pureboot: the classic megas and the word-addressed 1284P groundwork

Device: boot-section detection probes SPMCR beside SPMCSR, the link
picks any hardware USART through the instance-aware lookups (URSEL
chips included), WDRF reads MCUSR-or-MCUCSR, and the >64 KiB shape
lands — word-addressed wire flash (info flag bit 1, page byte 0 means
256, base as a word address), far reads through flash_load_far, a
single 32-bit byte-cursor page walk (the 256-byte page wraps its low
byte exactly), and slot arithmetic in words (the return address
already is one). Host: addresses stay bytes internally and scale at
the wire, the boot-fuse decode becomes a per-signature table (byte
index + BOOTSZ ladder — the m168A's lives in EXTENDED), and the
planner tests pin every chip's ladder plus the word-addressed info
decode. Tests: the device runner serves every mega over the USART pty,
pbapp banners over the right link, the update rehearsal synthesizes
its assumed fuses from the tool's own table, and the PI lint tracks
the renamed info symbol. All six classic-mega/168A targets pass the
full suite (size, PI, planner, protocol, reloc, self-update) at
466–504 B; the 1284P builds await a libavr far-path slimming to make
its 512.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/pbtest.py

@@ -90,11 +90,17 @@ def main():
     read_flash = os.path.join(workdir, "readback_flash.bin")
     read_eeprom = os.path.join(workdir, "readback_eeprom.bin")
 
-    # The geometry the host will discover, for computing the expected image.
+    # The geometry the host will discover, for computing the expected image:
+    # megas carry a boot section (no vector surgery), the large ones speak
+    # word addresses, and the page byte is the wire's 0-means-256.
+    mega = mcu.startswith("atmega")
+    word_flash = base + 512 > 0x10000
+    wire_base = base // 2 if word_flash else base
+    flags = (0 if mega else 1) | (2 if word_flash else 0)
     info = pb.Info(
-        bytes([ord("P"), ord("B"), 1, 0, 0, 0, page])
-        + bytes([base & 0xFF, base >> 8, eeprom_size & 0xFF, eeprom_size >> 8])
-        + bytes([0 if mcu == "atmega328p" else 1])
+        bytes([ord("P"), ord("B"), 1, 0, 0, 0, page & 0xFF])
+        + bytes([wire_base & 0xFF, wire_base >> 8, eeprom_size & 0xFF, eeprom_size >> 8])
+        + bytes([flags])
     )
 
     device = Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump)
@@ -150,8 +156,9 @@ def main():
         fail("loader region looks erased in the ground-truth dump")
 
     # The surgery, decoded independently: the patched vector must land on the
-    # loader, the trampoline on the application's own entry.
-    if mcu != "atmega328p":
+    # loader, the trampoline on the application's own entry (tinies only —
+    # the megas' word 0 stays the application's).
+    if not mega:
         flash_words = (base + 512) // 2
         app = open(app_bin, "rb").read()
         word0 = flash_true[0] | (flash_true[1] << 8)