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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 it’s worthwhile to know.
Endianness is a larger 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 important.
Presently, most private computer systems which are bigger than pocket sized use processors which are little-endian. Most of these processors have an instruction to effectively swap between byte orders. For instance bswap or rev. This kind of provision usually makes endianness comparatively unimportant for effectivity issues.
Present ARM processor designs are stated to be bi-endian. They will work with both endianness with equal effectivity.
Web
Web requirements specify that information 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 growth initiatives I’ve labored on, together with heterogeneous community purposes, endianness shouldn’t be a problem. Builders do not even want to pay attention to endianness. The compiler takes care of this for you. Commonplace software program libraries handle this for you.
Bitcoin core is an exception to this. There are most likely many others however it’s the solely venture I recall encountering the place this can be a huge situation that steadily 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 important 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 important digit.
Conclusion
The order you see in a hexadecimal illustration might need been ordered to go well with human interpretation or it is perhaps ordered to match, for instance, community order. Context ought to make this obvious.
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