The reason behind endianness?

Question

I was wondering, why some architectures use little-endian and others big-endian. I remember I read somewhere that it has to do with performance, however, I don't understand how can endianness influence it. Also I know that:

The little-endian system has the property that the same value can be read from memory at different lengths without using different addresses.

Which seems a nice feature, but, even so, many systems use big-endian, which probably means big-endian has some advantages too (if so, which?).

I'm sure there's more to it, most probably digging down to the hardware level. Would love to know the details.

Answer 1

I've looked around the net a bit for more information on this question and there is a quite a range of answers and reasonings to explain why big or little endian ordering may be preferable. I'll do my best to explain here what I found:

Little-endian

The obvious advantage to little-endianness is what you mentioned already in your question... the fact that a given number can be read as a number of a varying number of bits from the same memory address. As the Wikipedia article on the topic states:

Although this little-endian property is rarely used directly by high-level programmers, it is often employed by code optimizers as well as by assembly language programmers.

Because of this, mathematical functions involving multiple precisions are easier to write because the byte significance will always correspond to the memory address, whereas with big-endian numbers this is not the case. This seems to be the argument for little-endianness that is quoted over and over again... because of its prevalence I would have to assume that the benefits of this ordering are relatively significant.

Another interesting explanation that I found concerns addition and subtraction. When adding or subtracting multi-byte numbers, the least significant byte must be fetched first to see if there is a carryover to more significant bytes. Because the least-significant byte is read first in little-endian numbers, the system can parallelize and begin calculation on this byte while fetching the following byte(s).

Big-endian

Going back to the Wikipedia article, the stated advantage of big-endian numbers is that the size of the number can be more easily estimated because the most significant digit comes first. Related to this fact is that it is simple to tell whether a number is positive or negative by simply examining the bit at offset 0 in the lowest order byte.

What is also stated when discussing the benefits of big-endianness is that the binary digits are ordered as most people order base-10 digits. This is advantageous performance-wise when converting from binary to decimal.

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While all these arguments are interesting (at least I think so), their applicability to modern processors is another matter. In particular, the addition/subtraction argument was most valid on 8 bit systems...

For my money, little-endianness seems to make the most sense and is by far the most common when looking at all the devices which use it. I think that the reason why big-endianness is still used, is more for reasons of legacy than performance. Perhaps at one time the designers of a given architecture decided that big-endianness was preferable to little-endianness, and as the architecture evolved over the years the endianness stayed the same.

The parallel I draw here is with JPEG (which is big-endian). JPEG is big-endian format, despite the fact that virtually all the machines that consume it are little-endian. While one can ask what are the benefits to JPEG being big-endian, I would venture out and say that for all intents and purposes the performance arguments mentioned above don't make a shred of difference. The fact is that JPEG was designed that way, and so long as it remains in use, that way it shall stay.

Answer 2

I would assume that it once were the hardware designers of the first processors who decided which endianness would best integrate with their preferred/existing/planned micro-architecture for the chips they were developing from scratch.

Once established, and for compatibility reasons, the endianness was more or less carried on to later generations of hardware; which would support the 'legacy' argument for why still both kinds exist today.