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0xFF padding vs. extension tags #104

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0xFF padding vs. extension tags #104

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e73bc81
Update README.md to correct padding data description
mestrode Mar 9, 2026
94662af
Change padding bytes from zeros to ones
mestrode Mar 9, 2026
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7 changes: 4 additions & 3 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ All fields, except for data, are 32 bit unsigned little endian integers.
| 20 | 4 | Sequential block number; starts at 0 |
| 24 | 4 | Total number of blocks in file |
| 28 | 4 | File size or board family ID or zero |
| 32 | 476 | Data, padded with zeros |
| 32 | 476 | Data, padded with `0xFF` |
| 508 | 4 | Final magic number, `0x0AB16F30` |

The following C struct can be used:
Expand Down Expand Up @@ -284,7 +284,8 @@ The first byte of tag contains its total size in bytes (including the size byte
and type designation).
The next three bytes designate the type of tag (if you want to define custom
tags, pick them at random).
The last tag has size of `0` and type of `0`.
The last tag has a size of `0xFF` and a type of `0xFFFFFF`, which does match padding data.
For backward compatibility, also (size `0x00` and type `0x000000`) should be considered as last tag marker.

Standard tag designations follow:
* `0x9fc7bc` - version of firmware file - UTF8 semver string
Expand All @@ -300,7 +301,7 @@ named `ACME Toaster mk3` (line breaks added for clarity):
```
09 bc c7 9f 30 2e 31 2e 32 00 00 00
14 9d 0d 65 41 43 4d 45 20 54 6f 61 73 74 65 72 20 6d 6b 33
00 00 00 00
FF FF FF FF
```

Extension tags can, but don't have to, be repeated in all blocks.
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4 changes: 2 additions & 2 deletions utils/uf2conv.py
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Expand Up @@ -73,7 +73,7 @@ def convert_from_uf2(buf):
assert False, "Non-word padding size at " + ptr
while padding > 0:
padding -= 4
outp.append(b"\x00\x00\x00\x00")
outp.append(b"\xFF\xFF\xFF\xFF")
if familyid == 0x0 or ((hd[2] & 0x2000) and familyid == hd[7]):
outp.append(block[32 : 32 + datalen])
curraddr = newaddr + datalen
Expand Down Expand Up @@ -121,7 +121,7 @@ def convert_to_uf2(file_content):
global familyid
datapadding = b""
while len(datapadding) < 512 - 256 - 32 - 4:
datapadding += b"\x00\x00\x00\x00"
datapadding += b"\xFF\xFF\xFF\xFF"
numblocks = (len(file_content) + 255) // 256
outp = []
for blockno in range(numblocks):
Expand Down