Working Python version for Commodore.

c64view/viewer/assemble.py

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+"""Assemble the 6502 viewer stubs with `xa` and build self-contained viewer PRGs.
+
+Each viewer is a small ML stub originating at $0801 (behind a BASIC SYS 2061
+autostart).  The picture data is appended after the stub, zero-padded so the
+bitmap lands exactly at $2000, screen RAM at $3F40, etc.  The whole thing loads
+in a single pass -- no second disk access -- so it works identically on real
+hardware and in any emulator regardless of device configuration.
+
+`xa -o` emits raw bytes starting at the origin without the 2-byte CBM load
+address, which we prepend here.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+import tempfile
+
+VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))
+LOAD_ADDR = 0x0801
+DATA_ADDR = 0x2000               # where appended picture data must land
+
+# mode/viewer key -> source filename
+SOURCES = {
+    "hires": "hires.s",
+    "multicolor": "multicolor.s",
+    "fli": "fli.s",
+    "fli_ntsc": "fli_ntsc.s",
+    "interlace": "interlace.s",
+}
+
+_cache: dict[str, bytes] = {}
+
+
+class AssemblerError(RuntimeError):
+    pass
+
+
+def have_xa() -> bool:
+    return shutil.which("xa") is not None
+
+
+def assemble_stub(viewer_key: str) -> bytes:
+    """Assemble a viewer stub to raw bytes (origin $0801, no load-address prefix)."""
+    if viewer_key in _cache:
+        return _cache[viewer_key]
+    if not have_xa():
+        raise AssemblerError(
+            "The 'xa' (xa65) assembler was not found on PATH.\n"
+            "Install it with:  sudo apt install xa65   (Debian/Ubuntu)\n"
+            "or build from https://www.floodgap.com/retrotech/xa/")
+
+    src = os.path.join(VIEWER_DIR, SOURCES[viewer_key])
+    if not os.path.exists(src):
+        raise AssemblerError(f"viewer source missing: {src}")
+
+    with tempfile.TemporaryDirectory() as td:
+        out = os.path.join(td, "viewer.bin")
+        proc = subprocess.run(["xa", "-o", out, src], capture_output=True, text=True)
+        if proc.returncode != 0:
+            raise AssemblerError(f"xa failed for {src}:\n{proc.stdout}{proc.stderr}")
+        with open(out, "rb") as f:
+            raw = f.read()
+    _cache[viewer_key] = raw
+    return raw
+
+
+def build_viewer_prg(viewer_key: str, data: bytes, data_addr: int = DATA_ADDR) -> bytes:
+    """Combine the assembled stub + padding + picture ``data`` into one PRG.
+
+    ``data`` is the block that must reside from ``data_addr`` upward (bitmap,
+    screen, colour RAM, background, ...).
+    """
+    stub = assemble_stub(viewer_key)
+    pad_len = (data_addr - LOAD_ADDR) - len(stub)
+    if pad_len < 0:
+        raise AssemblerError(
+            f"viewer stub {viewer_key} is {len(stub)} bytes, exceeds the "
+            f"{data_addr - LOAD_ADDR} bytes available before ${data_addr:04x}")
+    payload = stub + bytes(pad_len) + bytes(data)
+    return bytes([LOAD_ADDR & 0xFF, (LOAD_ADDR >> 8) & 0xFF]) + payload