program.cs:57 and 99, RegistryTools.cs:15-50).
+      - os: windows
+        method: registry
+        key: 'HKCU\SOFTWARE\RetroBat'
+        value: LatestKnownInstallPath
+        bios_subdir: bios
       # installer.iss: DefaultDirName=C:\RetroBat, the first place to look.
       - os: windows
         method: path_exists
@@ -384,11 +413,18 @@ platforms:
       - os: linux
         method: path_exists
         path: $HOME/Emulation/bios
+      # functions/all.ps1:4-14 (emudeck-we 037460ef): the APPDATA settings,
+      # else the profile ones, and C:\Emulation when neither names a folder.
       - os: windows
         method: config_file
         config: '%APPDATA%\EmuDeck\settings.ps1'
         parse_key: $emulationPath
         bios_subdir: bios
+      - os: windows
+        method: config_file
+        config: '%USERPROFILE%\EmuDeck\settings.ps1'
+        parse_key: $emulationPath
+        bios_subdir: bios
       standalone_copies: *standalone_copies
   lakka:
     config: lakka.yml
@@ -475,6 +511,14 @@ platforms:
       - os: linux
         method: path_exists
         path: /romm/library/bios
+      # RomM 5.4 places firmware by filesystem.structure.firmware in this
+      # file, relative to /romm/library (rommapp/romm 5.4.0,
+      # backend/config/config_manager.py:58,101-109,264-265,564-568): a
+      # {platform}/bios library has no library/bios to find.
+      - os: linux
+        method: path_exists
+        path: /romm/config/config.yml
+        bios_path: /romm/library/bios
     cores:
     - atari800
     - clk
diff --git a/tests/test_install.py b/tests/test_install.py
index d266f555..cf752489 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -19,6 +19,7 @@ import threading
 import unittest
 from types import SimpleNamespace
 import unittest.mock
+import urllib.error
 from pathlib import Path
 
 REPO_ROOT = Path(__file__).resolve().parent.parent
@@ -1027,7 +1028,8 @@ class DetectionFollowsTheRegistry(unittest.TestCase):
         with unittest.mock.patch.object(install, "_parse_os_release", return_value={}), \
                 unittest.mock.patch.object(
                     Path, "exists",
-                    lambda p: str(p) in ("/etc/knulli-release", "/etc/batocera-version"),
+                    lambda p: str(p) in ("/usr/share/knulli/knulli.version",
+                                       "/usr/share/batocera/batocera.version"),
                 ):
             self.assertEqual(install._detect_embedded(), [("batocera", Path("/userdata/bios"))])
         # What ROCKNIX writes since 2025: OS_NAME and HW_DEVICE, no ID.
@@ -1623,6 +1625,9 @@ class TestStandaloneCopyTargetsAreUntrusted(unittest.TestCase):
         self.assertIsInstance(install._validate_manifest(manifest, "retroarch"), dict)
 
     def test_symlinked_destination_is_not_written_through(self):
+        """A link at the name was skipped in silence and counted as "dir
+        not found" while the folder existed: the emulator kept whatever the
+        link pointed at. The link now gives way to the file."""
         bios = Path(_tempdir(self))
         (bios / "boot.bin").write_text("rom")
         target = Path(_tempdir(self))
@@ -1630,12 +1635,17 @@ class TestStandaloneCopyTargetsAreUntrusted(unittest.TestCase):
         outside.write_text("untouched")
         (target / "boot.bin").symlink_to(outside)
 
-        copied, skipped, _ = install.do_standalone_copies(
-            self._manifest(str(target)), bios, "linux"
-        )
-        self.assertEqual(copied, 0)
-        self.assertEqual(skipped, 1)
+        out = io.StringIO()
+        with contextlib.redirect_stdout(out):
+            copied, skipped, failed = install.do_standalone_copies(
+                self._manifest(str(target)), bios, "linux"
+            )
+        self.assertEqual((copied, skipped, failed), (1, 0, 0))
         self.assertEqual(outside.read_text(), "untouched")
+        self.assertFalse((target / "boot.bin").is_symlink())
+        self.assertEqual((target / "boot.bin").read_text(), "rom")
+        self.assertIn("link replaced", out.getvalue())
+        self.assertEqual(sorted(p.name for p in target.iterdir()), ["boot.bin"])
 
 
 @unittest.skipUnless(shutil.which("pwsh"), "pwsh not available")
@@ -2283,6 +2293,29 @@ class ThePowerShellWrapperPassesTheStatusOn(unittest.TestCase):
                 self.assertNotIn("installer failed", proc.stdout + proc.stderr)
 
 
+class ThePowerShellWrapperPassesEveryArgument(unittest.TestCase):
+    """A [Parameter()] attribute made install.ps1 an advanced script, whose
+    common parameters took -v (as -Verbose), -vb, -d and -ea: the documented
+    `-v` never reached install.py."""
+
+    @unittest.skipUnless(shutil.which("pwsh"), "needs PowerShell")
+    def test_short_flags_reach_the_installer(self):
+        folder = Path(_tempdir(self))
+        shutil.copy(REPO_ROOT / "install.ps1", folder / "install.ps1")
+        (folder / "install.py").write_text(
+            "import json, sys\nprint(json.dumps(sys.argv[1:]))\n"
+        )
+        for args in (["-v", "--check"], ["-vb"], ["-Verbose"], ["-d", "-ea", "x"],
+                     ["--platform", "a b"], []):
+            with self.subTest(args=args):
+                proc = subprocess.run(
+                    ["pwsh", "-NoProfile", "-File", str(folder / "install.ps1"), *args],
+                    capture_output=True, text=True, timeout=120,
+                )
+                self.assertEqual(proc.returncode, 0, proc.stderr)
+                self.assertEqual(json.loads(proc.stdout), args)
+
+
 @unittest.skipIf(os.name == "nt" or os.geteuid() == 0, "needs a folder this user cannot write")
 class AnUnwritableFolderIsCodeTwo(unittest.TestCase):
     """A BIOS folder that could not be created ended with 1, while the same
@@ -3163,8 +3196,9 @@ class NoFallbackOutsideWhatAPlatformReads(unittest.TestCase):
         self.assertIsNone(install._default_dest("windows", "batocera"))
         self.assertIsNone(install._default_dest("windows", "retropie"))
         self.assertEqual(install._default_dest("linux", "batocera"), Path("/userdata/bios"))
+        # emudeck-we functions/all.ps1:12-14, not the profile folder.
         self.assertEqual(
-            install._default_dest("windows", "emudeck"), install.DEFAULT_DESTS["emudeck"]
+            install._default_dest("windows", "emudeck"), install._host_path("C:/Emulation/bios")
         )
 
     def test_a_forced_platform_without_a_folder_is_refused(self):
@@ -3601,5 +3635,471 @@ class DestAloneFollowsWhatIsFoundHere(unittest.TestCase):
         self.assertIn("Which platform's files go to /media/card?", out)
 
 
+class _FakeResponse:
+    """What urlopen returns, serving one payload from its start."""
+
+    def __init__(self, payload: bytes) -> None:
+        self.status = 200
+        self.headers = {"Content-Length": str(len(payload))}
+        self._body = io.BytesIO(payload)
+
+    def __enter__(self) -> _FakeResponse:
+        return self
+
+    def __exit__(self, *_exc: object) -> None:
+        return None
+
+    def read(self, size: int = -1) -> bytes:
+        return self._body.read(size)
+
+
+def _sharing_violation() -> PermissionError:
+    exc = PermissionError(13, "The process cannot access the file")
+    exc.winerror = 32
+    return exc
+
+
+class ACompletePartialIsHandedOffOnWindows(unittest.TestCase):
+    """Windows renames no open file, so the run that verified a large copy
+    released its lock before the rename. A second run claimed the copy in
+    that moment, found it complete and truncated it, and the first run's
+    rename failed with a sharing violation reported as "folder not
+    writable"."""
+
+    PAYLOAD = bytes(range(256)) * 200
+    ENTRY = {
+        "dest": "big.bin", "repo_path": "bios/big.bin", "size": len(PAYLOAD),
+        "sha256": hashlib.sha256(PAYLOAD).hexdigest(),
+    }
+
+    def _run(self, tmp: Path, seen: list[int]):
+        part = tmp / ".big.bin.part"
+
+        def urlopen(request, timeout=None):
+            seen.append(part.stat().st_size if part.exists() else -1)
+            return _FakeResponse(self.PAYLOAD)
+
+        with unittest.mock.patch.object(install, "RESUME_FROM_SIZE", 1024), \
+                unittest.mock.patch.object(install, "_NO_RENAME_WHILE_OPEN", True), \
+                unittest.mock.patch.object(install.urllib.request, "urlopen", urlopen):
+            return install._download_one(dict(self.ENTRY), tmp)
+
+    def test_the_other_runs_copy_is_left_whole(self):
+        tmp = Path(_tempdir(self))
+        (tmp / ".big.bin.part").write_bytes(self.PAYLOAD)
+        seen: list[int] = []
+        self.assertEqual(self._run(tmp, seen), ("big.bin", True, ""))
+        # The download ran while the complete copy was still whole.
+        self.assertEqual(seen, [len(self.PAYLOAD)])
+        self.assertEqual((tmp / "big.bin").read_bytes(), self.PAYLOAD)
+        # Nothing held it afterwards: it was a leftover, now removed.
+        self.assertFalse((tmp / ".big.bin.part").exists())
+
+    def test_the_rename_waits_for_the_other_run_to_let_go(self):
+        tmp = Path(_tempdir(self))
+        source = tmp / "copy"
+        source.write_bytes(b"x")
+        real_replace = os.replace
+        calls = []
+
+        def replace(src, dst):
+            calls.append(src)
+            if len(calls) < 3:
+                raise _sharing_violation()
+            return real_replace(src, dst)
+
+        with unittest.mock.patch.object(install.os, "replace", replace), \
+                unittest.mock.patch.object(install.time, "sleep"):
+            install._replace_released(source, tmp / "dest", {})
+        self.assertEqual(len(calls), 3)
+        self.assertEqual((tmp / "dest").read_bytes(), b"x")
+
+    def test_a_copy_the_other_run_placed_counts_once_verified(self):
+        tmp = Path(_tempdir(self))
+        dest = tmp / "dest"
+        dest.write_bytes(self.PAYLOAD)
+        expected = {"sha256": self.ENTRY["sha256"]}
+        install._replace_released(tmp / "gone", dest, expected)
+        dest.write_bytes(b"other bytes")
+        with self.assertRaises(FileNotFoundError):
+            install._replace_released(tmp / "gone", dest, expected)
+
+    def test_a_sharing_violation_is_no_refused_folder(self):
+        self.assertFalse(install._write_refused(_sharing_violation()))
+        self.assertTrue(install._write_refused(PermissionError(13, "denied")))
+
+
+class TheEmulatorNamesHaveTheirOwnListing(unittest.TestCase, _LoopbackRun):
+    """--emulator nope said "--list-cores lists all 441" and --list-cores
+    printed 189: the buildbot names --emulator accepts were listed nowhere."""
+
+    def setUp(self):
+        self.env = self.serve(self)
+
+    def test_the_hint_counts_what_the_listing_prints(self):
+        proc = _install_run(self.env, "--platform", "retroarch", "--no-input",
+                            "--emulator", "nope")
+        self.assertEqual(proc.returncode, 1, proc.stdout)
+        found = re.search(r"--list-emulators lists all (\d+)\.", proc.stderr)
+        self.assertIsNotNone(found, proc.stderr)
+        listing = _install_run(self.env, "--platform", "retroarch", "--no-input",
+                               "--list-emulators")
+        self.assertEqual(listing.returncode, 0, listing.stderr)
+        names = [re.split(r"\s{2,}", line.strip())[0]
+                 for line in listing.stdout.splitlines() if line.startswith("  ")]
+        manifest = json.loads((REPO_ROOT / "install" / "retroarch.json").read_text())
+        accepted = install.emulator_names(
+            manifest, manifest["files"], manifest.get("omitted_files", []))
+        self.assertEqual(int(found.group(1)), len(accepted))
+        self.assertTrue(set(accepted) <= set(names), set(accepted) - set(names))
+        self.assertIn("mednafen_psx", accepted)
+        self.assertRegex(listing.stdout, r"mednafen_psx\s+same as beetle_psx")
+
+    def test_every_listed_name_is_accepted(self):
+        manifest = {"core_names": {"beetle_psx": ["mednafen_psx"]}}
+        files = [{"dest": "a", "cores": ["pcsx_rearmed"]}]
+        names = install.emulator_names(manifest, files, [])
+        self.assertEqual(names, {"beetle_psx": "beetle_psx", "mednafen_psx": "beetle_psx",
+                                 "pcsx_rearmed": "pcsx_rearmed"})
+        self.assertEqual(install.resolve_choices(["MEDNAFEN_PSX"], sorted(names), "emulator"),
+                         ["mednafen_psx"])
+
+
+class ANetworkFailureNamesTheNetwork(unittest.TestCase):
+    """Every transfer cut by the connection ended on "Location that was being
+    written to" and "once that is resolved", naming the folder and nothing
+    about the connection or the proxy."""
+
+    def _fail(self, error: BaseException) -> str:
+        def urlopen(*_args, **_kwargs):
+            raise error
+
+        entry = {"dest": "a.bin", "repo_path": "bios/a.bin", "size": 10,
+                 "sha256": "0" * 64}
+        with unittest.mock.patch.object(install.urllib.request, "urlopen", urlopen), \
+                unittest.mock.patch.object(install, "_retry_delay", return_value=0):
+            _dest, ok, reason = install._download_one(entry, Path(_tempdir(self)))
+        self.assertFalse(ok)
+        return reason
+
+    def test_cuts_and_refusals_are_network_failures(self):
+        import http.client
+        import socket
+
+        for error in (
+            urllib.error.URLError(socket.gaierror(-2, "Name or service not known")),
+            http.client.IncompleteRead(b"abc", 10),
+            ConnectionResetError(104, "Connection reset by peer"),
+            socket.timeout("timed out"),
+            urllib.error.HTTPError("u", 503, "Service Unavailable", None, None),
+        ):
+            with self.subTest(error=error):
+                self.assertIsInstance(self._fail(error), install.NetworkReason)
+
+    def test_a_full_disk_and_a_missing_file_are_not(self):
+        import errno
+
+        for error in (OSError(errno.ENOSPC, "No space left on device"),
+                      urllib.error.HTTPError("u", 404, "Not Found", None, None)):
+            with self.subTest(error=error):
+                self.assertNotIsInstance(self._fail(error), install.NetworkReason)
+
+    def test_the_summary_points_at_the_connection(self):
+        failures = [("a.bin", install.NetworkReason("timed out")),
+                    ("b.bin", install.NetworkReason("Connection reset by peer"))]
+        out = io.StringIO()
+        with contextlib.redirect_stdout(out), self.assertRaises(SystemExit) as raised:
+            install._report_outcome([Path("/bios")], 0, 0, 2, 0, False, failures=failures)
+        self.assertEqual(raised.exception.code, 2)
+        text = out.getvalue()
+        self.assertIn("HTTPS_PROXY", text)
+        self.assertIn("Destination: /bios", text)
+        self.assertNotIn("being written to", text)
+
+
+class RetroBatIsFoundWhereItSaysItIs(unittest.TestCase):
+    """RetroBat on D:\\RetroBat was not detected, and --platform retrobat
+    wrote 7.6 GB into C:\\RetroBat\\bios, a folder it does not read.
+    RetroBat.exe writes its folder to the registry on every start."""
+
+    def test_the_table_is_the_registry(self):
+        import yaml  # noqa: PLC0415
+
+        registry = yaml.safe_load((REPO_ROOT / "platforms" / "_registry.yml").read_text())
+        rules = {
+            (rule["os"], name, rule["key"], rule["value"], rule["bios_subdir"])
+            for name, entry in registry["platforms"].items()
+            for rule in (entry.get("install") or {}).get("detect", [])
+            if rule["method"] == "registry"
+        }
+        self.assertEqual(set(install._REGISTRY_RULES), rules)
+
+    def test_the_registry_folder_wins_over_the_default(self):
+        folder = Path(_tempdir(self))
+        with unittest.mock.patch.object(install, "_read_registry",
+                                        return_value=str(folder) + "\\"):
+            found: list = []
+            install._detect_by_registry("windows", found)
+        self.assertEqual(found, [("retrobat", folder / "bios")])
+
+    def test_a_folder_since_removed_is_passed_over(self):
+        gone = Path(_tempdir(self)) / "moved"
+        with unittest.mock.patch.object(install, "_read_registry", return_value=str(gone)):
+            found: list = []
+            install._detect_by_registry("windows", found)
+        self.assertEqual(found, [])
+
+    def test_wsl_reads_it_through_reg_exe(self):
+        output = ("\nHKEY_CURRENT_USER\\SOFTWARE\\RetroBat\n"
+                  "    LatestKnownInstallPath    REG_SZ    D:\\Games\\RetroBat\\\n\n")
+        with unittest.mock.patch.dict(install._WSL, {"active": True, "root": "/mnt"}), \
+                unittest.mock.patch.object(install.os, "name", "posix"), \
+                unittest.mock.patch.object(
+                    install.subprocess, "run",
+                    return_value=SimpleNamespace(stdout=output, returncode=0)) as run:
+            value = install._read_registry("HKCU\\SOFTWARE\\RetroBat", "LatestKnownInstallPath")
+            self.assertEqual(value, "D:\\Games\\RetroBat\\")
+            self.assertEqual(install._host_path(value.rstrip("\\")),
+                             Path("/mnt/d/Games/RetroBat"))
+        self.assertEqual(run.call_args[0][0][:2], ["reg.exe", "query"])
+
+
+class EmuDeckWindowsIsFoundAsItsScriptsFindIt(unittest.TestCase):
+    """EmuDeck's Windows scripts read %APPDATA%\\EmuDeck\\settings.ps1, else
+    %USERPROFILE%\\EmuDeck\\settings.ps1, and take C:\\Emulation when neither
+    names a folder (functions/all.ps1:4-14). Only the first was read, and a
+    forced --platform emudeck wrote into %USERPROFILE%\\Emulation\\bios."""
+
+    def _env(self) -> tuple[Path, Path]:
+        root = Path(_tempdir(self))
+        appdata, profile = root / "appdata", root / "profile"
+        patcher = unittest.mock.patch.dict(
+            os.environ, {"APPDATA": str(appdata), "USERPROFILE": str(profile)})
+        patcher.start()
+        self.addCleanup(patcher.stop)
+        return appdata, profile
+
+    @staticmethod
+    def _settings(folder: Path, text: str, encoding: str = "utf-8-sig") -> None:
+        (folder / "EmuDeck").mkdir(parents=True, exist_ok=True)
+        (folder / "EmuDeck" / "settings.ps1").write_bytes(text.encode(encoding))
+
+    def test_the_profile_settings_are_read_when_appdata_has_none(self):
+        _appdata, profile = self._env()
+        self._settings(profile, '\n$emulationPath="/games/Emulation"\n')
+        self.assertEqual(install._emudeck_windows_bios(), Path("/games/Emulation/bios"))
+
+    def test_appdata_comes_first(self):
+        appdata, profile = self._env()
+        self._settings(profile, '$emulationPath="/b/Emulation"\n')
+        self._settings(appdata, '$emulationPath="/a/Emulation"\n', "utf-16")
+        self.assertEqual(install._emudeck_windows_bios(), Path("/a/Emulation/bios"))
+
+    def test_settings_naming_no_folder_mean_c_emulation(self):
+        appdata, _profile = self._env()
+        self._settings(appdata, '$device="Windows"\n')
+        self.assertEqual(install._emudeck_windows_bios(),
+                         install._host_path("C:\\Emulation") / "bios")
+
+    def test_a_network_install_is_its_share(self):
+        appdata, _profile = self._env()
+        self._settings(appdata, '$networkInstallation="true"\n'
+                       '$VolName="/nas/games"\n'
+                       '$emulationPath=$testdrive.ToString() + ":\\Emulation"\n')
+        self.assertEqual(install._emudeck_windows_bios(),
+                         Path("/nas/games\\Emulation") / "bios")
+
+    def test_no_settings_detects_nothing(self):
+        self._env()
+        self.assertIsNone(install._emudeck_windows_bios())
+
+    def test_a_forced_platform_falls_back_to_c_emulation(self):
+        self.assertEqual(install._default_dest("windows", "emudeck"),
+                         install._host_path("C:/Emulation/bios"))
+
+
+class MuOsIsReadFromItsRetroArchConfig(unittest.TestCase):
+    """muOS wrote to /mnt/mmc/MUOS/bios while RetroArch reads
+    /run/muos/storage/bios, bound to the second card when it holds MUOS/bios,
+    and --installed-cores found no core in /opt/muos/share/core."""
+
+    def _muos(self, *, user_config: bool) -> tuple[Path, Path]:
+        root = Path(_tempdir(self))
+        muos = root / "opt" / "muos"
+        system_dir, cores = root / "run" / "bios", root / "core"
+        cores.mkdir(parents=True)
+        configs = (muos / "share/info/config/retroarch.cfg",
+                   muos / "share/emulator/retroarch/retroarch.default.cfg")
+        default = configs[1]
+        default.parent.mkdir(parents=True)
+        default.write_text(f'libretro_directory = "{cores}"\n'
+                           f'system_directory = "{root / "default-bios"}"\n')
+        if user_config:
+            configs[0].parent.mkdir(parents=True)
+            configs[0].write_text(f'libretro_directory = "{cores}"\n'
+                                  f'system_directory = "{system_dir}"\n')
+        for patcher in (
+            unittest.mock.patch.object(install, "MUOS_ROOT", muos),
+            unittest.mock.patch.object(install, "MUOS_RETROARCH_CONFIGS", configs),
+            unittest.mock.patch.object(
+                install, "_detect_embedded",
+                lambda: [("retroarch", Path("/run/muos/storage/bios"))]),
+            unittest.mock.patch.dict(os.environ, {"HOME": str(root / "home")}),
+            unittest.mock.patch.dict(install._RETROARCH_CONFIGS, clear=True),
+        ):
+            patcher.start()
+            self.addCleanup(patcher.stop)
+        return (system_dir if user_config else root / "default-bios"), cores
+
+    def test_the_users_config_decides(self):
+        system_dir, cores = self._muos(user_config=True)
+        found = install.detect_platforms("linux")
+        self.assertIn(("retroarch", system_dir), found)
+        self.assertNotIn(("retroarch", Path("/run/muos/storage/bios")), found)
+        self.assertIn(cores, install.core_dirs("retroarch", system_dir))
+
+    def test_before_the_first_launch_the_default_does(self):
+        system_dir, cores = self._muos(user_config=False)
+        self.assertIn(("retroarch", system_dir), install.detect_platforms("linux"))
+        self.assertIn(cores, install.core_dirs("retroarch", system_dir))
+
+    def test_without_a_config_the_bound_folder_is_used(self):
+        rule = next(r for r in install._EMBEDDED_RULES if r[2] == ("/opt/muos",))
+        self.assertEqual(rule[3], "/run/muos/storage/bios")
+
+
+class BatoceraAndKnulliByWhatTheirBuildsWrite(unittest.TestCase):
+    """No Batocera build writes /etc/batocera-version (batocera-system.mk
+    writes /usr/share/batocera/batocera.version), and Knulli writes no
+    /etc/knulli-release and removes /usr/share/batocera: neither was
+    detected, and a Knulli board got every board's files without a word."""
+
+    def _detect(self, release: dict, files: tuple[str, ...]):
+        with unittest.mock.patch.object(install, "_parse_os_release", return_value=release), \
+                unittest.mock.patch.object(Path, "exists", lambda p: str(p) in files):
+            return install._detect_embedded()
+
+    def test_batocera_by_its_version_file(self):
+        self.assertEqual(self._detect({}, ("/usr/share/batocera/batocera.version",)),
+                         [("batocera", Path("/userdata/bios"))])
+        self.assertEqual(self._detect({}, ("/etc/batocera-version",)), [])
+
+    def test_knulli_by_its_os_release_or_version_file(self):
+        self.assertEqual(self._detect({"OS_NAME": "knulli"}, ()),
+                         [("batocera", Path("/userdata/bios"))])
+        self.assertEqual(self._detect({}, ("/usr/share/knulli/knulli.version",)),
+                         [("batocera", Path("/userdata/bios"))])
+
+    def test_a_knulli_board_is_named_and_not_narrowed(self):
+        board = Path(_tempdir(self)) / "knulli.arch"
+        board.write_text("a133")
+        with unittest.mock.patch.dict(install._DERIVATIVE_BOARDS,
+                                      {"batocera": ("Knulli", str(board))}):
+            self.assertEqual(install.derivative_board("batocera"), ("Knulli", "a133"))
+            self.assertIsNone(install.derivative_board("recalbox"))
+
+    def test_the_run_says_why_every_board_is_installed(self):
+        source = (REPO_ROOT / "install.py").read_text(encoding="utf-8")
+        block = source[source.index("board = detect_board(plat_name)"):]
+        block = block[:block.index("if target:")]
+        self.assertIn("has no", block)
+        self.assertIn("derivative_board(plat_name)", block)
+
+
+class RomMFirmwareFollowsItsTemplate(unittest.TestCase):
+    """RomM 5.4 reads firmware where filesystem.structure.firmware says, and
+    a {platform}/roms library is told to declare "{platform}/bios". The
+    installer only wrote bios/<platform>/, where such a library lists none."""
+
+    CONFIG = (
+        "# RomM\n"
+        "exclude:\n"
+        "  platforms: []\n"
+        "filesystem:\n"
+        "  structure:\n"
+        "    default: \"{platform}/roms/{game}\"\n"
+        "    firmware: \"{platform}/bios\"  # as RomM suggests\n"
+        "system:\n"
+        "  firmware: elsewhere\n"
+    )
+
+    def _romm(self, config: str | None) -> Path:
+        base = Path(_tempdir(self))
+        library = base / "library"
+        (library / "psx" / "roms").mkdir(parents=True)
+        if config is not None:
+            (base / "config").mkdir()
+            (base / "config" / "config.yml").write_text(config)
+        return library / "bios"
+
+    def test_templates_parse_as_romm_parses_them(self):
+        self.assertEqual(install.parse_romm_firmware("bios/{platform}"), (("bios",), ()))
+        self.assertEqual(install.parse_romm_firmware("{platform}/bios"), ((), ("bios",)))
+        self.assertEqual(install.parse_romm_firmware("/a/{platform}/b/c/"),
+                         (("a",), ("b", "c")))
+        for bad in ("", "bios", "{platform}/{game}", "{region}/{platform}",
+                    "../{platform}", "{platform}/{platform}"):
+            with self.subTest(template=bad), self.assertRaises(ValueError):
+                install.parse_romm_firmware(bad)
+
+    def test_the_template_is_read_from_config_yml(self):
+        bios = self._romm(self.CONFIG)
+        layout = install.romm_firmware_layout(bios, None)
+        self.assertEqual(layout, ((), ("bios",)))
+        files = [{"dest": "psx/scph5501.bin"}, {"dest": "3do/panafz1.bin"}]
+        moved, root = install.romm_relayout(files, bios, *layout)
+        self.assertEqual(root, bios.parent)
+        self.assertEqual([f["dest"] for f in moved],
+                         ["psx/bios/scph5501.bin", "3do/bios/panafz1.bin"])
+
+    def test_the_default_keeps_the_pack_layout(self):
+        bios = self._romm("filesystem:\n  structure:\n    firmware: bios/{platform}\n")
+        files = [{"dest": "psx/scph5501.bin"}]
+        self.assertEqual(install.romm_relayout(files, bios, *install.romm_firmware_layout(
+            bios, None)), (files, bios))
+
+    def test_a_platform_first_library_without_a_template_is_refused(self):
+        bios = self._romm(None)
+        with self.assertRaises(ValueError) as raised:
+            install.romm_firmware_layout(bios, None)
+        self.assertIn('--romm-firmware "{platform}/bios"', str(raised.exception))
+        self.assertEqual(install.romm_firmware_layout(bios, "{platform}/bios"),
+                         ((), ("bios",)))
+
+    def test_the_option_is_refused_where_romm_is_not_installed(self):
+        err = io.StringIO()
+        with unittest.mock.patch.object(install, "detect_os", return_value="linux"), \
+                unittest.mock.patch("sys.argv", [
+                    "install.py", "--platform", "batocera", "--dest", _tempdir(self),
+                    "--romm-firmware", "{platform}/bios", "--no-input"]), \
+                contextlib.redirect_stdout(io.StringIO()), \
+                contextlib.redirect_stderr(err), self.assertRaises(SystemExit) as caught:
+            install.main()
+        self.assertEqual(caught.exception.code, 1)
+        self.assertIn("--romm-firmware applies to RomM", err.getvalue())
+
+
+class RomMFirmwareInstallsUnderItsTemplate(unittest.TestCase, _LoopbackRun):
+    """The whole run, over loopback: config.yml beside the library decides."""
+
+    def setUp(self):
+        self.env = self.serve(self)
+
+    def test_lynx_lands_in_its_platform_folder(self):
+        base = self.home / "romm"
+        (base / "library" / "lynx" / "roms").mkdir(parents=True)
+        (base / "config").mkdir()
+        (base / "config" / "config.yml").write_text(
+            "filesystem:\n  structure:\n    default: \"{platform}/roms/{game}\"\n"
+            "    firmware: \"{platform}/bios\"\n")
+        proc = _install_run(self.env, "--platform", "romm", "--dest",
+                            str(base / "library" / "bios"), "--system", "atari-lynx",
+                            "--no-input")
+        self.assertEqual(proc.returncode, 0, proc.stdout + proc.stderr)
+        self.assertTrue((base / "library" / "lynx" / "bios" / "lynxboot.img").is_file())
+        self.assertFalse((base / "library" / "bios").exists())
+        self.assertIn("{platform}/bios", proc.stdout)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/tests/test_install_selection.py b/tests/test_install_selection.py
index a379e1a3..22b806a2 100644
--- a/tests/test_install_selection.py
+++ b/tests/test_install_selection.py
@@ -44,6 +44,7 @@ class OneRegionVocabulary(unittest.TestCase):
         self.assertEqual(install.REGION_ALIASES, region.ALIASES)
         self.assertEqual(install.WORLD_REGION, region.WORLD)
         self.assertEqual(install.REGIONS, region.REGIONS)
+        self.assertEqual(install.FILE_GROUP, region.FILE_GROUP)
 
 
 class TheRegionRuleIsTheBuilders(unittest.TestCase):
@@ -74,9 +75,11 @@ class TheRegionRuleIsTheBuilders(unittest.TestCase):
                     index[dest] = {"regions": regions,
                                    "has_untagged": not regions, "emulators": [],
                                    "priorities": {priority}}
-                groups.setdefault(f"g{g}", []).append((dest, dest))
-                if f"g{g}" not in entries[dest]["region_groups"]:
-                    entries[dest]["region_groups"].append(f"g{g}")
+                # A lone core file no system names gets a group of its own.
+                group = f"g{g}" if rng.random() < 0.85 else f"e{region.FILE_GROUP}{dest}"
+                groups.setdefault(group, []).append((dest, dest))
+                if group not in entries[dest]["region_groups"]:
+                    entries[dest]["region_groups"].append(group)
         for entry in entries.values():
             if not entry["regions"]:
                 del entry["regions"], entry["region_groups"]
@@ -84,17 +87,57 @@ class TheRegionRuleIsTheBuilders(unittest.TestCase):
         requested = rng.sample(["north-america", "europe", "japan", "uk", "canada",
                                 "taiwan", "asia-ntsc", "china", "asia-pal"],
                                rng.randint(1, 3))
-        return groups, index, list(entries.values()), requested
+        held = {dest for dest in entries if rng.random() < 0.7}
+        return groups, index, list(entries.values()), requested, held
 
     def test_two_thousand_random_platforms(self):
         rng = random.Random(20261010)
         for trial in range(2000):
-            groups, index, entries, requested = self._case(rng)
+            groups, index, entries, requested, held = self._case(rng)
             with self.subTest(trial=trial):
                 self.assertEqual(
                     install.region_drops(entries, requested),
                     region.resolve_region_drops(groups, index, requested),
                 )
+                self.assertEqual(
+                    install.region_drops(entries, requested, held=held),
+                    region.resolve_region_drops(groups, index, requested, held),
+                )
+
+    def test_narrow_passes_the_installable_files_as_held(self):
+        """The builder passes what the collection holds (generate_pack
+        _select_variants). An omission is the installer's unheld file."""
+        rng = random.Random(20261021)
+        for trial in range(500):
+            groups, index, entries, requested, held = self._case(rng)
+            files = [dict(e, size=1, cores=None) for e in entries if e["dest"] in held]
+            omitted = [dict(e, cores=None) for e in entries if e["dest"] not in held]
+            kept, still_omitted = install.narrow(files, omitted, [], [], requested)
+            drops = region.resolve_region_drops(groups, index, requested, held)
+            with self.subTest(trial=trial):
+                self.assertEqual(
+                    {f["dest"] for f in kept},
+                    {f["dest"] for f in files} - drops,
+                )
+                self.assertEqual(
+                    {o["dest"] for o in still_omitted},
+                    {o["dest"] for o in omitted} - drops,
+                )
+
+    def test_a_lone_core_file_leaves_its_systems_drop(self):
+        """Dolphin's GC/EUR/IPL.bin is a core extra no system names, plus the
+        GameCube's own declaration. The builder skips the one-file group, so
+        --region us drops it."""
+        entries = [
+            {"dest": "GC/USA/IPL.bin", "regions": ["north-america"],
+             "region_system_groups": ["nintendo-gamecube"]},
+            {"dest": "GC/EUR/IPL.bin", "regions": ["europe"],
+             "region_groups": [f"dolphin{region.FILE_GROUP}GC/EUR/IPL.bin"],
+             "region_system_groups": ["nintendo-gamecube"]},
+        ]
+        self.assertEqual(
+            install.region_drops(entries, ["north-america"]), {"GC/EUR/IPL.bin"}
+        )
 
 
 class Narrowing(unittest.TestCase):
@@ -141,12 +184,21 @@ class Narrowing(unittest.TestCase):
         self.assertIn("scph5501.bin", files)
         self.assertEqual(omitted, [])
 
-    def test_an_unavailable_match_still_wins_as_the_builder_counts_it(self):
+    def test_an_unavailable_match_never_displaces_a_held_file(self):
+        """The builder keeps the held BIOS when the requested one is missing
+        (region.resolve_region_drops, held). The missing one stays reported."""
         files, omitted = self._dests([], [], ["europe"])
-        self.assertNotIn("scph5500.bin", files)
-        self.assertNotIn("scph5501.bin", files)
+        self.assertIn("scph5500.bin", files)
+        self.assertIn("scph5501.bin", files)
         self.assertEqual(omitted, ["scph5502.bin"])
 
+    def test_a_missing_japanese_bios_keeps_the_american_one(self):
+        files = [dict(self.FILES[1])]
+        omitted = [dict(self.OMITTED[0], dest="scph5500.bin", regions=["japan"])]
+        kept, still = install.narrow(files, omitted, [], [], ["japan"])
+        self.assertEqual([f["dest"] for f in kept], ["scph5501.bin"])
+        self.assertEqual([o["dest"] for o in still], ["scph5500.bin"])
+
     def test_no_match_keeps_every_regional_file(self):
         files, _ = self._dests([], [], ["brazil"])
         self.assertIn("scph5500.bin", files)
diff --git a/wiki/installer.md b/wiki/installer.md
index b0b30f25..51a87086 100644
--- a/wiki/installer.md
+++ b/wiki/installer.md
@@ -231,10 +231,12 @@ what the options name, everything by default.
 | `--cores-dir PATH` | The folder RetroArch loads its cores from, for `--installed-cores` when it is not found. Implies `--installed-cores` |
 | `--system NAMES` | Install only these systems. Comma-separated or repeated. `--list-systems` prints the names, `other` included |
 | `--core NAMES` | Keep the platform's own list and only the extra files these cores load. `--list-cores` prints the names |
-| `--emulator NAMES` | Install what these emulators read on the detected platform, and nothing else: their own files and the platform's files of the systems they run. Each emulator page on the site gives this command. `--list-cores` prints the names |
+| `--emulator NAMES` | Install what these emulators read on the detected platform, and nothing else: their own files and the platform's files of the systems they run. Each emulator page on the site gives this command. `--list-emulators` prints the names, buildbot names such as `mednafen_psx` included |
 | `--region LIST` | Preferred regions, best first (`us,eu,jp`): keep one BIOS per region where a system has several. Accepts the names and aliases of `generate_pack.py --region` |
 | `--list-systems` | Print a platform's systems with the files and size each holds |
 | `--list-cores` | Print the cores whose extra files a platform installs |
+| `--list-emulators` | Print every name `--emulator` accepts, each buildbot name beside the emulator it stands for |
+| `--romm-firmware TEMPLATE` | RomM: where each platform's firmware goes in the library, as `filesystem.structure.firmware` in RomM's `config.yml` says (`{platform}/bios` for a library laid out as `{platform}/roms`). Read from `config.yml` beside the library when it is there. See [RomM firmware layout](#romm-firmware-layout) |
 | `--no-input` | Never ask a question, even at a terminal |
 | `--check` | Report and exit without writing |
 | `--dry-run` | Show what would be downloaded, by system and with sizes, without writing anything |
@@ -279,17 +281,21 @@ wins, since a machine is only one of them.
 | Evidence | Platform | BIOS directory |
 |----------|----------|----------------|
 | `/etc/os-release` `OS_NAME=ROCKNIX` (or `ID=rocknix` on images before 2025) | ROCKNIX | `/storage/roms/bios` |
-| `/etc/knulli-release` | Batocera | `/userdata/bios` |
+| `/etc/os-release` `OS_NAME=knulli` or `/usr/share/knulli/knulli.version` | Batocera (Knulli) | `/userdata/bios` |
 | `/etc/os-release` `ID=lakka` | Lakka | `/storage/system` |
-| `/etc/batocera-version` | Batocera | `/userdata/bios` |
+| `/usr/share/batocera/batocera.version` | Batocera | `/userdata/bios` |
 | `/recalbox/recalbox.version` or `/usr/bin/recalbox-settings` | Recalbox | `/recalbox/share/bios` |
-| `/opt/muos` or `/mnt/mmc/MUOS/` | RetroArch | `/mnt/mmc/MUOS/bios` |
+| `/opt/muos` | RetroArch (muOS) | `system_directory` of `/opt/muos/share/info/config/retroarch.cfg`, else of the default it is copied from: `/run/muos/storage/bios`, which muOS binds to `MUOS/bios` on the second card when that card holds one, else the first. Cores are read from that config's `libretro_directory` (`/opt/muos/share/core`) |
+| `/mnt/mmc/MUOS/` | RetroArch | `/mnt/mmc/MUOS/bios` |
 | `/home/ark` and `/opt/system` | RetroArch | `/roms/bios` |
 | `/mnt/vendor/bin/dmenu.bin` | RetroArch | `/mnt/mmc/bios` |
 
 Batocera, Recalbox, ROCKNIX and Lakka also name the board they were built for, and
 that board narrows the install unless `--target` or `--every-board` says
-otherwise (see the options above).
+otherwise (see the options above). A board with no core list of its own is
+named and gets the files of every board. Knulli names its board in
+`/usr/share/knulli/knulli.arch` but builds its own set of cores, so Batocera's
+per-board lists are not applied to it and the run says so.
 
 Some platforms keep their BIOS in a fixed directory, and that directory is the
 evidence. They are looked for after the configurations below, for a platform
@@ -302,6 +308,7 @@ not already found that way.
 | `~/Emulation/bios` | EmuDeck | the same |
 | `~/.var/app/net.retrodeck.retrodeck` | RetroDECK | `~/retrodeck` |
 | `/romm/library/bios` | RomM | the same |
+| `/romm/config/config.yml` | RomM | `/romm/library/bios`, laid out by its firmware template |
 | `~/RetroPie/BIOS` | RetroPie | the same |
 | `%USERPROFILE%\BizHawk\Firmware` | BizHawk | the same |
 | `EmuHawk.exe` or `EmuHawkMono.sh` in a `BizHawk*` folder of the home folder or one level down (`~/Games/BizHawk-2.11.1`), the newest | BizHawk | `Firmware` beside it |
@@ -318,8 +325,9 @@ WSL does not pass them. A forced `--platform retrobat` falls back to
 
 | Evidence | Platform | BIOS directory |
 |----------|----------|----------------|
+| `HKCU\SOFTWARE\RetroBat` `LatestKnownInstallPath`, which `RetroBat.exe` writes on every start (read through `reg.exe` under WSL) | RetroBat | that folder + `\bios`, before the fixed folders above |
 | `~/.config/EmuDeck/settings.sh` | EmuDeck | `emulationPath` + `/bios` |
-| `%APPDATA%\EmuDeck\settings.ps1` | EmuDeck | `$emulationPath` + `\bios` |
+| `%APPDATA%\EmuDeck\settings.ps1`, else `%USERPROFILE%\EmuDeck\settings.ps1` | EmuDeck | `$emulationPath` + `\bios`, `C:\Emulation\bios` when it names none (a network install: the `$VolName` share + `\Emulation\bios`) |
 | `~/.var/app/net.retrodeck.retrodeck/config/retrodeck/retrodeck.json` | RetroDECK | `paths.rd_home_path`, falling back to the pre-migration `retrodeck.cfg` and its `rdhome` |
 | `~/.var/app/org.libretro.RetroArch/config/retroarch/retroarch.cfg` | RetroArch (Flatpak) | `system_directory` |
 | `~/snap/retroarch/current/.config/retroarch/retroarch.cfg` | RetroArch (Snap) | `system_directory` |
@@ -362,7 +370,7 @@ When a platform named with `--platform` is not found on the machine, the
 destination falls back to the directory its detection looks for:
 `/userdata/bios` for Batocera, `/recalbox/share/bios` for Recalbox,
 `/storage/system` for Lakka, `~/retrodeck` for RetroDECK, `~/Emulation/bios`
-for EmuDeck, `/storage/roms/bios` for ROCKNIX, `/media/fat/games` for MiSTer,
+for EmuDeck (`C:\Emulation\bios` on Windows), `/storage/roms/bios` for ROCKNIX, `/media/fat/games` for MiSTer,
 `~/RetroPie/BIOS` for RetroPie, `/romm/library/bios` for RomM,
 `C:\RetroBat\bios` for RetroBat, and RetroArch's own default system
 directory for RetroArch. BizHawk has no such folder: it reads `Firmware`
@@ -405,9 +413,27 @@ run rather than a file to copy.
 
 Targets are per OS, WSL falling back to the Linux ones. A directory that does
 not exist is skipped rather than created, so nothing is copied for an emulator
-that is not installed. A destination that is already a symbolic link is skipped
-too: the copy leaves the tree the user opted into, and a link there would
-redirect the write somewhere else again.
+that is not installed. A destination that is already a symbolic link is
+replaced by the file rather than written through: a write through the link
+would land outside the folder the user opted into. The summary counts each
+copy as copied, skipped (folder absent) or failed, and names every failure.
+
+## RomM firmware layout
+
+RomM 5.4 places each platform's firmware where `filesystem.structure.firmware`
+in its `config.yml` says, relative to the library: `bios/{platform}` by
+default, the layout of the pack. A library laid out as `{platform}/roms` makes
+RomM stop until `config.yml` declares its templates, and RomM suggests
+`firmware: "{platform}/bios"` for it.
+
+The installer reads that value from `config.yml` in the `config` folder beside
+the library, `/romm/config/config.yml` inside the container, and places each
+file accordingly: with `{platform}/bios`, `psx/scph5501.bin` goes to
+`library/psx/bios/scph5501.bin`. `--dest` still names the library's `bios`
+folder, whose parent is the library. Run from the host of a container, where
+`config.yml` sits elsewhere, `--romm-firmware` gives the same value. A
+library laid out as `{platform}/roms` with no template found is refused
+rather than filled in the wrong place, and the message gives the command.
 
 ## Android
 
@@ -479,8 +505,8 @@ user's own layout, and EmuDeck links `bios/shadps4/sys_modules` into shadPS4's
 data directory where the emulator actually reads it. The boundary is lexical,
 not a resolved-path comparison, which is what keeps that case working.
 Standalone copies are stricter, since they write outside the tree the user
-selected: a symlink already sitting at the destination is skipped rather than
-followed.
+selected: a symlink already sitting at the destination is replaced by the
+file rather than followed.
 
 ## Changing the installer
 
@@ -514,7 +540,9 @@ fields, the installer refuses to narrow rather than install everything.
 
 `install.ps1` passes the installer's code on unchanged: run as a file it exits
 with it, run through `irm \| iex` it sets `$LASTEXITCODE` (an `exit` there
-would close the PowerShell window). Its body runs in a script block of its own,
+would close the PowerShell window). It declares no parameter of its own, so
+every argument reaches `install.py` as typed, short forms such as `-v`
+included. Its body runs in a script block of its own,
 so `irm \| iex` leaves neither its variables nor its `$ErrorActionPreference`
 in the session.