Hey all,

I ran into a situation recently where I was reading from an I2C device, and the byte ordering was in the opposite endianness that I was expecting. Although I fixed the problem by reversing the bytes before returning them, I went off to research exactly what endian was. I've spent numerous hours testing, reading and more testing, but I still can't grasp it fully. I'm hoping just one or two more examples will have it 'click'.

So, I'll start off with a couple of examples here to see if I have the basics down. Please feel free to elaborate with other examples or comments etc.

Set up our number, and two byte scalars (full version copy/pastable at the bottom of the post):

use warnings;
use strict;
use feature 'say';

my $num = 1023; # 0x03ff
my ($b1, $b2);

[download]

Now, if I do the following bit shifting, the printf() is printing the bytes in big endian format, correct?

$b1 = ($num & 0xff00) >> 8;
$b2 = $num & 0xff;

printf("%x, %x\n", $b1, $b2); # 3, ff

[download]

Likewise, if I reverse the operations/bytes, this one will print in little endian format, right?

$b1 = $num & 0xff;
$b2 = ($num & 0xff00) >> 8;

printf("%x, %x\n", $b1, $b2); # ff, 3

[download]

Full code:

use warnings;
use strict;
use feature 'say';

my $num = 1023;

my ($b1, $b2);

$b1 = ($num & 0xff00) >> 8;
$b2 = $num & 0xff;

printf("%x, %x\n", $b1, $b2);

$b1 = $num & 0xff;
$b2 = ($num & 0xff00) >> 8;

printf("%x, %x\n", $b1, $b2);

[download]

In reply to Understanding endianness of a number by stevieb

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