Programming-Guides/Intrinsics_Reference/ch_intro.xml

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<chapter version="5.0" xml:lang="en" xmlns="http://docbook.org/ns/docbook" xmlns:xi="http://www.w3.org/2001/XInclude"
xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="section_intro">
  
  <!-- Chapter Title goes here. -->
  <title>Introduction to Vector Programming on POWER</title>

  <section>
    <title>A Brief History</title>
    <para>
      The history of vector programming on POWER processors begins
      with the AIM (Apple, IBM, Motorola) alliance in the 1990s.  The
      AIM partners developed the Power Vector Media Extension (VMX) to
      accelerate multimedia applications, particularly image
      processing.  VMX is the name still used by IBM for this
      instruction set.  Freescale (formerly Motorola) used the
      trademark "AltiVec," while Apple at one time called it "Velocity
      Engine."  While VMX remains the most official name, the term
      AltiVec is still in common use today.  Freescale's AltiVec
      Technology Programming Interface Manual (the "AltiVec PIM") is
      still available online for reference (see <xref
      linkend="VIPR.intro.links" />).
    </para>
    <para>
      The original VMX specification provided for thirty-two 128-bit
      vector registers (VRs).  Each register can be treated as
      containing sixteen 8-bit character values, eight 16-bit short
      integer values, four 32-bit integer values, or four 32-bit
      single-precision floating-point values (the "VMX data types").
      Furthermore, the integer data types have signed, unsigned, and
      boolean variants.  An extensive set of arithmetic, logical,
      comparison, conversion, memory access, and permute class
      operations were specified to operate on these registers.
    </para>
    <para>
      The AltiVec PIM documents intrinsic functions to be used by
      programmers to access the VMX instruction set.  Because similar
      operations are provided for all the VMX data types, the PIM
      provides for overloaded intrinsics that can operate on different
      data types.  However, such function overloading is not normally
      acceptable in the C programming language, so compilers compliant
      with the AltiVec PIM (such as <code>gcc</code> and
      <code>clang</code>) were required to add special handling to
      their parsers to permit this.  The PIM suggested (but did not
      mandate) the use of a header file,
      <code>&lt;altivec.h&gt;</code>, for implementations that provide
      AltiVec intrinsics.  This is common practice for all compliant
      compilers today.
    </para>
    <para>
      The first chips incorporating the VMX instruction set were
      introduced by Freescale in 1999, and used primarly in Apple
      desktop computers.  IBM's last desktop CPU (the PowerPC 970)
      also included AltiVec support, and was used in the Apple
      PowerMac G5.  IBM initially omitted support for VMX from its
      server-class computers, but added support for it in the POWER6
      server family.
    </para>
    <para>
      IBM extended VMX by introducing the Vector-Scalar Extension
      (VSX) for the POWER7 family of processors.  VSX adds 64 logical
      Vector Scalar Registers (VSRs); however, to optimize the amount
      of per-process register state, the registers overlap with the
      VRs and the scalar floating-point registers (FPRs) (see <xref
      linkend="VIPR.intro.unified" />).  The VSRs can represent all
      the data types representable by the VRs, and can also be treated
      as containing two 64-bit integers or two 64-bit double-precision
      floating-point values.  
    </para>
    <para>
      Both the VMX and VSX instruction sets have been expanded for the
      POWER8 and POWER9 processor families.  Starting with POWER8,
      a VSR can now contain a single 128-bit integer; and starting
      with POWER9, a VSR can contain a single 128-bit floating-point
      value.  The VMX and VSX instruction sets together may be
      referred to as the POWER SIMD (single-instruction,
      multiple-data) instructions.
    </para>
    <section>
      <title>Little-Endian Linux</title>
      <para>
	The POWER architecture has supported operation in either
	big-endian (BE) or little-endian (LE) mode from the
	beginning.  However, IBM's POWER servers were only shipped
	with big-endian operating systems (AIX, Linux, i5/OS) prior to
	the introduction of POWER8.  With POWER8, IBM began
	supporting little-endian Linux distributions for the first
	time, and introduced a new application binary interface (the
	64-Bit ELFv2 ABI Specification <xref
	linkend="VIPR.intro.links" />) that can be used for either
	big- or little-endian support.  In practice, the ELFv2 ABI is
	currently used only for little-endian Linux.
      </para>
      <para>
	Although POWER has always supported big- and little-endian
	memory accesses, the introduction of vector register support
	added a layer of complexity to programming for processors
	operating in different endian modes.  Arrays of elements
	loaded into a VR or VSR will be indexed from left to right in
	the register in big-endian mode, but will be indexed from
	right to left in the register in little-endian mode.  However,
	the VMX and VSX instructions originally assumed that elements
	will always be indexed from left to right in the register.
	This is an inconvenience that needs to be hidden from the
	application programmer wherever possible.  To this end, IBM
	developed a bi-endian vector programming model (see <xref
	linkend="VIPR.biendian" />).  The intrinsic functions provided
	for the bi-endian vector programming model are described in
	<xref linkend="VIPR.vec-ref" />.
      </para>
    </section>
  </section>

  <section xml:id="VIPR.intro.unified">
    <title>The Unified Vector Register Set</title>
    <para>filler</para>
  </section>

  <section xml:id="VIPR.intro.links">
    <title>Useful Links</title>
    <para>filler</para>
  </section>

</chapter>