Bitfield diagram explanation
This document explains how to read the bitfield diagrams used in the [logs][log_format] and [inspect][inspect_format] formats.
The following example bitfield diagram is broken down into its component parts:
.---------------------------------------------------------------.
| |1|1|1|1|1|2|2|2|2|2|3|3|3|3|3|4|4|4|4|4|5|5|5|5|5|6|6|
|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|
|---+---+-------+-----------------------+-----------------------|
| O | R | Type | Parent index | Name index |
|---------------------------------------------------------------|
| Reference count (optional) |
'---------------------------------------------------------------'
O = Order
R = Reserved, must be 0
Type = {3,10}
Top row
In the top row, each column denotes the starting bit index, and each space corresponds to two consecutive bits.
For example, the first "0" in the left-most column denotes that the space is reserved for the bits at index 0 and 1. The next "2" at index 2 and 3, and so on. The first column with two numbers should be read as "10", the next "12", and so on.
.---------------------------------------------------------------.
| |1|1|1|1|1|2|2|2|2|2|3|3|3|3|3|4|4|4|4|4|5|5|5|5|5|6|6|
|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|4|6|8|0|2|
|---+-----------+-----------------------------------------------|
Bottom rows
The bottom rows of the diagram illustrate the physical layout of the fields. Each row consists of 64 bits.
|---+---+-------+-----------------------+-----------------------|
| O | R | Type | Parent index | Name index |
|---------------------------------------------------------------|
| Reference count (optional) |
'---------------------------------------------------------------'
In the example, we are representing 128 bits. The first 64 bits contain:
O: 4 bits at indexes[0, 4)(left inclusive, right exclusive, so in this case the indexes{0, 1, 2, 3}).R: 4 bits at indexes[4, 8)Type: 8 bits at indexes[8, 16)Parent index: 24 bits at indexes[16, 40)Name index: 24 bits at indexes[40, 64)
The second 64 bits contain a single value, the Reference count.
Notes
At the end of the diagram, there is a legend to make it clearer what some fields mean or what
restrictions they might have. In the example above, R means Reserved and O means Order but
there wasn’t enough space in the diagram to fit those words. Type will only contain values in the
set {3, 10}.