diff --git a/README.md b/README.md
index 50b6848..2642c53 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,7 @@ Discs are matched by hash first (most precise), then serial, then filename.
   `openvgdb-covers.zip` offline cover bundles, and the legacy cover cache.
 - `data/base.PBP` — a PSN PS1 classic EBOOT (any game) used as the encrypted
   loader when converting PS1 discs to `EBOOT.PBP` (git-ignored, copyrighted).
+  Prefer a 6.60/6.61-era classic; 3.xx-era loaders can fail on modern CFW.
 - State dir (`~/.local/state/psp-preserve/` by default, or `$XDG_STATE_HOME/psp-preserve`):
   - `library.sqlite` — the catalog + library database.
   - `covers/` — downloaded cover cache.
@@ -188,16 +189,20 @@ PS1 discs (pending) and converted EBOOTs also show up in the **Library** tab
 - **Serial detection** — reads `SYSTEM.CNF` to derive the game ID (e.g.
   `SLUS-00001`) and writes it into `PARAM.SFO` + the PSAR header.
 - **base.pbp** — conversion needs a PSN PS1 classic `EBOOT.PBP` (any game) to
-  supply Sony's encrypted `DATA.PSP` loader. Copy one to `data/base.PBP` and
-  point **Settings → General → PS1 base.pbp** at it.
+  supply Sony's encrypted `DATA.PSP` loader (POPS emulator) and `STARTDAT`.
+  Copy one to `data/base.PBP` and point **Settings → General → PS1 base.pbp**
+  at it. **Prefer a 6.60/6.61-era classic**: very old 3.xx-era classics ship a
+  POPS loader that fails to link on modern CFW (`80020148` "game could not be
+  started"). Any PS1 classic works structurally — the converter finds
+  `STARTDAT` at `PSAR + p1` regardless of PSAR alignment.
 - **Output** — a folder `<out>/<Game ID>/` containing a single `EBOOT.PBP`
-  (the popstation way: uncompressed level-0 PSAR, no `KEYS.BIN`/`DOCUMENT.DAT`),
+  (the popstation way: raw-DEFLATE-compressed PSAR, no `KEYS.BIN`/`DOCUMENT.DAT`),
   ready to copy to `<PSP>/PSP/GAME/<Game ID>/`. Compatible with CFW POPS
-  (ARK-4).
+  (ARK-4/ARK-5).
 
 Configure the folders in **Settings → General** (or `PSP_PS1_ROOTS` /
 `PSP_PS1_OUTPUT_DIR`). Current limits: single-disc games only (no multi-disc
-PBP), and images are stored uncompressed (LZRC compression is on the roadmap).
+PBP); disc data is DEFLATE-compressed (level 1 by default, `--complevel 0-9`).
 
 ## Store — archive.org and local folder
 
diff --git a/app/ps1_convert.py b/app/ps1_convert.py
index e001cd2..fe05fb4 100644
--- a/app/ps1_convert.py
+++ b/app/ps1_convert.py
@@ -375,6 +375,46 @@ def extract_icon0(base_pbp: Path) -> bytes:
     return read_pbp_section(base_pbp, header, idx)
 
 
+def _sfo_entry(base_sfo: bytes, key: str) -> Optional[bytes]:
+    """Return the raw data of an SFO entry, or ``None`` if absent."""
+    if len(base_sfo) < 20 or base_sfo[:4] != b"\x00PSF":
+        return None
+    _, _, key_table_off, data_table_off, nitems = struct.unpack("<IIIII", base_sfo[:20])
+    for i in range(nitems):
+        entry_off = 20 + i * 16
+        koff, _, _, max_len, doff = struct.unpack("<HHIII", base_sfo[entry_off:entry_off + 16])
+        kend = base_sfo.find(b"\x00", key_table_off + koff)
+        k = bytes(base_sfo[key_table_off + koff:kend]).decode("ascii", "replace")
+        if k == key:
+            return base_sfo[data_table_off + doff: data_table_off + doff + max_len]
+    return None
+
+
+def _warn_old_base(base_pbp: Path) -> None:
+    """Warn when the base PBP carries a very old POPS loader.
+
+    PS1 classics from the 3.xx firmware era ship a POPS loader that often
+    fails to link on 6.60/6.61 CFW (the PSP reports error 80020148), so
+    newer base PBPs are strongly preferred.
+    """
+    try:
+        raw = _sfo_entry(extract_sfo(base_pbp), "PSP_SYSTEM_VER")
+        if not raw:
+            return
+        version = raw.rstrip(b"\x00").decode("ascii", "replace")
+        major = version.split(".")[0]
+        if major.isdigit() and int(major) < 6:
+            import sys
+            print(
+                f"warning: base PBP is a {version}-era PS1 classic; its POPS loader "
+                "may fail to load on 6.60/6.61 CFW (error 80020148). "
+                "Prefer a newer base.pbp.",
+                file=sys.stderr,
+            )
+    except Exception:
+        pass
+
+
 def patch_sfo(base_sfo: bytes, title: str, game_id: str) -> bytes:
     """Patch ``TITLE`` and ``DISC_ID`` in a base PARAM.SFO in place.
 
@@ -481,16 +521,18 @@ def extract_data_psp(base_pbp: Path) -> bytes:
 def extract_startdat(base_pbp: Path) -> bytes:
     """Extract the trailing STARTDAT block (boot.png + PGD) from a base PBP.
 
-    The STARTDAT offset is discovered the same way the reference writer does:
-    read the u32 at ``psar + 0x0C``, add ``0x50000``, and expect b"STARTDAT"
-    there. Everything from that marker to EOF is copied verbatim.
+    STARTDAT sits immediately after the disc data: at ``PSAR + p1`` where
+    ``p1`` is the u32 at ``PSAR + 0x0C``. (The original popstation computes
+    ``p1 + 0x50000`` as an absolute offset, which only works for bases whose
+    PSAR happens to be aligned to 0x50000; using ``PSAR + p1`` works for any
+    PS1 classic.)
     """
     header = read_pbp_header(base_pbp)
     psar_offset = header[SECTION_OFFSET["psar"] // 4]  # header array index
     with open(base_pbp, "rb") as fh:
         fh.seek(psar_offset + 0x0C)
         (x,) = struct.unpack("<I", fh.read(4))
-        marker_off = x + 0x50000
+        marker_off = psar_offset + x
         fh.seek(marker_off)
         if fh.read(8) != b"STARTDAT":
             raise ConversionError(f"cannot find STARTDAT in {base_pbp} (not a valid PS1 base PBP?)")
@@ -825,6 +867,7 @@ def convert(
     # Resources required from the base PBP.
     data_psp = extract_data_psp(base_pbp)
     startdat = extract_startdat(base_pbp)
+    _warn_old_base(base_pbp)
 
     # Reuse the base PBP's SFO, patched with our title + game id (the
     # popstation approach), rather than rebuilding it from scratch.