Bits in ABNF

Introduction This specification adds to Terminal values by adding an optional bit width to the binary, decimal, and hexadecimal terminals:

The width is a positive integer expressed in decimal. This width is the number of bits in the terminal.
The width must be equal to or greater than one (1).
When the left side has a width then, the number of bits on the left of the equal sign must be the total of the widths on the right side of the equal sign.
When the right side of the equal sign has widths and the left side does not. Then the right side can be a rule with a variable length.
The left side only needs a width when it is significant and relevant to the protocol. A left side with no width may be present even when the right side is fixed with.
The addition of a %p padding indicator with a required bit-width.
An alteration of the "rulename" ABNF as shown in .

A caution to the implementors of code generated from ABNF that specifies bit widths for signed integer values. Many computer languages will convert a narrower bit value into a wider bit value and move the sign bit to the most significant position. So when preparing a signed bit value, be sure to clamp the value and adjust the sign to the correct bit position before packing the bits. This would apply to signed integer values and not unsigned integer values. The right most elements are placed into the least signification part of the value. The left most bits being more significant than the bits to the right of them as shown in examples and . The result are these existing definitions with the added optional width indicators.

Extended ABNF Terminal Values In example, "device-status" is 8-bits wide, and the three values to the right of the equal sign have a total width of 8-bits.

"device-status" light-on:1 (A) | status:3 (B) | | switch-position:4 (C) | | | | .-' '-. .---' '---. | | | | | v v v v v +---+---+---+---+---+---+---+---+ | A | B | B | B | C | C | C | C | +---+---+---+---+---+---+---+---+ ^ ^ | | '-------------. .-------------' | device-status:8 in "email-status" is defined to be 8-bits wide, and only three bits are used, so five zero bits have been defined to be at the most significant positions. The total to the right of the equal sign is 8-bits:

"email-status" pad:5 (0) | seen:1 (A) | | flagged:1 (B) .-----' '-----. | | deleted:1 (C) | | | | | v v v v v +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | 0 | A | B | C | +---+---+---+---+---+---+---+---+ ^ ^ | | '-------------. .-------------' | email-status:8 Figure shows an example where "c-string" on the left sides has no predefined width and the right side has a variable number of 8-bit wide values. And "collection" has no predefined with and the right side is set of values with various and variable widths.

Padding in the Middle The padding does not need to be at the left. it can be in the middle and could have many zero bits as shown in .

Padding in the Middle Most significant 16 bits of trajectory:64 ----------------------------------------- 16 most significant bits of %p:18 (0) | | .---------------------------' '---------------------------. | | v v +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ ^ ^ | | '-----------------------------. .-----------------------------' | 16-bits Next most significant 16 bits of trajectory:64 ---------------------------------------------- Two least Top two most significant significant bits of vector-x:10 bits of | (X) %p:18 rotation:10 (R) %p:2 | | | | | . | . . / \ .--------------' '----------------. / \ / \ | | | | | | | | v v v v v v v v +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | 0 | 0 | R | R | R | R | R | R | R | R | R | R | 0 | 0 | X | X | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ ^ ^ | | '-----------------------------. .-----------------------------' | 16-bits Second least significant 16 bits of trajectory:64 ------------------------------------------------- 8 least significant bits %p:2 6 most significant bits bits of vector-x:10 (X) | of vector-y:10 (Y) | . | .------------' '----------. / \ .-------' '--------. | | | | | | v v v v v v +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | X | X | X | X | X | X | X | x | 0 | 0 | Y | Y | Y | Y | Y | Y | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ ^ ^ | | '-----------------------------. .-----------------------------' | 16-bits Least significant 16 bits of trajectory:64 ------------------------------------------ 4 least significant bits of vector-y:10 (Y) | | %p:2 vector-z:10 (Z) | . | .---' '---. / \ .---------------' '---------------. | | | | | | v v v v v v +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Y | Y | Y | Y | 0 | 0 | Z | Z | Z | Z | Z | Z | Z | Z | Z | Z | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ ^ ^ | | '-----------------------------. .-----------------------------' | 16-bits