What’s the default endianness of the arith_uint256 sort?
tl;dr: The one chosen by the libraries and compiler you’re utilizing for the processor you focused? However that is not what you want to know.
Endianness is a higher supply of confusion in Bitcoin than in some other software program venture I’ve encountered.
CPU
Some traditionally standard processors, 68K, SPARC, PowerPC, have been big-endian – however these are now not vital.
At present, most private computer systems which can be bigger than pocket sized use processors which can be little-endian. Most of these processors have an instruction to effectively swap between byte orders. For instance bswap or rev. This form of provision usually makes endianness comparatively unimportant for effectivity issues.
Present ARM processor designs are mentioned to be bi-endian. They will work with both endianness with equal effectivity.
Web
Web requirements specify that knowledge transmitted over the Web ought to transmit numbers in big-endian order. Nevertheless Bitcoin doesn’t. This is also a supply of confusion.
Software program
In a lot of the software program improvement initiatives I’ve labored on, together with heterogeneous community functions, endianness isn’t a difficulty. Builders do not even want to pay attention to endianness. The compiler takes care of this for you. Commonplace software program libraries care for this for you.
Bitcoin core is an exception to this. There are in all probability many others however it’s the solely venture I recall encountering the place it is a massive situation that incessantly journeys individuals up and causes confusion so typically.
People
Most numbers introduced to people in hexadecimal notation are introduced as big-endian. This implies probably the most vital digits are on the left, in order that in a left-to-right writing system the offset or position-index is smallest for probably the most vital digit.
Conclusion
The order you see in a hexadecimal illustration may need been ordered to go well with human interpretation or it is likely to be ordered to match, for instance, community order. Context ought to make this obvious.
