The loader note said every record gains a 02 00 prefix in RAM. It does not:
the word is the record's own leading field, already on the disc at every
record. What the loader changes is the pointer -- it addresses the word, two
bytes earlier than the disc's table -- and that pointer shift is what made the
bytes look inserted.
Measured, in order:
- the two bytes before a payload are identical in core_data and in RAM, record
for record: 02 00 on level 0's 90-record chunk at 0x100b302, 08 00 on the
level's first chunk. Nothing is inserted;
- the RAM table's low 24 bits are the payload's minus 2, consistently, so the
record is [leading u16][two index bytes][payload] and the loaded table
addresses the leading word. That is also why the index bytes read as +2 from
the pointer -- the same +2 the earlier RAM result quotes;
- the word is a field, not a pad: uniform across a chunk's records on 1,439 of
1,509 chunks (None on the 70 that vary, reported rather than averaged),
fixed within all 71 chunk types, and only ever 1, 2, 4, 6, 8 or 10 -- 554
chunks carry 2, 520 carry 4.
Two readings of the value were measured and disproved, so they are recorded as
disproved rather than left as candidates: it is not an entry count (the payload
divided by it leaves 57 distinct fractional "sizes", and 162 chunks do not
divide at all), and it is not the number of distinct leading bytes (agrees on 0
of 1,506 chunks).
Chunk gains `prefix: Option<u16>`, and the disc test asserts the uniformity,
the per-type fixity and the value set. The scratch example that produced the
per-type class-ID table is dropped now that the same scan is a committed test,
which also lands here: moby class IDs sit in one body slot per chunk type, with
the slot varying by type and the distinct >= 4 filter load-bearing -- without it
the top slots are constants that read as 305x because any value below the moby
count counts as a hit.
ReaperOfVeriod
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