First public commit.

lenser/apple/dsk.py

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+"""Write a bootable Apple II .dsk (DOS 3.3 sector order, 143360 bytes).
+
+The .dsk stores sectors in DOS *logical* order, but the Disk II boot ROM reads
+*physical* sectors, so we place each chunk of the bitmap at the logical slot that
+maps to the physical sector our loader will read -- making the loader's
+physical-sequential reads come out contiguous in memory.
+"""
+
+from __future__ import annotations
+
+# DOS 3.3 physical-sector -> logical-sector (the .dsk read interleave).
+PHYS2LOG = [0, 7, 14, 6, 13, 5, 12, 4, 11, 3, 10, 2, 9, 1, 8, 15]
+
+SECTOR = 256
+SPT = 16
+TRACKS = 35
+DISK_SIZE = TRACKS * SPT * SECTOR        # 143360
+
+
+class DskError(RuntimeError):
+    pass
+
+
+def _offset(track: int, phys_sector: int) -> int:
+    return (track * SPT + PHYS2LOG[phys_sector]) * SECTOR
+
+
+def build_boot_dsk(boot: bytes, payload: bytes) -> bytes:
+    """boot = assembled boot0 (origin $0800); payload = data pages the loader reads
+    in order: track 0 sectors 1-15, then whole tracks 1, 2, ... (physical order)."""
+    if len(payload) % SECTOR:
+        raise DskError("payload must be a whole number of 256-byte sectors")
+    npages = len(payload) // SECTOR
+    disk = bytearray(DISK_SIZE)
+    disk[0:SECTOR] = (bytes(boot) + bytes(SECTOR))[:SECTOR]   # boot0 at T0 phys 0
+
+    assigns = [(0, p) for p in range(1, 16)]                  # track 0 (skip boot)
+    track = 1
+    while len(assigns) < npages:
+        assigns += [(track, p) for p in range(16)]
+        track += 1
+    if track > TRACKS:
+        raise DskError("payload exceeds disk capacity")
+    for page, (trk, phys) in enumerate(assigns[:npages]):
+        off = _offset(trk, phys)
+        disk[off:off + SECTOR] = payload[page * SECTOR:(page + 1) * SECTOR]
+    return bytes(disk)
+
+
+def write_dsk(path: str, data: bytes) -> str:
+    with open(path, "wb") as f:
+        f.write(data)
+    return path