Summary: the cores compared

Below, you find a comparison of the Intel Xeon/Pentium 4, the Opteron/Athlon 4, the G5 and the previous CPU in the Apple Power: the G4 of Motorola.

CPU feature

Motorola G4+

G5 (IBM PowerPC 970)

Intel Xeon P4 Irwindale

AMD Opteron Troy

Process technology

0.18 µ CU SOI

0.09 µ CU SOI

0.09 µ CU

0.09 µ CU SOI

GP Register Width (bit)





Number of transistors (Million)





Die Size (mm²)



+/-130 (112   for 1 MB L2)


Maximum Clockspeed (MHz)


2700 (liquid cooled)



Pipeline Stages ( fp)


16 (21)

31 - 39*

12 (17)

issue rate (Instruction per clockcycle)

3 + 1 Branch

4 + 1 branch

4 ports, max. 6 (3 sustained)

6 (3 sustained)

Integer issue rate (IPC)

3 + 1 Branch


4 (3 sustained)


Floating point issue rate (IPC)





Vector  issue rate (IPC)

2-4 ( Altivec)

2-4 ( Altivec, velocity)

4  Single(SSE-2/3)

4  Single(SSE-2/3)

2 Double (SSE-2/3)

2 Double (SSE-2/3)

Load & Store units





"instructions in flight" (OOO Window)


215 (100)



Branch History Table size (entries)





L1-cache (Instruction/Data)

32 KB/32 KB

64 KB/32 KB

12k µops (+/- 8-16 KB)/16 KB

64 KB/64KB


256 KB

512 KB

2048 KB

1024 KB


2 MB DDR SRAM 64 bit at 1/4 th of core clock




Front Side Bus (MHz)


1350 (675 DDR)

800 (200 Quad)


Front Side Bus (GB/s)

1.3 Half Duplex

10,8 Full Duplex

6.4 Half Duplex


Memory Bandwidth (GB/s)





Core Voltage


1,1V ?



Power Dissipation

30W at 1 GHz

+/- 59 (Typical) -80 Watt (max)

110 W (Typical)

92,6W (Max)

*31 is branch misprediction pipeline length, 39 is the length of the total pipeline including decoding stages before the trace cache.

Let us summarize: in theory, the PowerPc 970FX is a very wide, deeply pipelined superscalar monster chip, with excellent Branch prediction and fantastic features for streaming applications. And let us not forget the two parallel FPUs and the SIMD Altivec unit, which can process up to 4 calculations per clock cycle.

The disadvantages are the rather coarse way that the 970FX handles the instruction flow and the high latency to the RAM.

Enough theory. Let us see how the G5 2.5 GHz and 2.7 GHz compares to the 3.6 GHz Xeon Irwindale and Opteron 250 (2.4 GHz). The Opteron 852 arrived just a day before my deadline, but I think that you will know how the 252 performs compared to the 250. Before we tackle performance, here are a few quick notes about power dissipation.

Power to the PowerPC

How power thirsty is this PowerPC 970FX? His predecessor, the 0.13µ SOI PowerPC 970 was a pretty cool chip. It consumed about 42W at 1.8 GHz (1.3v). The newer 0.09µ SOI PowerPC 970FX CPU is reported to dissipate about 55-59W at 2.5 GHz. However, a few annotations must be made.

First of all, IBM and Apple tend to increase the core voltage when running at higher clock speed. This makes the needed power increase more than linearly. For example, the 1.8 GHz PowerPC 970 consumed 42 Watt, but the 2 GHz version (both 0.13µ CPUs) needed 66 Watt.

Secondly, the TDP IBM talks about is typical , not maximum like AMD's.

Let us clarify this by checking IBM's and Apple's numbers. For the 90 nm, IBM's own documents tell us that the PowerPC 970FX only consumes 24.5 Watt at 2 GHz (1V). However, the same 0.09µ SOI PowerPC970FX is reported to consume about 55W at 2.3 GHz (1.1V?) in the Xserve, according to Apple's own website. Typically, you would expect the G5 to consume about 28 Watt (24.5 * 2.3 / 2) at 2.3 GHz, when using the 24.5 Watt at 2 GHz as a reference. Apple talks about "at most" (maximum), and IBM about "typical".

Still, that is a huge gap between "typical" and "maximum" power dissipation. The 55 Watt number seems to indicate that the core voltage must have been increased significantly at 2.3 GHz. The maximum power dissipation of the 2.5/2.7 GHz G5 inside the liquid-cooled PowerMacs might thus be quite a bit higher than in the 1U Xserve, probably around 80 Watt for the 2.7 GHz. That is a lot of power for a 66 mm² CPU, and it probably explains why Apple introduced liquid cooling. The liquid cooling system inside our PowerMac wouldn't get warm and wouldn't be necessary at all if the two 2.5 GHz CPUs were only dissipating a 59 Watt maximum.

IBM PowerPC 970FX: Superscalar monster Benchmark configuration
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  • michaelok - Saturday, June 4, 2005 - link

    "with one benchmark showing that the PowerMac is just a mediocre PC while another shows it off as a supercomputer, the unchallenged king of the personal computer world."

    Well, things are a little different when you connect, say, 32768 processors together, i.e. you go from running MySQL to Teradata, so yes, the Power architecture seems to dominate, and the Virginia Tech supercomputer is still up there, at 7th.

    " The RISC ISA, which is quite complex and can hardly be called "Reduced" (The R of RISC), provides 32 architectural registers"

    'Reduced Instruction Set' is misleading, it actually refers to a design philosophy of using *smaller, simpler* instructions, instead of a single complex instruction. This is to be compared with the Itanium for example, which Intel calls 'EPIC' (Explicit Parallel Instruction Computing), but it is essentially derived from VLIW (Very Long Instruction Word).

    Anyway, nice article, certainly much more to discuss here, such as SMT (Simultaneous Multithreading), (when that is available for the Apple :), vs. Intel's Hyperthreading. We'll still be comparing Apples to Oranges but isn't that why everybody buys the Motor Trend articles, i.e. '68 Mustang vs. '68 GT?

  • psychodad - Saturday, June 4, 2005 - link

    I agree. Recently I read a review which pitted macs against pcs using software blatantly optimized for macs. If you have ever used unoptimized software, you will know it. It is slow, often unstable and not at all usable, especially if you're after productivity.
  • Viditor - Friday, June 3, 2005 - link

    IntelUser2000 - "about the AMD TDP number, they never state that its max power, they say its maximum power achievable under most circumstances, its not absolute max power"

    Not true at all...AMD's datasheet clearly states that it's not only max power, but max theoretical power.
  • trooper11 - Friday, June 3, 2005 - link

    I think its hard enough comparing a G5 to PC systems. I dont belive there will ever be a 'fair' comparison that satisfies everyone on both sides. There are too few general programs to compare and people will always complain about using or not using optimized apps for either platform. many of the varibles are subjective and the benchmarks to be compared are so heavily debated without a clear answer.

    I think this was a good attempt, but I gave up trying to 'fairly' compare the two a long time ago. Anyhting that sheds a bit of light is a good thing, but i never expect an end to the contreversy, too many questions that cant be answered.

    I would though love to see the addition of dual core amd chips since they are out there and would be serious competition, of course it would fly in server applications. hopefully the numbers for that could be added in a later article.
  • psychodad - Friday, June 3, 2005 - link

    Fascinating. You run these tests using a compiler that Apple does not use (unless it is Yellow Dog) against software generally optimized for x86 architectures and you make conclusions. This makes your data tainted (actually biased) and your conclusions faulty. I would suggest that in fairness you make your tests more "real world" by using the software compiled by compilers that the rest of us nontechnical people use on a daily basis.
  • smitty3268 - Friday, June 3, 2005 - link

    Oh, I assumed he was using the Apple version of gcc. If not, then I see what you mean.
  • crimsonson - Friday, June 3, 2005 - link

    This article may be moot by Monday
  • Garyclaus16 - Friday, June 3, 2005 - link

    " Oh and the graph on page 5 doesnt display correctly in firefox. "

    AND you are using firefox for what reason? deserve to view pages incorrectly
  • Rosyna - Friday, June 3, 2005 - link

    smitty3268, that's part of the problem. Almost no one uses GCC 3.3.3 (stock, from the main gcc branch) for Mac OS X development because it really sucks at optimizing for the PPC. On the other hand, OS X was compiled with the Apple shipped GCC 3.3/GCC 4.0.
  • smitty3268 - Friday, June 3, 2005 - link

    I think its fair to use the compilers most people are going to be using. That would be gcc on both platforms. As far as autovectorization in 4.0, don't expect very much from it. Obviously it will be better than 3.3, but the real work is being added now in 4.1.

    I'll join the other 50 posters who would have liked to see at least 1 page showing the G5's performance under linux compared to OSX. That and maybe a few more real world benchmarks. But your article was very informative and answered a lot of questions. It was frustrating that there really wasn't anything done like this before.

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