The G5 is a 64-bit member of the PowerPC processor family that is fully compatible with 32-bit code. The third generation of G5 chips were first used in the Late-2005 Power Mac G5. These were the very final PowerPC chips ever used in any Mac, and were the only ever Dual-Core PowerPC chips.

This chip was announced on July 7th, 2005 in Tokyo, at the Power Everywhere forum, The codename for the PowerPC 970MP is “Antares”, which is named after a red supergiant star in the constellation scorpius.

The third G5 CPU, the PowerPC 970MP, was built based off of the second-generation PowerPC 970fx CPU. The 970MP is considered to consume far more power than the prior generation, making it far less suitable for thinner profile computers such as the iMac G5. As a result, the iMac G5 iSight has a 970fx chip instead of a 970MP. “Due to high power requirements, IBM discontinued units above 2.0 GHz.” it says, on Wikipedia.

• See: IBM_PowerPC-970MP.pdf.zip (Download, 747 KB)
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• See: Power旋風が吹いたマルチコアプロセッサ (Translation: Multi-core processor with a whirlwind of Power) Fall Processor Forum 2005レポート

The 970MP came with 64 KB instruction cache and 32 KB data cache – a step forward from using 32 KB for each throughout the G3 and G4 eras, just like the 970 and 970fx before it.
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PowerPC 970MP Key Features and Notes

What sets it apart from prior generations in addition to being Dual Core, is the increase in L2 cache from 512 KB to 1 MB. The Power4 was designed with a balanced system/bus throughput design in mind, with native 32-bit compatibility, and a high frequency. The 970MP took these design goals a step further, focusing on a smaller die with lower power consumption than the Power4, enough to fit into a desktop.
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(Copyright International Business Machines Corporation 2005.)

(Copyright International Business Machines Corporation 2005.)
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“The PowerPC G5 usually knows the answer before it asks the question, using branch prediction and speculative operation to increase efficiency. Like finishing someone else’s sentences, branch prediction anticipates which instruction should go next, and speculative operation causes that instruction to be executed.” as mentioned on applemuseum.dk.
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Chip Power Draw

Official documentation shows 70 to 100 watts at 2 GHz in full operation, and 125 watts at 2.5 GHz. Keep in mind this is per-CPU, and may ramp up higher than this when maximally stressed. Despite being on the third iteration, this G5 chip never hit the 3 GHz goal Apple set out. It managed to push performance in other ways, at the expense of thermal management, power consumption, and portability. The PowerPC 970 MP unfortunately solidified the G5 as a power-hugry chip.
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(Copyright International Business Machines Corporation 2005.)

• “When one of the cores is idle, it will enter a “doze” state and shut down.” (Wikipedia)
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Late 2005 Power Mac G5 Logic Boards

Below is a picture of how the logic boards are laid out on a Late 2005 Power Mac G5. Note that in the (parentheses) where it says “Quad configuration only”, it refers to logic boards having a second processor socket if and only if it’s a Quad G5 model. This is because Late 2005 Dual G5 models refer to single-CPU PowerPC 970MP boards, which are Dual-Core.
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Edit: Using IBM’s own CPC945 Datasheet, I was able to find out the 16-Bit HyperTransport bus runs at 12.8 GB/s. There seemed to be an internal change from GB/s to MT/s at some point, and perhaps someone mislabeled the chart. I can’t find anything on the mid-bridge GB/s and don’t want to provide inaccurate info. If indeed the HT is 12.8 GB/s, that’s a massive jump up from the CPC925 Northbridge chip.
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Inside the PowerPC 970MP Third-Generation (Diagram)

Upon closer inspection of this official IBM diagram of the PowerPC 970MP, we can see it’s strikingly identical to the 970fx except with increased L2 cache.

All three PowerPC G5 chips pictured

CPC945 Northbridge Chip

This is a redesigned Northbridge chip specifically for this new Dual-Core PowerPC 970MP CPU, capable of vastly superior technical specs than the predecessor CPC925. Apple calls it the U4 chip. It’s also capable of talking to the PowerPC 970fx second-generation G5 chip, which means it may be found in other Late 2005 Macs.

• Capable of supporting two chips, like the CPC925 (prior one)
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• Has two, 625 MHz Unidirectional CPU buses (typical), but the processor interface bus in a Late 2005 Power Mac G5 runs at half the processor speed instead.
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• Two memory controllers: Which support up to 64 GB of 533 MHz DDR2 SDRAM with ECC.
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• A 800 MHz 16-Bit HyperTransport Tunnel.

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PowerPC 970GX, another “G5” chip, theoretically

The PowerPC 970GX was merely a single core version of the 970MP, retaining the full 1 MB of L2 cache, and consuming lesser power from having only one core. This chip was produced in October 2006, after the final PowerPC Mac shipped, yet, it finally hit the 3 GHz mark Apple was looking for. This chip ranged from 1.2 to 3 GHz, consumed 16W at 1.6 GHz and 85W at 3 GHz.
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Going beyond 16GB of RAM

Since the controller can see up to 64 GB of RAM, some attempts have been made (and appeared to be successful but are unconfirmed) in seeing up to 32 GB RAM. The main issue in the past was RAM density and cost, although 8 GB 667 MHz ECC sticks are known to exist and work in a Mac Pro 1,1 and 2,1. Back in 2011, a user named Iroquois in a MacRumors Thread posted screenshots of their G5 Quad on Tiger with 32 GB of RAM.

Officially, the Power Mac G5 Quad can support up to 16 GB of RAM. Unofficially, people have gotten 32 GB of RAM to work but it’s not fully confirmed or proven, and yet the controller chip supports up to 64 GB.

(Credit: Iroquois from MacRumors. URL to original post.)
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Condensed CPU Specs