MxCh

MxCh is the identifier for a Lego Island data chunk. They are seen extensively in SI Files to allow interleaving of several different data types.

MxCh chunks contain partial data of various different data types intended to be joined together continuously to form a complete file (an MxDa section). The chunks can also contain solely header data. A chunk will almost never contain a complete file on its own.

Specification
The MxCh header is 22 bytes long and specifies the length of the chunk with other data. MxCh chunks must start on an even offset or they will not be skipped by LEGO Island.

Flags?
The full purpose of the 2-byte "Flags" value above is not entirely known, however the following appear to be accurate:

Split Chunk
In addition to the data being split into chunks, the chunks themselves are sometimes split into two. If a chunk is split, both chunks will have the Flags? section set to 16, and they'll also both have the same Milliseconds? value. The "Chunk Size" will be accurate to each chunk's size, but the "Chunk Data Size" of the first chunk appears to be the total size of both chunks' data. The second chunk's "Chunk Data Size" is accurate to its own chunk data size.

The necessity of the split chunks appears to be due to the way Lego Island streams SI files; it appears to only be able to stream 20000h (131072 bytes) at a time and the SI file must conform to this limitation. A chunk can have no data that extends over a multiple of 20000h. Doing so causes Lego Island to crash as it tries to read beyond the 20000h buffer it's allocated for streaming. Therefore if a chunk is going to extend beyond a 20000h multiple, it must be split at that point and then another chunk must be written directly afterwards with the remainder of the data. This is a big obstacle to inserting audio that is larger than the existing audio, since literally all subsequent chunks of all subsequent songs must be shifted to fit around the 20000h multipliers or Lego Island will crash. This essentially necessitates a reconstruction of most, if not all, of the file.