source: prologue/architecture.xml@ 8eb3fe4

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<sect1 id="pre-architecture">
  <?dbhtml filename="architecture.html"?>

  <title>LFS Target Architectures</title>

<para>The primary target architectures of LFS are the AMD/Intel x86 (32-bit)
and x86_64 (64-bit) CPUs.  On the other hand, the instructions in this book are
also known to work, with some modifications, with the Power PC and ARM CPUs. To
build a system that utilizes one of these CPUs, the main prerequisite, in
addition to those on the next few pages, is an existing Linux system such as an
earlier LFS installation, Ubuntu, Red Hat/Fedora, SuSE, or other distribution
that targets the architecture that you have. Also note that a 32-bit
distribution can be installed and used as a host system on a 64-bit AMD/Intel
computer.</para>

<para>Some other facts about 64-bit systems need to be added here. When
compared to a 32-bit system, the sizes of executable programs are slightly
larger and the execution speeds of arbitrary programs are only slightly faster.
For example, in a test build of LFS-6.5 on a Core2Duo CPU based system, the
following statistics were measured:</para>

<screen><computeroutput>Architecture Build Time     Build Size
32-bit       198.5 minutes  648 MB
64-bit       190.6 minutes  709 MB</computeroutput></screen>

<para>As you can see, the 64-bit build is only 4% faster and is 9% larger than
the 32-bit build.  The gain from going to a 64-bit system is relatively
minimal. Of course, if you have more than 4GB of RAM or want to manipulate
data that exceeds 4GB, the advantages of a 64-bit system are substantial.</para>

<note><para>The above discussion is only appropriate when comparing 
builds on the same hardware.  Modern 64-bit systems are considerably
faster than older 64-bit systems and the LFS authors recommend building
on a 64-bit system when given a choice.</para></note>

<para arch="default">The default 64-bit build that results from LFS is
considered a "pure" 64-bit system. That is, it supports 64-bit executables only.
Building a "multi-lib" system requires compiling many applications twice, once
for a 32-bit system and once for a 64-bit system. This is not directly supported
in LFS because it would interfere with the educational objective of providing
the instructions needed for a straightforward base Linux system.  You can refer
to the <ulink url="http://trac.clfs.org/">Cross Linux From Scratch</ulink>
project for this advanced topic.</para>

<!-- This does not appear to be valid for LFS/BLFS any more
<para>There is one last comment about 64-bit systems. There are some older
packages that cannot currently be built in a "pure" 64-bit system or require
specialized build instructions. Generally, these packages have some embedded
32-bit specific assembly language instructions that fail when building on a
64-bit system.  This includes some Xorg drivers for some legacy video cards at
<ulink url="http://xorg.freedesktop.org/releases/individual/driver/">
http://xorg.freedesktop.org/releases/individual/driver/</ulink>. Many of these
problems can be worked around, but may require some specialized procedures or
patches.</para>
-->

<para arch="multilib">You are reading the
<emphasis>LFS-&version;-multilib</emphasis> book. Building a multilib system
requires compiling many applications three times: once for the primary
64-bit architecture; once for the minor 32-bit architecture; and once again for
the 32-bit architecture with 64-bit memory extensions.</para>

<para arch="multilib">Multilib goes a bit beyond the basic educational approach
of LFS which is to teach you how to build a Linux system. If you haven't
previously built by the standard LFS book, you are encouraged to do so before
attempting this edition.</para>

<para arch="multilib">Continue only if you meet the following requirements:
<itemizedlist>
  <listitem><para>you have a x86_64 compatible machine</para></listitem>
  <listitem><para>you already have experience with LFS</para></listitem>
  <listitem><para>you have a need for 32-bit support</para></listitem>
</itemizedlist>
</para>

</sect1>