This white paper discusses issues of interest
to those who are thinking of upgrading their Macs with G4
processors. The G4 is the latest generation of PowerPC processors
from Motorola, originally announced by the company in October
1998.
What are the advantages of the G4 over the
G3?
The G4 will incorporate several new technologies
that will improve its performance over the G3, including:
AltiVec technology. AltiVec technology is Motorola's
name for the G4's vector-processing unit, which can carry
out as many as 16 simultaneous calculations. Apple calls it
the Velocity Engine, which is how this document will refer
to it. The Velocity Engine fulfills the same role as DSP add-in
cards designed to accelerate specific functions that require
a lot of computing power. Examples of such functions include
Photoshop filters, video compression and 3D rendering. The
Velocity Engine can improve the speed of such operations dramatically.
On some computing tasks, software modified to take advantage
of the Velocity Engine is expected to run as much as 30 times
as fast on a G4 as it does on a G3. (Typical real-world speed
gains will likely be less, but will still be very impressive.)
Multiprocessing. The G4 is well suited to multiprocessing
(MP), i.e., systems containing more than one processor. The
PowerPC 604 also had this capability, and many applications
were MP-enabled when Apple, DayStar and UMAX were selling
multiple-604 systems. The G3, on the other hand, is not an
effective processor for multiprocessing. MP-enabled software
will be able to take immediate advantage of systems that have
two or more G4 processors installed in them.
Faster FPU. The G4's FPU will be up to 40%
faster than the G3's for processors running at the same clock
speed. Any software that makes heavy use of floating-point
processing, such as a 3D-rendering application, will see an
immediate benefit from the G4. Software will not need to be
modified to take advantage of the G4's increased FPU speed.
Larger L2 cache. The G4 supports L2 cache sizes
up to 2 MB. The cache-size limit for the G3 is 1 MB. It is
not yet clear how significant a benefit this will turn out
to be. Applications that re-use large amounts of data, such
as image-processing applications, stand to benefit most from
larger caches. The Power Macintosh G4 systems that Apple began
shipping in September 1999 contain 1 MB of L2 cache.
128-bit system bus. There will be two versions
of the G4 processor: a 64-bit version and a 128-bit version.1
The 128-bit version will be able to process information at
up to twice the speed of the 64-bit version. All software
should benefit to some extent from a wider system bus. Applications
that handle large images, audio and video streams, or 3D animations
will benefit most.
But there's a catch. Existing Macs upgraded
with G4 processors will be restricted to using the 64-bit
version of the chip. Only Macs with newly designed logic boards
that contain a 128-bit system bus will be able to utilize
the 128-bit version of the G4 chip. Apple has said nothing
about when it might offer such systems.
When will G4 systems and upgrade cards be available?
Apple began shipping Power Macintosh G4 systems
in September 1999. At present, however, G4 processor chips
are in extremely short supply, and only 400 MHz Power Mac
G4s are available.
Most companies currently manufacturing G3 upgrades,
including XLR8, have tested, have announced and plan to ship
G4 processor-upgrade cards for desktop systems in Q4 1999,
as soon as G4 chip supplies increase. No company has publicly
announced support for G4 upgrades for PowerBooks. See the
section on Compatibility, below, for a detailed list of supported
systems.
When will dual-G4 systems be available?
Apple has not made any public statement regarding
its future plans for MP systems. XLR8 is currently investigating
the possibility of developing G4 upgrade cards for PCI-based
Power Macs and Mac clones.
I've heard that only Motorola will produce
G4s. What are IBM's plans?
At present, only Motorola is manufacturing G4s
with AltiVec technology. IBM instead plans to focus on using
its advanced chip-manufacturing process to produce faster
G3 processors. Using this process, IBM expects to achieve
processor speeds as high as 670 MHz by the end of 1999, and
1 GHz in 2001.
Was the G4 designed specifically for the Macintosh?
No. Motorola initially designed the G4 as an
embedded processor, for integration into its communications
devices. Many computing functions required by communications
devices ? digital quantization of audio signals and encryption
are two examples ? can benefit from the Velocity Engine. Because
vector processing can benefit multimedia functions as well,
Apple is promoting the G4's Velocity Engine as a means of
boosting the Mac's multimedia performance.
Compatibility
Can I upgrade my current desktop Mac with a
G4 processor?
You will definitely be able to upgrade any PCI-based
Power Mac, Mac clone or original (beige) Power Mac G3 with
a G4 processor. The initial version of the G4 processor will
be pin-compatible with G3s.1 Apple has not publicly committed
to supporting G4 upgrades for blue & white Power Mac G3s.
XLR8 and other vendors, however, will offer G4 upgrades for
the blue & white G3. These upgrades are expected to begin
shipping in Q4 1999.
NuBus-based Power Macintoshes.No upgrade company
has announced G4 upgrades for these original Power Macs. The
NuBus standard on these systems may pose problems for future
upgrades because Apple no longer supports NuBus.
PCI-based Apple Power Macintoshes. These will
be fully supported by all major upgrade card vendors. Note
that there are two ways of upgrading these systems. The traditional
approach is with a dedicated processor-upgrade card whose
processor is soldered directly onto the card. The more recent
approach is to upgrade a system with what is known as a "carrier"
card, a processor upgrade card that in turn contains socket
for a ZIF processor daughtercard. With a carrier-type upgrade
card, it is easier and less expensive than with a traditional
card to upgrade a computer with a series of ever-more-powerful
processors as they become available, because only the daughtercard
needs to be replaced. Information on XLR8's CarrierZIF upgrade
adapters can be found at the company's Web site: www.xlr8.com.
Mac OS Clone Systems. In general, clone systems
based on Apple's PowerSurge (removable CPU daughtercard) Power
Macintosh architecture can be upgraded using the same upgrade
strategies as with G3 upgrades. Supported clone systems include
Power Computing's PowerWave and PowerTower Pro; UMAX's S900
and J700; and DayStar's Genesis MP and Millennium. G4 upgrades
have not been announced for any of the Motorola StarMax clones,
the UMAX C500 or the Power Computing PowerBase, PowerCenter,
PowerCenter Pro and PowerTower.
Power Macintosh G3 (beige). All major upgrade
companies have announced G4 upgrades for these machines. This
version of the Power Macintosh G3 can be upgraded to G4 using
standard ZIF (Zero Insertion Force) socket cards, similar
to the card used in the 400 MHz Power Macintosh G4.
Power Macintosh G3 (blue & white). Current
firmware on Apple's blue & white G3 systems does not support
the G4 processor. Nevertheless, XLR8 and PowerLogix have announced
that they are developing G4 ZIF upgrades for these machines.
XLR8's G4 upgrades for blue & white G3s will be bundled with
a software patch enabling G4 support.
Power Macintosh G4. Current G4 systems use
the same ZIF socket as the blue & white G3s. As faster G4
processors become available, it will be possible to upgrade
these systems using standard ZIF processor daughtercards.
My current Mac has a slower system bus than
today's Power Mac G3s. Will that matter for G4 upgrades?
No more than for G3 upgrades. Of course, any
processor will perform better on a system with a faster bus
than it will on a system with a slower bus. On the other hand,
the backside L2 caches present on G3s and G4s minimize the
importance of system-bus speed. Most of the benefit of a processor
upgrade can be achieved even with a relatively slower bus.
What is more important is that your Mac's system
bus speed sets an upper limit on how fast a processor you
can install in it. The maximum processor speed supported by
a system is the product of two factors: the computer's rated
system-bus speed multiplied by the processor's maximum CPU
multiplier.
For example, consider an old (pre-G3) Power
Mac or Mac clone with a 50-MHz bus, upgraded with a G3 processor.
Until recently, the maximum CPU multiplier on G3s was 8x;
this limited to 400 MHz (50 MHz x 8) the speed at which these
older Power Macs could run. Even if you installed a faster
G3 in such a system, it would not reliably be able to run
faster than 400 MHz.
Recently, however, IBM began shipping G3s that
support a 10x CPU multiplier. This makes it possible to push
systems with 50-MHz buses to run at 500 MHz (50 MHz x 10).
Higher multipliers are expected in the future. Motorola has
not yet announced what the G4's highest CPU multiplier will
be.
What versions of the Mac OS will work with a
G4?
G4 processors should work with all Mac OS versions
7.5.2 or later, including Mac OS X. Mac OS 9 will be the first
version of the Mac OS that incorporates Velocity Engine acceleration.
Will I have to upgrade to new software if I
upgrade to a G4?
That depends. Although there will no doubt be
a few wrinkles to iron out, software that works well on a
G3 processor should continue to work on a G4 processor. But
if you want to take advantage of the G4's Velocity Engine
and multiprocessing capability you'll need updated software
that has been modified explicitly to support these features.
See the Performance section below for more details.
Will my current desktop Mac support a dual-G4
upgrade card?
Any Mac that can support a G4 upgrade should
be able to support a dual-G4 upgrade as well.
Performance
The Benefits of the G4 Velocity Engine
How does the Velocity Engine work?
To answer that question, it helps to understand
that processors traditionally work by performing a single
operation on a single element of data at a time. To perform
a Gaussian blur on an image on a system with a G3 processor,
for example, Photoshop must operate on each 8-bit channel
of each pixel one at a time.
In contrast, a Velocity Engine-enhanced version
of Photoshop (running on a system with a G4 processor, of
course) could pack 8 bits of data from 16 separate pixels
at once into the G4's Velocity Engine and operate on all them
with a single instruction. Similarly, the G4's Velocity Engine
could speed up a 3D-rendering application by performing four
simultaneous floating-point instructions. In addition, some
Velocity Engine instructions combine several steps of an operation
into one. This approach allows for performance gains far beyond
those obtained by simply operating on multiple data elements
at the same time.
AltiVec technology (the Velocity Engine) is
built-in to the G4 processor, along with more-typical integer
and floating-point processing units. All three units can operate
in parallel.
What software will benefit from the G4's Velocity
Engine?
Apple plans to take advantage of the Velocity
Engine in the Mac OS itself. QuickTime in particular will
get an immediate boost from the G4. Video compression is well
suited to parallel processing. Supporting G4 in QuickTime
codecs (compressor/decompressors) will enable any video application
that uses QuickTime to capture video with higher frame rates,
larger images and better image quality.
Many types of third-party applications will
also benefit from being modified for the Velocity Engine.
Image-processing, video-compression, video-effects-processing,
and 3D-rendering applications should all benefit greatly from
implementing support for the Velocity Engine.
What will software vendors have to do to take
advantage of the Velocity Engine?
Applications must be modified to capitalize
on the Velocity Engine. To utilize the Velocity Engine, software
developers will need to isolate portions of their applications
that are mathematically intensive and repetitive. Developers
will need to rewrite the code for these operations using the
G4's new vector instructions in place of the older integer
(or, in rare cases, floating-point) instructions.
When will applications that take advantage of
AltiVec be available?
Adobe has produced plug-ins for Photoshop 5.5
that take advantage of the Velocity Engine to accelerate many
filter operations and other functions. At present, however,
the only way to obtain these plug-ins is to purchase a Power
Mac G4 from Apple; they are bundled with the new systems.
Adobe has not yet made the plug-ins available for download
from its Web site or for bundling with third-party processor
upgrades. Until they do, or until third-party G4 upgrade vendors
develop plug-ins of their own, users upgrading older system
to G4 processors will not be able to take advantage of Velocity
Engine acceleration in Photoshop.
Several other vendors, including Avid Technology
Inc., Bungie Software, Casady & Greene, Macromedia, Media
100 and Terran Interactive have also announced their intention
to support the Velocity Engine in future software releases.
You can expect to see this list grow in the coming months.
The Benefits of Multiprocessing
What software will benefit from the G4's multiprocessing
capability?
Many of the same applications that will benefit
from the Velocity Engine can will also benefit from MP when
dual-G4 systems become available. Moreover, many computation-intensive
applications already have been modified to take advantage
of MP. Adobe Photoshop, Premiere and After Effects, Strata
Studio 3D and Deneba Canvas are among these MP-enabled applications.
Moreover, the MP implementation in Mac OS 8
is more limited than MP will be in Mac OS X. Apple's Mac OS
8 MP implementation only allows specialized functions within
applications to be parceled out to different processors. Mac
OS X, on the other hand, is designed from the ground up so
that the OS itself can take advantage of multiple processors.
What will software vendors have to do to take
advantage of multiprocessing?
Modifying an application to enable MP is much
like modifying it to enable AltiVec technology. Both involve
rewriting portions of the code so that it can be parceled
out in small chunks. MP requires somewhat larger chunks than
AltiVec, but the basic concept is much the same.
When will software that takes advantage of
multiprocessing be available?
As mentioned above, a number of applications
are already MP-enabled. When G4-based MP systems or upgrade
cards are available, these applications will be able to take
immediate advantage of them. Likewise, when Mac OS X ships,
it will be able to take immediate advantage of MP.
Can the same application support both MP and
the Velocity Engine?
Absolutely. Applications that do so will get
a double speed boost from dual-G4 systems. Photoshop, which
is already MP-enabled and which is being modified to support
the Velocity Engine as well, is likely to be the first application
to see this type of performance gain.
The Benefits of the G4's FPU and Cache
What software will benefit from the G4's faster
FPU?
Any application that uses floating-point instructions
will benefit automatically from the G4's enhanced FPU speed.
Applications that will benefit most are 3D-rendering applicationss
like Bryce and math-and-science apps like Mathematica. If
these programs are modified to take advantage of the Velocity
Engine as well ? remember, the Velocity Engine can perform
floating-point calculations four at a time ? they will see
an even greater speed boost from the G4.
Note, however, that most applications, even
many computationally intensive ones, use primarily integer,
not floating-point instructions. For example, it is a common
misconception that Excel, because it is math-intensive, uses
floating-point instructions. For the most part, this is not
true; Excel uses floating-point only for a limited set of
operations, such as sines and cosines.
What software will benefit from the G4's larger
L2 cache?
It's too early to tell. Cache sizes have increased
steadily from zero to 1 MB over the course of the past few
years. With each increase has come a corresponding speed boost
in both system- and application-software performance. It's
not clear, however, that a similar benefit will be achieved
by the increase from 1 to 2 MB. The final word on this question
will have to wait until extensive testing can be performed
with real applications running on G4 processors with these
larger caches.
Choosing the Best Upgrade
How do I decide whether a G4 or a high-speed
G3 is best for my needs?
This is a tricky one. No one answer will work
for everyone. Here are some points to consider:
1.Velocity Engine-enhanced software. As of
early October 1999, almost no Velocity Engine-enchanced software
is available. By the end of 1999, however, Mac OS 9, which
takes advantage of the Velocity Engine, and several enhanced
third-party applications should be available. At that point,
a G4 upgrade will be more enticing.
2.System bus speed. Although IBM plans to produce
G3 chips with ultra-fast processor speeds, at the present
time (October 1999), 500 MHz is the maximum speed announced
for either G3 or G4 chips. Once faster-speed G3 chips become
available, if your computer's bus speed is high enough, you
may be able to install a G3 chip that outperforms the fastest
available G4. Although even a super-fast G3 chip won't speed
up Velocity Engine-enabled apps as much as a G4 would, it
will speed up everything else you do on your computer more
than a G4 would. If you don't plan to use many Velocity Engine-enabled
applications, a faster G3 may be a better choice.
3.Price. At present, G4 upgrades are far more
expensive than G3 upgrades. If you're looking to upgrade now,
your best bet may be to upgrade to a G3. Later, when G4 prices
drop, you can take advantage of XLR8's trade-up program.
4.Compatibility. As with all new technologies,
minor compatibility problems will no doubt crop up as software
developers test their programs with G4 systems. Apple and
third-party G4-upgrade vendors will do all they can to ensure
a smooth transition. Still, you may want to verify that your
existing software is compatible, or that updates are readily
available, before upgrading your system with a G4 processor.
The G3/G4 horse race promises to be a very entertaining
one in the coming year. It's likely that the landscape will
shift many times as Motorola and IBM try to outdo each other.
The value of G4 processors will depend largely on how quickly
developers get Velocity Engine-enabled applications to market.
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