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DATE 2016-12-01

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MESSAGE
DATE 2016-12-06
FROM Christopher League
SUBJECT Re: [Learn] png data format
From learn-bounces-at-nylxs.com Tue Dec 6 17:14:21 2016
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Tue, 6 Dec 2016 17:14:15 -0500 (EST)
From: Christopher League
To: Ruben Safir , Hangout ,
learn-at-nylxs.com
In-Reply-To:
References: <66105244-4afa-4330-b0c2-0661bde965fd-at-mrbrklyn.com>
<87bmwpf857.fsf-at-contrapunctus.net> <878trtf7qw.fsf-at-contrapunctus.net>

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Date: Tue, 06 Dec 2016 17:14:12 -0500
Message-ID: <87h96gemdn.fsf-at-contrapunctus.net>
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Subject: Re: [Learn] png data format
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Ruben Safir writes:

> There is an aspect of this I just don't understand. Why does the order
> remain intact without ntohl when you look byte by byte through the
> allocated memory, but the order is reversed when you evaluate it as a
> continuous 32 bit unsigned int variable

Memory is essentially just an array of *bytes*, because memory is
byte-addressable. The addressable unit is never smaller than a byte --
you can't have a pointer just to a bit or three.

And if you THINK have a pointer to something bigger than a byte, you
REALLY DON'T. You STILL just have a pointer to a byte, but when you read
from it we're going to also read the next several bytes.

So when you do that, the way that larger values (such as 32-bit
integers) are stored as bytes MATTERS. And different machines do it
differently. (BTW, historically there isn't just big- and little-endian,
some machines used other alternatives that are often called
"middle-endian"! Thank your $deity (if any) that those architectures are
now extinct.)

I think this page helps:


CL

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1.0, user-scalable=3Dyes">




Ruben Safir ruben-at-mrbrklyn.com=
writes:



There is an aspect of this I just don=E2=80=99t understand. Why does the=
order remain intact without ntohl when you look byte by byte through the a=
llocated memory, but the order is reversed when you evaluate it as a contin=
uous 32 bit unsigned int variable



Memory is essentially just an array of bytes, because memory is=
byte-addressable. The addressable unit is never smaller than a byte =E2=80=
=93 you can=E2=80=99t have a pointer just to a bit or three.


And if you THINK have a pointer to something bigger than a byte, you REA=
LLY DON=E2=80=99T. You STILL just have a pointer to a byte, but when you re=
ad from it we=E2=80=99re going to also read the next several bytes.


So when you do that, the way that larger values (such as 32-bit integers=
) are stored as bytes MATTERS. And different machines do it differently. (B=
TW, historically there isn=E2=80=99t just big- and little-endian, some mach=
ines used other alternatives that are often called =E2=80=9Cmiddle-endian=
=E2=80=9D! Thank your $deity (if any) that those architectures are now exti=
nct.)


I think this page helps: /cmsc311/Notes/Data/endian.html" class=3D"uri">https://www.cs.umd.edu/class=
/sum2003/cmsc311/Notes/Data/endian.html


CL





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