Google is committed to advancing racial equity for Black communities. See how.

Bitfield diagram explanation

This document explains how to read the bitfield diagrams used in the logs and inspect 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}.