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When
I first heard that AMD had plans to extend the
x86 ISA to 64 bits, I thought it was a terrible
idea. Though x86 is the world's most successful
ISA, it's also the world's most widely disparaged.
Programmers, analysts, architecture buffs, and
enthusiasts often see x86 as a leaden albatross
around the neck of the entire computing industry,
and like the Mariner's albatross we were all hoping
it would just fall off at some point and slip
quietly into the deep. But in spite of such hopes,
I really knew better. In fact, I've argued elsewhere
that x86 isn't going away anytime soon, and it
no longer makes any real sense to gripe about
its quirks from a performance perspective. I won't
recap that argument here, but I can sum it up
briefly. Most of us would probably assent to the
following statement: "there's a huge global market
for mainstream business and consumer software,
and the overwhelming majority of that software
just so happens to use the x86 ISA." This statement
is true, as far as it goes, but framing x86's
role in the software industry this way misses
an important point. In my article "The Future
of x86 and the Concept of the ISA," I argue that
a statement like the following would provide a
more relevant assessment of the true state of
the software industry: "There's a huge global
market for mainstream business and consumer x86
software, and several smaller markets for software
written to other ISAs." All discussions of the
desktop prospects of widely ported operating systems
(i.e. Linux) or of the possible effects of greater
open source market penetration aside, this statement
should still ring true to anyone who's acquainted
with the present realities of the installed base
of IT and consumer software. If we think realistically
about most of the world's commercial software
not as "software" in the abstract but as x86 binary
code, then it becomes apparent that improvements
to the x86 ISA represent one of the most practical
and cost-effective ways to advance and expand
the x86 software market. Indeed, Intel's continuing
extensions of additions to the x86 ISA prove just
this point. Consider the move from 16 bits to
32 bits, the addition of the x87 floating-point
instructions, and the addition of integer and
then floating-point SIMD instructions. All of
these modifications of x86 helped bring new capabilities
to the PC, allowing it to find new applications
and enter new markets. Thus, the ongoing adaptation
of the x86 ISA to ever newer technological contexts
has been one of the essential subplots in the
past two decades' story of the "information revolution."
The present article outlines what AMD hopes is
the next step in x86's evolution: x86-64. As we'll
see, x86-64 is more than just a 64-bit extension
to the 32-bit x86 ISA; it adds some new features,
as well, while getting rid of some obsolete ones.
Note that this article deals with the x86-64 ISA
only. The sequel will cover the specific implementations
(Hammer, Opteron, etc.). And note also that the
general discussions of 64-bit computing that make
up the first half of the article are applicable
to 64-bit platforms, not just x86-64. So those
of you interested in the implications of a possible
Apple move to a 64-bit platform like the PPC 970
might want to read at least the first half of
the article.
Why
64 bits?
The question of why we need 64-bit computing is
often asked but rarely answered in a satisfactory
manner. That this is so is evidenced by the fact
that the question keeps coming up again and again
in online discussions of AMD's upcoming Hammer
processor. There are good reasons for the confusion
surrounding the question, the first of which is
the rarely acknowledged fact that "the 64-bit
question" is actually two questions: 1) how does
the existing 64-bit server and workstation market
use 64-bit computing, and 2) what use would the
consumer market have for 64-bit computing. People
who ask the 64-bit question are usually asking
for the answer to question 1 in order to deduce
the answer to question 2. This being the case,
we'll first look at question 1 before tackling
question 2.
What
is 64-bit computing?
If you've read my introduction to the basic concepts
in microprocessor technology, "Understanding the
Microprocessor," then you're familiar with the
code/data distinction and its implications. (If
you haven't read that article, you might want
to at least skim it and look at the diagrams before
going any further.) Simply put, the labels "16-bit,"
"32-bit" or "64-bit," when applied to a microprocessor,
characterize the processor's data stream. Although
you may have heard the term "64-bit code," this
designates code that operates on 64-bit data.
In more specific terms, the labels "64-bit," 32-bit,"
etc. designate the number of bits that each of
the processor's general-purpose registers (GPRs)
can hold. So when someone uses the term "64-bit
processor," what they mean is "a processor with
GPRs that store 64-bit numbers." And in the same
vein, a "64-bit instruction" is an instruction
that operates on 64-bit numbers.
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