2019-08-05 21:35:27 UTC
interminable, and has generated more heat than light.
Historically, since computers were developed in the Western world, storage of
chracters in computer words was from "left" to "right", that is, from the most
significant part to the least significant part, basically without thought - it
was simply assumed to be the natural way to do it, as it corresponded to how
numbers and words were written by us on paper.
When advances in technology made it useful to build small-sized computers, with
a word length less than that of numbers it might be desired to calculate with on
those computers, some of those computers (one example is the Honeywell 316 and
related machines) would put the least significant part of a 32-bit number in the
first 16-bit word making it up.
This was done for the practical reason that adding two numbers together could
then proceed simply: go to the addresses where they're stored, get the least
significant parts first, add them, then use the carry from that for adding the
two most significant parts together.
The PDP-11 computer was a 16-bit computer made by Digital Equipment Corporation.
This company made aggressively priced minicomputers, but the PDP-11 was intended
to be more than just a run-of-the-mill minicomputer; instead of using an old-
fashioned ISA like that of DEC's own PDP-8 and PDP-4/7/9/15, or 16-bit machines
like the Honeywell 316 or the Hewlett-Packard 2115A, it was to be modern and
advanced, and in some ways similar to mainframes like the IBM System/360.
So, because it was to have a low price, it handled 32-bit numbers with their
least significant 16 bits first.
To make it less like an ordinary minicomputer, and more like a mainframe, the
byte ordering within a 32-bit integer was going to be consistent - like on the
System/360. It couldn't be ABCD, but instead of being CDAB, it could be DCBA.
Just put the first character in a 16-bit word in the least significant part, and
give that part the lower byte address!
This is how the idea of little-endian byte order was born.
The general belief today among computer scientists, it seems to me, is that the
industry should standardize on little-endian. Yes, big-endian seems more
familiar to people out of habit, but byte-order is just a convention.
Personally, I like big-endian myself. But I can see that it's an advantage that,
given that we address objects in memory by their lowest-addressed byte, the
place value for the first part is independent of length and thus always
But having the world of binary numbers used for arithmetic follow one rule, and
numbers as strings of ASCII characters that are read in and printed follow the
opposite rule... only works if those to worlds really are separate.
When the IBM System/360 came out, of course the PDP-11, which it partly helped
to inspire, didn't exist yet. So little-endian didn't exist as an alternative to
But despite that, if there had been a choice, big-endian would have been the
right choice for System/360.
The IBM System 360 mainframe, just like the microcomputer on your desktop,
handled character strings in its memory, even if they were in EBCDIC instead of
ASCII, and it also handled binary two's complement integers. (That was a bit of
a departure for IBM, as the 7090 used sign-magnitude.)
But it also had something else that Intel and AMD x86-architecture processors
only give very limited support to. Packed decimal numbers.
A packed decimal number is a way to represent numbers inside a computer as a
sequence of decimal digits, each one represented by four binary bits with the
binary-coded-decimal representation of that number.
The System/360 had pack and unpack instructions, to convert between numbers in
character string form and numbers in packed decimal form. Obviously, having both
kinds of numbers in the same order, with the first digit in the lowest address,
made those instructions easier to implement.
But the System/360 also *did arithmetic* with packed decimal quantities. And
they often used the same ALU, with a decimal adjust feature using nibble carries
turned on, to perform that arithmetic. So having the most significant part on
the same side of a number for binary numbers and packed decimal numbers made
Packed decimal numbers as an arithmetic data type, therefore, form a bridge
between numbers in character representation and numbers in binary
representation. As there are benefits from their byte ordering matching the byte
ordering of _both_ of those forms of numbers, their presence in a computer
architecture makes big-endian ordering advantageous for that architecture.