[BIP Draft] 24 bits for nVersion nonce space instead of 16

[Matt Corallo]

<pre>
BIP: ?
Title: 24 nVersion bits for general purpose use
Author: Matt Corallo <bi...@bluematt.me>
Status: Draft
Type: Specification
Created: 2026-02-26
License: BSD-3-Clause
CC0-1.0
Replaces: 320
</pre>

==Abstract==

24 bits are reserved in the nVersion field as extra nonce space for miners, providing for additional
hashrate with header-only mining without relying on rolling nTime more often than once per second.

==Motivation==

BIP 320 defined 16 bits of nVersion as nonce space for additional nonce space. It turns out that
this isn't enough, as some devices have started using 7 bits from nTime for extra nonce space (see
https://github.com/stratum-mining/sv2-spec/discussions/187). Given there's limited utility in 16
bits of nVersion space for signaling, instead here we offer 24 bits of nVersion space as extra nonce
space.

==Specification==

24 bits from the block header nVersion field, starting from 5 and ending at 28 inclusive
(0x1fffffe0), are reserved for nonce use and removed from BIP8 and BIP9 specifications. A mask of
0xe000001f should be applied to nVersion bits so bits 5-28 inclusive will be ignored for soft-fork
signalling and unknown soft-fork warnings.

==Rationale==

Headers-only mining avoids mining devices (either ASICs or the firmware) from having to concern
itself with the vast space of consensus logic (handling transactions, merkle trees, etc). It is
widely deployed in ASICs, but requires a substantial number of jobs fed across an entire device,
keeping the ASIC controller busy. Providing additional nonce space for the ASICs to roll without
needing fresh work from the controller may simplify ASIC design somewhat, and as been apparently
adopted in some miners by using extra space in nTime as extra nonce space. Doing so in nVersion
instead is preferable to using nTime.

==Backwards Compatibility==

Non-upgraded nodes will interpret the reserved bits of this proposal as signals for soft forks, and
may additionally activate the warning system for unknown soft forks.

At the time of writing no known soft forks are pending using any of 24 bits reserved in this BIP,
and given that a non-trivial percentage of the hashrate is already making uses of those bits,
future soft forks SHOULD NOT utilize those bits for activation signalling.

==Acknowledgements==

Timo Hanke and Sergio Lerner for originally proposing 15-bit extra nNonce2. Btc Drak for writing BIP
320.

==Changelog==

==Copyright==

This document is dual licensed as BSD 3-clause, and Creative Commons CC0 1.0 Universal.

[Antoine Poinsot]

Makes sense.

The current 16 bits in the header allow to serve fresh job to an ASIC only once every second up to ~280TH/s. There are nowadays ASICs that are able to perform more than 1PH/s[^0], and 8 more bits would extend that limit up to 72PH/s without nTime rolling.

This leaves 5 bits available for concurrent deployments. With upgrades becoming less frequent as Bitcoin matures, i think this is more than enough.


[^0]: https://shop.bitmain.com/product/detail?pid=00020250707194812615L4HTKEMY064E

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