pico.write must happen at 256 byte boundaries

there's no good way to tell if the payload should be padded or if it
should just be realigned, so raise an exception and make the caller
figure it out

gp2040ce_bintools/pico.py

@@ -32,6 +32,15 @@ PICO_COMMANDS = {
 }
 
 
+#################
+# LIBRARY ITEMS #
+#################
+
+
+class PicoAlignmentError(ValueError):
+    """Exception raised when the address provided for an operation is invalid."""
+
+
 def get_bootsel_endpoints() -> tuple[usb.core.Endpoint, usb.core.Endpoint]:
     """Retrieve the USB endpoint for purposes of interacting with a Pico in BOOTSEL mode.
 
@@ -189,6 +198,9 @@ def write(out_end: usb.core.Endpoint, in_end: usb.core.Endpoint, location: int,
         location: memory address of where to start reading from
         content: the data to write
     """
+    if (location % 256) != 0:
+        raise PicoAlignmentError("writes must start at 256 byte boundaries, please pad or align as appropriate!")
+
     # set up the data
     command_size = 8
 

tests/test_pico.py

@@ -5,6 +5,7 @@ import sys
 import unittest.mock as mock
 from array import array
 
+import pytest
 from decorator import decorator
 
 import gp2040ce_bintools.pico as pico
@@ -144,3 +145,36 @@ def test_write():
     ]
     end_out.write.assert_has_calls(expected_writes)
     assert end_in.read.call_count == 6
+
+
+def test_misaligned_write():
+    """Test that we can't write to a board at invalid memory addresses."""
+    end_out, end_in = mock.MagicMock(), mock.MagicMock()
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE001, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE008, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE010, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE020, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE040, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE080, b'\x00\x01\x02\x03')
+    with pytest.raises(pico.PicoAlignmentError):
+        _ = pico.write(end_out, end_in, 0x101FE0FF, b'\x00\x01\x02\x03')
+
+    _ = pico.write(end_out, end_in, 0x101FE100, b'\x00\x01\x02\x03')
+
+    expected_writes = [
+        mock.call(struct.pack('<LLBBxxLL12x', 0x431fd10b, 1, 0x1, 1, 0, 1)),
+        mock.call(struct.pack('<LLBBxxL16x', 0x431fd10b, 1, 0x6, 0, 0)),
+        mock.call(struct.pack('<LLBBxxLLL8x', 0x431fd10b, 1, 0x3, 8, 0, 0x101FE100, 4)),
+        mock.call(struct.pack('<LLBBxxL16x', 0x431fd10b, 1, 0x6, 0, 0)),
+        mock.call(struct.pack('<LLBBxxLLL8x', 0x431fd10b, 1, 0x5, 8, 4, 0x101FE100, 4)),
+        mock.call(b'\x00\x01\x02\x03'),
+        mock.call(struct.pack('<LLBBxxLL12x', 0x431fd10b, 1, 0x1, 1, 0, 0)),
+    ]
+    end_out.write.assert_has_calls(expected_writes)
+    assert end_in.read.call_count == 6