ID3 uses unsigned integers encoded in one, two, three, and four bytes. If you first write a general binary type that takes the number of bytes to read as an argument, you can then use the short form of define-binary-type to define the specific types. The general unsigned-integer type looks like this:

Another type you’ll need to be able to read and write is the 28-bit value used in the header. This size is encoded using 28 bits rather than a multiple of 8, such as 32 bits, because an ID3 tag can’t contain the byte #xff followed by a byte with the top 3 bits on because that pattern has a special meaning to MP3 decoders. None of the other fields in the ID3 header could possibly contain such a byte sequence, but if you encoded the tag size as a regular , it might. To avoid that possibility, the size is encoded using only the bottom seven bits of each byte, with the top bit always zero.3

The definition of id3-tag-size is then trivial.