First public commit.

lenser/pet/viewer/assemble.py

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+"""Assemble the PET viewer with `xa` and build the loadable .prg.
+
+The PRG loads at the PET BASIC start ($0401): a BASIC stub ``10 SYS1056`` then
+the 6502 viewer (at $0420 = 1056) then the screen data (at $0500).  Running it
+(or SYS 1056) copies the screen codes to $8000.
+"""
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+import tempfile
+
+VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))
+
+BASIC_START = 0x0401
+ML_ORG = 0x0420                # 1056
+DATA_ORG = 0x0500
+
+# BASIC: 10 SYS1056  (bytes from $0401)
+_STUB = bytes([0x0B, 0x04, 0x0A, 0x00, 0x9E,
+               0x31, 0x30, 0x35, 0x36, 0x00, 0x00, 0x00])
+
+
+class AssemblerError(RuntimeError):
+    pass
+
+
+def have_xa() -> bool:
+    return shutil.which("xa") is not None
+
+
+def _assemble(pages: int) -> bytes:
+    if not have_xa():
+        raise AssemblerError("The 'xa' (xa65) assembler was not found (apt install xa65).")
+    wrapper = (f"#define DATA ${DATA_ORG:04X}\n"
+               f"#define PAGES {pages}\n"
+               '#include "viewer.s"\n')
+    with tempfile.TemporaryDirectory() as td:
+        out = os.path.join(td, "v.bin")
+        fd, wrap = tempfile.mkstemp(suffix=".s", prefix="_wrap_", dir=VIEWER_DIR)
+        try:
+            with os.fdopen(fd, "w") as f:
+                f.write(wrapper)
+            proc = subprocess.run(["xa", "-o", out, os.path.basename(wrap)],
+                                  capture_output=True, text=True, cwd=VIEWER_DIR)
+            if proc.returncode != 0:
+                raise AssemblerError(f"xa failed:\n{proc.stdout}{proc.stderr}")
+            with open(out, "rb") as f:
+                return f.read()
+        finally:
+            os.unlink(wrap)
+
+
+def build_prg(screen: bytes) -> bytes:
+    """screen = screen-RAM bytes (1000 for 40-col, 2000 for 80-col)."""
+    pages = (len(screen) + 255) // 256
+    data = bytes(screen) + bytes(pages * 256 - len(screen))     # pad to whole pages
+    code = _assemble(pages)
+    mem = bytearray()
+    mem += _STUB
+    mem += b"\x00" * (ML_ORG - BASIC_START - len(mem))
+    mem += code
+    if len(mem) > DATA_ORG - BASIC_START:
+        raise AssemblerError("viewer code overruns the data area")
+    mem += b"\x00" * (DATA_ORG - BASIC_START - len(mem))
+    mem += data
+    return bytes([BASIC_START & 0xFF, BASIC_START >> 8]) + bytes(mem)

lenser/pet/viewer/viewer.s

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+; Commodore PET screen viewer (6502).  Copies PAGES*256 bytes of precomputed
+; screen codes from DATA into the PET screen RAM at $8000, then loops.  The PET
+; video hardware continuously displays $8000 as text, so the picture stays up.
+;
+; #defines from viewer/assemble.py --  DATA = source address, PAGES = 256-byte
+; pages to copy (4 for 40-col / 1000 bytes, 8 for 80-col / 2000 bytes).
+
+        * = $0420
+
+src = $fb
+dst = $fd
+
+start:
+        lda #<DATA
+        sta src
+        lda #>DATA
+        sta src+1
+        lda #$00
+        sta dst
+        lda #$80
+        sta dst+1               ; dest = $8000 (screen RAM)
+        ldx #PAGES
+        ldy #$00
+cp:
+        lda (src),y
+        sta (dst),y
+        iny
+        bne cp
+        inc src+1
+        inc dst+1
+        dex
+        bne cp
+hang:
+        jmp hang