First public commit.

lenser/crt.py

@@ -0,0 +1,68 @@
+"""Read/write C64 .crt cartridge images (CCS64/VICE format) for a 16K ROM at $8000.
+
+VICE and MAME disagree on how a 16K Generic cart must be described:
+  * VICE attaches it only as ONE 16K CHIP packet at $8000 (two 8K packets are
+    rejected with "Failed to attach image").
+  * MAME maps each CHIP at its own load address and treats a single 16K CHIP as
+    8K -- it needs TWO 8K packets (ROML $8000 + ROMH $A000) to map $A000-$BFFF.
+So `split=False` (default) targets VICE; the MAME preview path uses `split=True`.
+"""
+
+from __future__ import annotations
+
+import struct
+
+CART_SIZE = 0x4000
+
+
+def _chip(load_addr: int, payload: bytes) -> bytes:
+    c = bytearray(16)
+    c[0:4] = b"CHIP"
+    struct.pack_into(">I", c, 4, 16 + len(payload))    # packet length
+    struct.pack_into(">H", c, 8, 0x0000)               # chip type: ROM
+    struct.pack_into(">H", c, 10, 0x0000)              # bank
+    struct.pack_into(">H", c, 12, load_addr)
+    struct.pack_into(">H", c, 14, len(payload))
+    return bytes(c) + payload
+
+
+def write_crt(rom: bytes, path: str, name: str = "8bitlenser",
+              split: bool = False) -> str:
+    """Wrap a 16K cart `rom` ($8000-$BFFF) in a .crt (16K config: EXROM & GAME
+    both 0).  ``split`` False = one 16K CHIP (VICE); True = two 8K CHIPs (MAME)."""
+    if len(rom) != CART_SIZE:
+        raise ValueError(f"expected a 16K cart ROM, got {len(rom)} bytes")
+
+    header = bytearray(64)
+    header[0:16] = b"C64 CARTRIDGE   "
+    struct.pack_into(">I", header, 16, 64)        # header length
+    struct.pack_into(">H", header, 20, 0x0100)    # version 1.0
+    struct.pack_into(">H", header, 22, 0x0000)    # hardware type: normal/generic
+    header[24] = 0                                # EXROM line (asserted)
+    header[25] = 0                                # GAME line (asserted) -> 16K
+    nm = name.encode("ascii", "replace")[:32]
+    header[32:32 + len(nm)] = nm
+
+    with open(path, "wb") as f:
+        f.write(header)
+        if split:
+            f.write(_chip(0x8000, rom[:0x2000]))      # ROML
+            f.write(_chip(0xA000, rom[0x2000:]))      # ROMH
+        else:
+            f.write(_chip(0x8000, rom))               # one 16K CHIP
+    return path
+
+
+def read_rom(path: str) -> bytes:
+    """Extract the raw 16K cart ROM from a .crt (handles one- or two-CHIP)."""
+    with open(path, "rb") as f:
+        data = f.read()
+    hdr_len = struct.unpack_from(">I", data, 16)[0]
+    rom = bytearray()
+    pos = hdr_len
+    while pos + 16 <= len(data) and data[pos:pos + 4] == b"CHIP":
+        plen = struct.unpack_from(">I", data, pos + 4)[0]
+        size = struct.unpack_from(">H", data, pos + 14)[0]
+        rom += data[pos + 16:pos + 16 + size]
+        pos += plen
+    return bytes(rom)