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author Rob Landley <rob@landley.net>
date Tue, 08 Dec 2009 20:22:53 -0600
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<title>Target architectures</title>
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<hr /><h1><a name="arm" \><center>ARM</center></h1>

<p>The ARM processor is popular in the embedded space because it has the best
power consumption to performance ratio, meaning it has the longest battery
life and smallest amount of heat generated for a given computing task.</p>

<h2>Processor vs archtecture</h2>

<p>Although ARM hardware has many different processor designs with varying
clock speeds, cache sizes, and integrated peripherals, from a software
perspective what matters is ARM
<a href=http://www.arm.com/products/CPUs/architecture.html>architectures</a>, which are the different
instruction sets a compiler can produce.  The architecture names have a "v"
in them and the processor designs don't, so "ARM922T" is a hardware processor
design which implements the "ARMv4T" software instruction set.</p>

<p>The basic architectures are numbered: ARMv3, ARMv4, ARMv5, ARMv6, and
ARMv7.  An ARMv5 capable processor can run ARMv4 binaries, ARMv6 can run ARMv5,
and so on.  Each new architecture is a superset of the old ones, and the main
reason to compile for newer platforms is efficiency: faster speed and better
battery life.  (I.E. they work about like i386, i486, i586, and i686 do in
the x86 world.  Recompiling ARMv4 code to ARMv5 code provides about a 25%
speedup on the same hardware.)</p>

<p>The oldest architecture this compatability goes back to is ARMv3 (which
introduced 32-bit addressing), but that hardware is now obsolete.  (Not just no
longer being sold, but mostly cycled out of the installed base.)  The oldest
architecture still in use is "ARMv4", which should run on any ARM hardware
still in use today (except ARMv7M, which is ARM in name only).</p>

<h2>Architecture extensions</h2>

<p>ARM architectures can have several instruction set extensions, indicated
by letters after the ARMv# part.  Some (such as the letter "E" denoting the
"Jazelle" bytecode interpreter, which provides hardware acceleration for
running Java bytecode) can safely be ignored if you're not using them, and
others are essentially always there in certain architectures (such as the DSP
extension signified by the letter "E" which always seems to be present in
ARMv5).  But some are worth mentioning:</p>

<p>The "Thumb" extension (ARMv4T) adds
a smaller instruction set capable of fitting more code in a given amount
of memory.  Unfortunately thumb instructions sometimes run more slowly, and
the instruction set isn't complete enough to implement a kernel, so they
supplement rather than replace the conventional ARM instruction set.  Note
that all ARMv5 and later processors include Thumb support by default, only
ARMv4T offers it as an extension.  Also, a new "Thumb2" fixes most of the
deficiencies of the original Thumb, and is part of the ARMv7 architecture.</p>

<p>The VFP (Vector Floating Point) coprocessor provides hardware floating
point acceleration.  There are some older hardware floating point options,
and some newer ones backwards compatible with VFP, but in general you can
treat a chip as either "software floating point" or "VFP".</p>

<p>The other detail is "l" vs "b" for little-endian and big-endian.
In theory ARM can do both (this is a compiler distinction, not a hardware
distinction), but in practice little-endian is almost universally
used on ARM, and most boards are wired up to support little-endian only even if
the processor itself can theoretically handle both.</p>

<p>So for example, "armv4tl" is ARMv4 with Thumb extensions, little endian.</p>

<h2>Application Binary Interface</h2>

<p>Linux initially implemented a somewhat ad-hoc ABI for ARM with
<a href=http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/Why-ARMs-EABI-matters/>poor
performance and several limitations</a>, and when ARM got around to documenting
a new one the main downside was that it was incompatible with the old
binaries.</p>

<p>So ARM has two ABIs that can run on the same hardware, the old one is
called OABI and the new one is
<a href=http://wiki.debian.org/ArmEabiPort>EABI</a>.  (This is a bit like
the way BSD binaries won't run under Linux even though the hardware's the
same, or the long ago switch from
<a href=http://www.linuxjournal.com/article/1059>aout to ELF</a> executable
formats.</p>

<p>The oldest hardware that can run EABI is ARMv4T, so ARMv4 hardware
without the Thumb extensions still has to use OABI.  (The kernel, C
library, and compiler must all agree which ABI is in use or the binaries
won't run.)</p>

<h2>Further Reading</h2>

<p>In theory the best reference for ARM processors is the
<a href=http://www.arm.com>website of ARM Limited</a>, but unfortunately
they make it somewhat hard to find information about their
<a href=http://www.arm.com/products/CPUs/ARM920T.html>older products</a>
and <a href=http://www.arm.com/products/CPUs/families/ARM7Family.html>many
of their pages</a> are more concerned with advertising their newest products
than giving the requested information.  Wikipedia may be less frustrating,
and occasionally even accurate.</p>

<hr /><h1><a name="m68k" \><center>Motorola 68000</center></h2>
<hr /><h1><a name="mips" \><center>Mips</center></h1>
<hr /><h1><a name="ppc" \><center>PowerPC</center></h1>
<hr /><h1><a name="sh4" \><center>Super Hitachi</center></h1>
<hr /><h1><a name="sparc" \><center>Sparc</center></h1>
<hr /><h1><a name="x86" \><center>x86</center></h1>

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