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Establish the SBlk 40-byte record layout

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Range measured: level 0 SBlk, 24..19508, aligned at +0 -- 487 whole records.

Method: for each of 40 candidate alignments, count byte positions that are
near-constant across records, and keep the best. Alignment +0 wins with 10.

Four positions are exactly constant and all zero: 7, 11, 31 and 35.
Distinct values per position run from 1 to 155, with position 29 the most
variable by a wide margin -- 155 against 21 to 48 elsewhere -- and positions
2, 10, 27, 34 and 39 low at 2 to 8.

The first record carries the SBlk magic and the DAW tag rather than descriptor
data, so it is a header rather than an entry. That matters before reading any
field at a fixed offset.

Not decoded: the layout is established and the constant positions are known,
but the fields are not named and which of the 40 bytes are an index, a length
and an offset is still open.
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2026-10-11 04:06:51 +02:00
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docs/formats/core-data-regions.md
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@@ -617,3 +617,26 @@ The earlier "they disagree" framing was wrong in one respect: the marker
spacing never claimed to be the record size. Read as a claim about markers
inside records rather than about the record itself, the two measurements were
never in conflict.
## SBlk record layout at 40 bytes
**Range measured:** level 0 `SBlk`, 24..19508, aligned at +0 — 487 whole
records.
**Method:** for each of 40 candidate alignments, record the number of byte
positions that are near-constant across records, and keep the best.
**Result:** alignment +0 is best, with 10 near-constant positions. Four are
exactly constant and all zero: positions **7, 11, 31 and 35**.
Distinct values per byte position across the 487 records runs from 1 to 155,
with position 29 the most variable by a wide margin (155 against 21-48
elsewhere) and positions 2, 10, 27, 34 and 39 low (2 to 8).
The first record carries the `SBlk` magic and the `DAW` tag rather than
descriptor data, so it is a header rather than an entry — which is worth
accounting for before reading any field at a fixed offset.
**Not decoded.** The layout is established and the constant positions are
known; the fields are not named, and which of the 40 bytes are an index, a
length and an offset is still open.