PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

4.3 AMD Processors

Until
late 1999, Intel had the desktop processor market largely to itself.
There were competing incompatible systems such as the Apple Mac,
based on processors from Motorola, IBM, and others, but those systems
sold in relatively small numbers. Some companies, including Cyrix,
IDT, Harris, and AMD itself, made Intel-compatible processors, but
those were invariably a step behind Intel's flagship
processors. When those companieswhich Intel calls
"imitators"were producing
enhanced 286s, Intel was already shipping the 386 in volume. When the
imitators began producing enhanced 386-compatible processors, Intel
had already begun shipping the 486, and so on. Each time Cyrix, AMD,
and the others got a step up, Intel would turn around and release its
next-generation processor. As a result, these other
companies' processors sold at low prices and were
used largely in low-end systems. No one could compete with Intel in
its core market.

All of that changed dramatically in late 1999, when AMD began
shipping the Athlon processor. The Athlon didn't
just match the best Intel processors. It was faster than the best
Intel could produce, and was in many respects a more sophisticated
processor. Intel had a fight on its hands, and it does to this day.

If you ever take a moment to appreciate how much processor you can
get for so little money nowadays, give thanks to AMD. Without AMD,
we'd all still be running sixth-generation Intel
processors at 750 MHz or so. An entry-level Intel processor would
cost $200 or $250, and a high-end one (that might run at 1 GHz) would
probably cost $1,000 or more. The presence of AMD as a worthy
competitor meant that Intel could no longer play the game of
releasing faster processors in dribs and drabs at very high prices.
Instead, Intel had to fight for its life by shipping faster and
faster processors at lower and lower prices. We all have AMD to thank
for that, and Intel should thank AMD as well. Although
we're sure Intel wishes AMD would just disappear
(and vice versa), the fact is that the competition has made both
Intel and AMD better companies, as well as providing the obvious
benefits to us, the users.

The following sections describe current and recent AMD processor
models.

4.3.1 The AMD Athlon Family

The AMD Athlon, which was originally
code-named the K7 and began shipping in August 1999, was the first
Intel-compatible processor from any maker that could compete on an
equal footing with mainstream Intel processors of the time.
First-generation Athlon processors matched or exceeded Katmai-core
Pentium III processors in most respects, including (for the first
time ever) floating-point performance. Intel finally had a real fight
on its hands.

Although AMD represented the Athlon as the first seventh-generation
processor, we regard the K7 Athlon as essentially an enhanced
sixth-generation processor. Athlon has, in theory, several advantages
relative to the aging Intel sixth-generation architecture, including
the ability to perform nine operations per clock cycle (versus five
for the Pentium III); more integer pipelines (three versus two); more
floating-point pipelines (three versus one); a much larger L1 cache
(128 KB versus 32 KB); more full x86 decoders (three versus one); and
a faster FSB (100 MHz double-pumped to 200 MHz by transferring data
on both the rising and falling edges of the clock cycle versus the
single-pumped Intel 100/133 MHz bus, which transfers data only once
during a clock cycle). While all that was very nice, tests showed
that in practice the K7 Athlon and Pentium III were evenly matched at
lower clock speeds, with the Pentium III sometimes showing a slight
advantage in integer performance, and the Athlon a slight advantage
in floating-point performance. At higher clock speeds, however, where
the Pentium III L2 cache running at full CPU speed comes into play,
the Coppermine Pentium III won most benchmarks handily.

AMD produced two variants of the first-generation Athlon, both in
Slot A form. The earliest Athlons used the 0.25m K7 core,
but AMD transitioned within a few months to the improved
0.18m K75 core, which was code-named Pluto for speeds
lower than 1 GHz and Orion in the 1 GHz model. Although the K7 and
K75 Athlons were good processors, they had the following drawbacks:

Initial acceptance of the Athlon was hampered because the only
chipset available was the AMD-750, which was originally intended as a
technology demonstrator rather than as a production chipset. The VIA
KX133 chipset, originally planned to ship at the same time as the
Athlon, was significantly delayed, and motherboards based on the
KX133 began shipping in volume only in the second quarter of 2000.
Many motherboard manufacturers delayed introducing Athlon
motherboards, and their first products were crude compared to the
elegant motherboards available for the Pentium III. In addition to
indifferent quality, stability, compatibility, performance, and
features, first-generation Athlon motherboards were in short supply
and relatively expensive compared to comparable models for the
Pentium III. In addition, KX133-based motherboards had problems of
their own, including their inability to support Slot A
Thunderbird-core Athlons. AMD soon made it clear that Slot A was an
interim solution and that it would quickly transition to Socket A, so
manufacturers devoted little effort to improving orphaned Slot A
motherboards.

Like the Deschutes-core Pentium II and the Katmai-core Pentium III,
K7 Athlons run L2 cache at half CPU speed. Unlike the Coppermine
Pentium III, which uses on-die L2 cache running at full CPU speed,
the Athlon uses discrete L2 cache chips, which AMD had to buy from
third parties. The Athlon architecture allows running L2 cache at
anything from a small fraction of CPU speed to full CPU speed. AMD
took advantage of this as it introduced faster versions of the Athlon
by reducing the speed of L2 cache relative to processor speed,
allowing the company to use less expensive L2 cache chips. The
Athlon/700 and slower run L2 cache at 1/2 CPU speed; The Athlon/750,
/800, and /850 run L2 cache at 2/5 CPU speed. the Athlon/900 and
faster run L2 cache at 1/3 CPU speed. Unfortunately, compared to the
full-speed Pentium III Coppermine L2 cache, the slow L2 cache used on
fast Athlons decreases performance substantially in many
applications.

Early Athlon processors were power-hungry, with some 0.25m
models consuming nearly 60 watts. In comparison, typical Intel
processors used one-half to one-third that amount. High power
consumption and the resulting heat production had many implications,
including the requirement for improved system cooling and larger
power supplies. In fact, for the Athlon, AMD took the unprecedented
step of certifying power supplies for use with its processor. If you
built a system around a first-generation Athlon, you had to make sure
that both cooling and power supply were adequate to meet the
extraordinarily high current draw and heat dissipation of the
processor.

Until mid-2001, no multiprocessor Athlon systems existed. Although
all Athlon processors from the earliest models have been SMP-capable
(and in fact use the superior point-to-point SMP method rather than
Intel's shared bus method), dual-processor Athlon
systems had to wait for the release of the AMD-760MP chipset
(originally designated the AMD-770) in mid-2001. This early absence
of SMP support hurt Athlon acceptance in the critical corporate
markets, not so much because there was a huge demand for SMP but
because the lack of SMP support led buyers to consider the Athlon a
less advanced processor than Intel's offerings.

With the exception of SMP support, which was never lacking in the
processor, these faults were corrected in the second generation of
Athlon CPUs, which are based on the enhanced K75 core code-named
Thunderbird. All early Athlon models used Slot A, which is physically
identical to Intel's SC242 (Slot 1), but uses EV-6
electrical signaling rather than the GTL signaling used by Intel.
Figure 4-10 shows a Slot A Athlon processor.

Figure 4-10. AMD Slot A Athlon processor

Table 4-3 lists the important characteristics of first- and
second-generation Slot A Athlon variants (Model 3 is missing because
it was assigned to the Duron processor). All Slot A variants use the
double-pumped 100 MHz FSB, for an effective 200 MHz FSB speed.
First-generation (K7- and K75-core) Athlons are characterized by
their use of 512 KB L2 cache running at a fraction of CPU speed and
by their use of split core and I/O voltages. Second-generation
(Thunderbird-core) Athlons are characterized by their use of a
smaller 256 KB L2 cache that operates at full CPU speed and by the
elimination of split voltages for core and I/O. Thunderbird
processors were produced in very small numbers in Slot A for OEM use,
and so are included in this table for completeness, but
we've never actually seen a Slot A Thunderbird and
don't know anyone who has.

Like Intel, which shifted from Slot 1 to Socket 370 for low-end
processors, AMD recognized that producing cartridge-based slotted
processors was needlessly expensive for the low end, and made it more
difficult to compete in the value segment. Also, improvements in
fabrication made it possible to embed L2 cache directly on the
processor die rather than using discrete cache chips. Accordingly,
AMD developed a socket technology, analogous to Socket 370, which it
called Socket A. AMD had never denied that Slot A was a stopgap
technology, and that Socket A was its mainstream technology of the
future. AMD rapidly phased out Slot A during 2000, and by late 2000
had fully transitioned to Socket A. AMD has to date produced four
major Athlon variants in Socket A. From earliest to latest, these
include:

The Thunderbird Athlon was originally designated
Athlon Professional and targeted at the
mainstream desktop and entry-level workstation market, in direct
competition with the Intel Pentium III and Pentium 4. The first
Thunderbird processors used an 0.18m process with aluminum
interconnects, but by late 2000 AMD had transitioned to a
0.18m process with copper interconnects. During that
transition, AMD phased out Slot A Thunderbird models, and shifted
entirely to Socket A. Early Thunderbirds used the 100 MHz FSB
(double-pumped to 200 MHz), with later models also available in 133
MHz FSB variants. Figure 4-11 shows a Socket A
Athlon Thunderbird processor.

Figure 4-11. AMD Socket A Athlon Thunderbird processor

There was to have been another variant of the Thunderbird-core Athlon, code-named Mustang and formally named Athlon Ultra, but that processor shipped only as samples. Mustang was to be a Socket A part, targeted at servers and high-performance workstations and desktops. It was to be an enhanced version of Thunderbird, with reduced core size, lower power consumption, and large, full-speed, on-die L2 cache, probably 2 MB or more. Mustang was to have used a 133 MHz DDR FSB, yielding an effective FSB of 266 MHz. It was intended to use a 0.18m process with copper interconnects from the start, and to require the AMD-760 chipset or later. Alas, the Mustang never shipped. It would have been a wonderful processor for its time.

AMD originally intended to name the Palomino-core Athlon the
Athlon 4, for obvious reasons. In fact, the
first Palomino-core Athlons that shipped were the Mobile Athlon 4 and
the 1.0 GHz and 1.2 GHz versions of the Athlon MP. Instead, given
Microsoft's schedule for introducing Windows XP, AMD
decided its new processor might tag along on the coattails of the new
Windows version. Accordingly, AMD finally named the Palomino-core
Athlon the Athlon XP. Various architectural changes from the
Thunderbird core, detailed later in this section, allow the Athlon XP
to achieve considerably higher performance at a given clock speed
than a comparable Thunderbird. The Athlon XP is also the first recent
AMD processor to use a model designation unrelated to its actual
clock speed. All Palomino-core Athlons use the 133/266 MHz FSB. Figure 4-12 shows a Palomino-core Athlon XP processor.

Figure 4-12. AMD Athlon XP processor (image courtesy of Advanced Micro Devices, Inc.)

The Thoroughbred core, introduced in June 2002, is really just a die
shrink of the Palomino core. In reducing the fabrication process size
from 0.18m to 0.13m, AMD was able to shrink the
die from 128 mm² to
81mm² (although that increased to
84mm² for the XP 2200+ and faster models).

There were no significant architectural changes from the Palomino
core to the Thoroughbred core, so performance did not increase with
the change to the new core. Transistor count did increase somewhat,
from 37.2 million to 37.5 million. AMD also increased the number of
metal layers from seven in the Palomino core to eight in the
Thoroughbred core, which increases manufacturing complexity and cost,
but allows improved routing by optimizing electrical paths within the
processor, allowing closer placement of components and faster clock
speeds. (For comparison, the Intel Northwood-core Pentium 4 uses only
six layers.) The die shrink also allows using lower voltages, which
reduces power consumption and heat output significantly. For example,
the Palomino-core Athlon XP 2100+ dissipates 72.0W maximum, while the
Thoroughbred-core Athlon XP 2100+ dissipates only 62.1W. All
Thoroughbred-core Athlons use the 133/266 MHz FSB.

In August 2002, AMD introduced the Thoroughbred
"B" core, which increased the
number of metal layers to nine, again to allow faster clock speeds.
From a functional standpoint, the major change is support for the
166/333 MHz FSB, which was first used with the Athlon XP 2400+
processor. Other than FSB, the only noticeable difference between the
Thoroughbred and Thoroughbred "B"
cores is that the former reports a CPUID string of 680, while the
later reports 681.

The Barton core, introduced in February 2003 with the Athlon XP
3000+, uses the same 0.13m fab size as the Thoroughbred
core, but the transistor count increases from 37.5 million to 54.3
million. That boost in transistor count increases die size from 84
mm² to 101 mm².
Most of the increase in transistor count and die size is a result of
L2 cache size being boosted from 256 KB to 512 KB. Other than the
larger cache and larger die size, the Barton core is essentially the
same as the Thoroughbred B core.

Despite the doubling of L2 cache size, the Barton core is a less
significant upgrade to the Thoroughbred core than one might expect.
Benchmarking a Willamette-core Pentium 4 with 256 KB of L2 cache
against a Northwood-core Pentium 4 with 512 KB L2 cache running at
the same clock speed typically shows performance increases in the 10%
to 25% range, and often more. Those who expect a similar improvement
going from a 256 KB Thoroughbred-core Athlon to a 512 KB Barton-core
Athlon will be disappointed. Differences in processor bandwidth and
caching technologies mean that the Athlon benefits much less from the
larger L2 cache than does the Pentium 4. On most benchmarks, a
Barton-core Athlon shows only a 1% to 5% performance improvement
relative to a Thoroughbred-core Athlon running at the same clock
speed.

Barton-core processors were initially available only with a 166/333
MHz FSB. Later Barton-core processors, including the Athlon XP 3200+,
will ship with the 200/400 MHz FSB.

The really significant changes took place in the upgrade to the
Thunderbird and Palomino cores. Other than the reduction from
0.18m to 0.13m and the substitution of copper
interconnects for aluminum ones, the subsequent changes to the Athlon
core, particularly those to Thoroughbred and Barton, are largely
minor tweaks that allow incrementally faster processor speeds. Faced
with Intel's modern Pentium 4 core, AMD has been
forced to squeeze as much as possible from its aging Athlon
technology in order to remain competitive.

By updating the Athlon core and using such marketing gimmicks as
naming its processors with model numbers higher than their actual
clock speeds, AMD has generally remained competitive. But the Barton
is almost certainly the last gasp for the Athlon. In order to counter
faster Pentium 4 models from Intel, AMD has no choice. It must
relegate the Athlon to the entry level and grab significant market
share quickly for its forthcoming Hammer-series processors. The
alternative doesn't bear thinking about.

AMD actually first shipped Palomino-core Athlon processors some
months before the Athlon/XP desktop processor in the Athlon 4 mobile
variant and the Athlon MP/1.0G and Athlon MP/1.2G variants, all of
which were designated by their actual clock speeds. Subsequent
Palomino-core Athlon processors are all designated using the
QuantiSpeed performance rating rather than their actual clock speeds.
For example, the Athlon XP/1500+, XP/1600+, XP/1700+, XP/1800+, and
XP/1900+ actually run at clock speeds of 1333, 1400, 1466, 1533, and
1600 MHz, respectively, as do the similarly badged Athlon MP
SMP-capable variants.

Although Palomino-core processors use the same 0.18m
fabrication process used for Thunderbird-core processors, AMD made
several improvements in layout and architecture. Relative to the
Thunderbird-core Athlon, Palomino-core Athlons (including the Athlon
XP, the Athlon MP, and the Mobile Athlon 4) provide 3% to 7% faster
performance clock for clock, and include the following enhancements:

This allows the CPU, without being instructed to do so, to use
otherwise unused FSB bandwidth to prefetch data that it thinks may be
needed soon. This single feature accounts for most of the performance
improvement in the Palomino core relative to the Thunderbird, and
also increases the processor's dependence on a
high-speed FSB/memory bus. Better data prefetch most benefits
applications that require high memory bandwidth and have predictable
memory access patterns, including video editing, 3D rendering, and
database serving.

Translation Look-aside Buffers
(TLBs) cache translated memory addresses.
Translation is needed for the CPU to access data in main memory.
Caching translated addresses makes finding data in main memory much
faster. Palomino-core Athlons include the following three
enhancements to the TLBs:

Palomino-core Athlons increase the number of L1 Data TLBs from 32 to
40. The larger number of TLB entries increases the probability that
the needed translated address will be cached, thereby improving
performance. Even with 40 entries, though, the Palomino-core Athlon
has fewer L1 TLB entries than the Intel Pentium III or Pentium 4, and
the benefit of this small increase is minor.

In Thunderbird-core Athlons, the L1 and L2 TLBs are nonexclusive,
which means that data cached in the L1 TLB is also cached in the L2
TLB. With the Palomino core, AMD uses an exclusive TLB architecture,
which means that data cached in the L1 TLB is not replicated in the
L2 TLB. The benefit of exclusive caching is that more entries can be
cached in the L2 TLB. The drawback is that using exclusive caching
results in additional latency when a necessary address is not cached
in the L2 TLB. Overall, exclusive TLB caching again results in a
minor performance increase.

Speculative reloading means that if an address is not present in the
TLB, the processor can load the address into the TLB before the
instruction that requested the address has finished executing,
thereby making the cached address available without the latency
incurred by earlier Athlon cores, which could load the TLB entry only
after the requesting instruction had executed. Once again,
speculative reloading provides a minor performance improvement.

Palomino-core Athlons support the full Intel SSE instruction set,
which AMD designates 3DNow! Professional.
Earlier Athlon processors supported only a subset of SSE and so could
not set the processor flag to indicate full support. That meant that
SSE-capable software could not use SSE on AMD processors, which in
turn meant that AMD processors ran SSE-capable software much more
slowly than did Intel SSE-capable processors. Palomino-core Athlons
set the SSE flag to true, which allows software to use the full SSE
instruction set (but

not the SSE2 instruction
set supported by Intel Pentium 4 processors). Also note that although
Palomino-core Athlons support the full SSE instruction set, all that
means is that they can run SSE-enabled software. It does not
necessarily mean that they run SSE-enabled software as fast as a
Pentium III or Pentium 4 processor does.

Palomino-core Athlons have an improved design that reduces power
consumption by 20% relative to Thunderbird, which reduces heat
production and allows the Palomino core to achieve higher clock
speeds than the Thunderbird core.

Rather oddly, Morgan-core Durons (based on the Athlon Palomino core) actually draw more current than the older Spitfire-core Durons (based on the Athlon Thunderbird core). In fact, Morgan-core Durons draw the same current as Palomino-core Athlons operating at the same clock speed, which leads us to believe that Morgan-core Durons are literally simply Palomino-core Athlons with part of the L2 cache disabled.

Palomino-core Athlons are the first AMD processors that include a
thermal diode, which is designed to prevent damage to the processor
from overheating by shutting down power to the processor if it
exceeds the allowable design temperature. Intel processors have
included a thermal diode for years. It is nearly impossible to damage
an Intel Pentium III or Pentium 4 processor by overheating, even by
so extreme a step as removing the heatsink/fan from the processor
while it is running. Pentium III systems crash when they overheat
badly, but the processor itself is protected from damage. Pentium 4
systems don't even crash, but simply keep running,
albeit at a snail's pace. The AMD thermal diode,
alas, is an inferior implementation. Although the thermal diode on an
AMD processor can shut down the CPU safely when heat builds gradually
(as with a failed CPU fan), it does not react quickly enough to
protect the processor against a catastrophic overheating event, such
as the heatsink falling off.

The Godzilla-size heatsink/fan units used on modern high-speed processors cause catastrophic heatsink/fan unit failures more often than you might think. Whereas Pentium 4 processors use a heatsink/fan retention mechanism that clamps securely to the motherboard, AMD processors still depend on heatsink/fan units that clamp to the CPU socket itself, which isn't designed to support that much weight, particularly in a vertical configuration such as a mini-tower system. If the heatsink/fan unit comes loose, as it may do when the system is shipped or moved, an AMD processor will literally burn itself to a crisp within a fraction of a second of power being applied. We're talking smoke and flames here. This problem is one of the major causes of AMD systems arriving DOA, but may also occur anytime you move an AMD system. So, if you move an AMD system or if you've just received a new AMD system, always take the cover off and make sure the heatsink/fan unit is still firmly attached before you apply power to the system. You have been warned.

Although the Athlon XP included some significant
technical enhancements over the Thunderbird-core Athlon, the change
that received the most attention was AMD's decision
to abandon clock speed labeling and instead designate Athlon XP
models with a Performance Rating
(PR) system

AMD K7-, K75-, and
Thunderbird-core Athlon processors were labeled with their actual
clock speeds. AMD Palomino-core and later Athlon XP processors use
AMD's QuantiSpeed designations, which are simply a
revival of the hoary PR system. Although AMD claims that these PR
numbers refer to relative performance of Palomino-core processors
versus Thunderbird-core processors, most observers believe that AMD
hopes consumers will associate Athlon XP model numbers with Pentium 4
clock speeds. For example, although the AMD Athlon XP/2800+ processor
actually runs at 2250 MHz, we think AMD believes buyers will at least
subconsciously associate the 2800+ model number with the Pentium
4/2.8G, which does in fact run at a 2800 MHz clock speed.

AMD has gone to great pains to conceal the actual clock speed
of Athlon MP processors from users. For example, it mandates that the
actual clock speed not appear in advertisements, and has actually
gone so far as to insist that system and motherboard makers modify
the BIOS to ensure that it reports only the model number and not the
actual clock speed. It's interesting that AMD
trumpeted its faster clock speeds until Intel overtook AMD and left
AMD in the dust in terms of clock speeds. Now that AMD can no longer
match Intel's clock speeds, clock speeds are no
longer important. Or so says AMD.

Table 4-4 lists the important characteristics of Socket
A Athlon variants as of July 2003. Note that AMD has produced two
Thoroughbred B processors using the same 2600+ designation. One runs
at 2133 MHz on a 266 MHz FSB and the other at 2083 MHz on a 333 MHz
FSB. All Socket A Athlon variants use a 64-bit backside (L2 cache)
bus running at full CPU speed and use a shared voltage rail for
V_CORE and V_I/O. For
more information about these processors, see http://www.amd.com.

4.3.1.1 Other AMD processors

AMD has produced two
special-purpose variants of the Athlon, the Duron and the
SMP-certified Athlon MP:

The Duron was AMD's answer to the low-end Intel
Celeron. Just as Intel introduced the Celeron in an attempt to
maintain a high average selling price for its flagship Pentium III
and Pentium 4 processors, AMD introduced the Duron as a
"value" version of the Athlon. AMD
has produced two models of the Duron:

The Duron, code-named Spitfire and for a short time designated
Athlon Value, was targeted at the value desktop
market, and was to be a Celeron-killer. With it AMD straddled a fine
line between matching Celeron clock speeds and performance on the one
hand, versus avoiding cannibalizing sales of Athlon processors on the
other. Accordingly, AMD differentiated the Duron by limiting the
clock speed of the fastest current Duron to one step below the clock
speed of the slowest current Athlon, by using a smaller and less
efficient L2 cache, and by making the Duron only in 100 MHz FSB
versions (versus the 133 MHz or higher FSB available on some Athlon
models). The Spitfire-core Duron was an excellent processor for its
time. It unquestionably offered more bang for the buck than any other
processor sold by AMD or Intel. Although it achieved reasonable sales
volumes in Europe, the Duron never really took off in the U.S.
because of the absence of high-quality integrated Duron motherboards.

The Morgan-core Duron is simply a refresh of the Spitfire Duron to
use the newer Palomino core. The advantages of the Morgan-core Duron
over the Spitfire-core Duron are analogous to the advantages of the
Palomino-core Athlon over the Thunderbird-core Athlon. The Morgan
core is essentially a Palomino core with a smaller and less efficient
L2 cache. As it did with the Spitfire, AMD carefully managed the
Morgan to prevent cannibalizing sales of the Athlon XP. The fastest
current Morgan was always at least one step slower than the slowest
current Athlon XP. In terms of absolute performance clock for clock,
the Morgan slightly outperforms the Coppermine-core Pentium III and
the Tualatin-core Celeron.

The Appaloosa-core Duron, based on the Thoroughbred-core Athlon XP,
was announced but later canceled. The Duron was a victim of
AMD's success with the Athlon. As faster Athlons
were introduced at lower prices, the Duron was simply squeezed out of
its market niche. The Duron is still available as of July 2003, but
is likely to disappear before year end. Figure 4-13
shows an AMD Duron processor.

Figure 4-13. AMD Duron processor (image courtesy of Advanced Micro Devices, Inc.)

Even the first Athlon processors had the
circuitry needed to support dual-processor operation. That feature
was useless until the introduction of the AMD-760MP chipset because
no prior Athlon chipset supported dual processors. In mid-2001, Tyan
shipped its 760MP-based Thunder motherboard. It supported dual
Athlons, but was expensive and required a special power supply. In
late 2001, Tyan shipped the inexpensive Tiger MP dual Athlon board,
which used a standard power supply. Suddenly, dual Athlon systems
were affordable, and many enthusiasts set out to build them.

AMD capitalized on this new market by introducing Athlon XP
processors certified for dual-processor operation, which they named
the Athlon MP. Athlon MP processors are binned (hand-picked and
individually tested) for reliable SMP operation, or so the rumor has
it. We have our doubts. We and many of our readers have run dual
Athlon XPs successfully. Alas, AMD has disabled SMP operation on
recent Athlon XP processors. If you want a dual Athlon system using
current products, the only option is to use SMP-certified (and more
expensive) Athlon MP processors. AMD has made Athlon MP processors
using two cores:

The first Athlon MP models used the Palomino core. They shipped in
June 2001, four months before AMD introduced the first Palomino-core
Athlon XP models. At that time, AMD had not yet decided to use model
numbers rather than clock speeds to designate its processors, so the
first two Athlon MP models were designated the Athlon MP/1.0G and the
Athlon MP/1.2G. Those numbers accurately reflect their true clock
speeds of 1000 MHz and 1200 MHz, respectively. By October 2001, when
AMD began rolling out the new Palomino-core Athlon XPs, it had
decided to designate the first model the Athlon XP/1500+, even though
its actual clock speed was only 1333 MHz. All subsequent Athlon MP
processors are designated by model number rather than clock speed.
Functionally, the Palomino-core Athlon MP is identical to the
Palomino-core Athlon XP.

Functionally, the Thoroughbred-core Athlon MP is identical to the
Thoroughbred-core Athlon XP. When AMD transitioned to
Thoroughbred-core Athlon XPs, it did not immediately introduce Athlon
MP processors based on the Thoroughbred core. Instead, AMD began the
staged introduction of Athlon MP processors that continues today. For
example, in June 2002, AMD introduced Thoroughbred-core Athlon XP
models 1700+ through 2200+. It was not until late August that AMD
introduced Thoroughbred-core Athlon MP models at 2000+ and 2200+,
just days after it introduced the Athlon XP 2400+ and 2600+. AMD says
the delay is needed to certify faster models for SMP operation, which
seems to us a reasonable explanation.

In May 2003 AMD shipped the Athlon MP 2800+, the first Athlon MP
based on the Barton core. The 2800+ may also be the final Athlon MP
model, because AMD now devotes all of its attention to the Opteron.
Functionally, the Barton-core Athlon MP is identical to the
Barton-core Athlon XP, including the increase from 256 KB to 512 KB
of L2 cache. Interestingly, a few examples of the Athlon MP 2800+
with 333 MHz FSB have surfaced. We don't understand
why AMD would produce such a processor. The 760MPX (the only Athlon
chipset that supports SMP) supports a maximum FSB speed of 266 MHz,
which seems to render a 333 MHz FSB Athlon MP pointless. We can only
speculate that AMD plans a refresh of the 760MPX to add support for
the 333 MHz FSB.

Table 4-5 lists the important characteristics of
Socket A Duron and Athlon MP variants as of July 2003. For more
information about these processors, see http://www.amd.com.