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Our start state has a DataOutputStream which is 'direct' meaning attached to an actual Java JVM output stream. Because DFDL and Daffodil are very bit-oriented we implement bit-level I/O on top by accompanying this output-stream with a "frag byte", a single byte which contains from 0 to 8 7 bits which are the fragment of a byte that is not yet complete and ready to be written to the actual output stream. The frag byte never contains all 8 bits (except transiently) because when it has all 8 bits we write it to the byte stream and clear it.

We'll depict a data output stream like so. This shows a data output stream where 4 whole bytes + 2 additional bits have been written.

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