JEFF BROWN TODAY HAD the lucky task of outing the Xbox360 CPU chip.
So said Jeff, an IBM chip developer, but as you know, the project is in conjunction with Microsoft. Add in a lot of work from Chartered, the company fabbing it. Microsoft definitely had input at all stages.
The chip itself is a three way SMP PPC with specialised function VMX extensions and two threads per core. It has 1MB of L2 cache, a FSB of 21.6GB/s. It has 165M transistors, and is built on IBM's 10KE 90nm SOI process.
The L1 Icache is 32K 2 way set associative, and has a 128 byte cache line size. It can issue 2 instructions per clock, in order, but can do delayed execution to cover load to use delays. The chip, still somewhat unnamed, has 2 fixed point units, and has a 2 cycle op latency. The Dcache is also 32K but is 4 way set associative, and is non-blocking. The FPU is combined with the VMX unit and can also handle two threads.
The full pipeline is 11 FO4 in length, and has a 10 cycle Scalar DP FPU latency, 2 cycle load latency, 4 cycles for simple VMX and 14 for dot product VMX. This is important because of the target for the chip, gaming. The VMX extensions are going to be heavily used here, and part of the MS mods were upping the number of VMX registers from 32 to 128. It also adds Direct3D pack and unpack instructions.
The 1MB L2 cache is shared by all three cores, and is 8-way set associtive. It is ECC protected, and supports the MESI coherency protocol. The FSB beyond that is specific to the Xbox360, and was designed for the machine itself. It connects to the ATI GPU at 10.8GBps in each direction, hence the 21.6GBps noted earlier. Interestingly, the IBM chip runs it's link layer at 1.35GHz with an 8 bit width, and ATI does it at 675MHz at 16 bits of width.
Despite the complexity of this chip, the first silicon was fully functional and running a test game a week after they got first power on. IBM and MS had an extensive testing program and it paid off. The chip was delivered on time and hit the quality targets. It is now in volume production, and you should be able to buy one in less than a month. They won't sell them to you without a console to wrap it in, but if they did, it would look like this.
Some last few tech rather miscellaneous specs. The project took about 24 months from signing on the dotted line to finish. 8 months of that was the time from tape out to volume production, meaning about 16 months was design and verification. The die is about 160 square mm, but they wouldn't talk about power. The work there is ongoing, so if you have a closed cabinet to keep them in, skip the first generation Xbox360s. To make matters worse, the mostly unnamed chip has a name, but they wouldn't give it out. Aaargh. The name on top of the chips will probably give Intel the shivers though.
The INQuirer
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