GPU/CPU on SOI: the Xbox 360 did it first
Posted date : May 11, 2011

Microsoft and IBM moved the CPU and the GPU of the best-selling game console in North America onto a single SoC – a year ahead of the pack.

There’s a lot of excitement about the “latest trend” of integrating both the computing chip – the CPU and the graphics chip – the GPU – into a single chip. But in fact, the first GPU/CPU system-on-chip (SoC) came out in June 2010. It’s on 45nm SOI, is produced by multiple foundries, and is at the heart of the hottest selling game console in North America: the Xbox 360.


Xbox 360 GPU/CPU SoC Chip Statistics:
• 372M transistors
• 45nm SOI, Ultra-low k dielectric
• 10 levels of metal
• 153 array types, ~1000 instances
• 1.8 million flip flops
• 6 PLLs
• 12 clock domains
• Compared to 2005 CPU GPU:
• >60% Power Reduction
• >50% Silicon Area Reduction
(Courtesy: IBM & Microsoft)

The technical community refers to it as the Xbox 360 S – “S” for “slim”, because the new chip enabled a host of slimming effects. The two most important were a slimmed-down power budget (43% less than the previous generation) and a serious reduction in the bill-of-materials (always good news for the bottom line).

Power down

You might think that the GPU/CPU combo also provided a major performance boost. But in fact, for game console lifetimes, one thing you can’t do is toy much with performance. Game developers count on having a stable platform – they need it to work just the way they first planned it for the entire console life cycle.

One of the advantages SOI gives to chip designers is that it’s a powerful “knob to turn” – they can ratchet up performance (and keep about the same some power budget), or drastically reduce power (in exchange for a less dramatic performance increase), or they can find a balance somewhere in between.

In the case of the Xbox 360 GPU/CPU, one can surmise that since they couldn’t boost performance too much, they had the luxury of turning the knob way down for power. And that translates into a whole lot of benefits.

But first let’s look at what they actually did.

2 for 1

The first Xbox 360 came out in 2005, with a CPU on 90nm SOI (see ASN #6) and a GPU on 90nm bulk. A few years later, the chips were migrated to 65nm. Then in 2010, the two were combined on a single chip using 45nm SOI.

For the IBM and Microsoft chip design team, the latest challenge involved both a port (the bulk GPU to SOI), a shrink (to 45nm) and a complete redesign of the GPU (which had originally been designed by ATI). However, from a graphics standpoint, the resulting chip had to remain functionally identical to the old GPU, to ensure the backward compatibility of the games.

It also involved removing the front-side bus (FSB), which handles functional intercommunication between the CPU and GPU blocks.

The Xbox 360 CPU and GPU process migration history. (Courtesy: IBM and Microsoft)

Compared to having two chips, putting the two units into one chip saved 60% in power and 50% in area compared to the 90nm versions.

Savings abound

In terms of silicon, the savings are obvious when you’re fabbing one chip instead of two. However, the ramifications for savings extend far beyond the silicon.

Consider thermal design, for example. Instead of cooling two chips, you just have one: so one heatsink; one fan. And with good thermal management, the fan speed is lower – so it’s significantly quieter and needs less power for the fan.


Left: The final 35mm x 35mm package includes the integrated GPU/CPU and an eDRAM. (Courtesy: IBM & Microsoft)
Right: The GPU/CPU SoC simplified the motherboard layout. (Courtesy: Microsoft & IBM)

This simplifies the motherboard layout and power delivery. And of course, between the lower power chip and the slower fan speeds, they could use a smaller power supply unit.

All these factors combined to enable a reduction in console size. However, the new chip is robust enough to also seamlessly handle the high-performance processing for Microsoft’s Kinect – the new motion sensor that replaces the controller.


The new Xbox 360 GPU/CPU also handles the processing for the Kinect motion sensor. (Courtesy: Microsoft)

In January 2011, Microsoft announced that the new Xbox 360 had just capped off six consecutive months as the best-selling console in North America. More than 50 million Xbox 360 consoles have been sold worldwide – double the amount of the previous generation Xbox.

Sales are up 27% year on year, and Microsoft has indicated that this console should keep going strong through 2015. Which shows that great things happen – and keep happening – on SOI.

A special thanks to Bob Drehmel of IBM for his technical guidance on this article.

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