Second Processor for SuperMac S900/910
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A unique feature of the SuperMac S900 and S910 is
the second processor slot. This accepts a proprietary CPU card and
allows these machines to function as dual-processor computers
without the expense of a dual-processor card.
In a dual-processor system, the speed of the secondary processor is determined by the processor/bus speed ratio of the system -- which, in turn, is determined by the speed of the primary processor.
In short, it is possible to mix and match primary and secondary processors of different speeds.
The second processor was available in speeds of 180, 200, and 233 MHz. The primary processor came in speeds of 150, 180, 200, 225, 233, and 250 MHz. Note that you cannot combine a secondary processor with a G3 or G4 upgrade. Also note that the 150 MHz CPU is not recommended, since it uses the older 604 processor, not the 604e used in faster cards.
However, best results will be achieved whan the primary and secondary processor speeds are identical. Still, even dual processors will not double overall performance, since the computer must also spend some time coordinating the work of both processors.
For instance, if the primary processor runs at 225 MHz and the secondary processor is a 200 MHz 604e, the primary processor will set a 5:1 processor:bus speed ratio. This results in a bus speed of 45MHz for the system. The secondary processor must run at a full or half multiple of that speed, so the 200 MHz card will run at 180MHz (that is, at a 4:1 bus ratio).
The following table lists the combinations of processor speeds which can be used together and the actual operating speeds of the secondary processor resulting from each individual combination. Remember that secondary processors achieves optimal performance when paired with a primary processor of the same speed.
| Primary CPU | System Bus | Secondary CPU |
Secondary Actual Speed |
| 180 MHz | 51.4 MHz | 180 MHz | 180 MHz |
| 200 MHz | 50 MHz | 180 MHz | 175 MHz |
| 200 MHz | 50 MHz | 200 MHz | 200 MHz |
| 200 MHz | 50 MHz | 233 MHz | 225 MHz |
| 225 MHz | 45 MHz | 180 MHz | 156 MHz |
| 225 MHz | 45 MHz | 200 MHz | 180 MHz |
| 225 MHz | 45 MHz | 233 MHz | 225 MHz |
| 233 MHz | 46.6 MHz | 180 MHz | 163 MHz |
| 233 MHz | 46.6 MHz | 233 MHz | 233 MHz |
| 250 MHz | 50 MHz | 200 MHz | 200 MHz |
I've just ordered a 180 MHz primary processor and 200 MHz secondary processor for my S900. Although not listed in the above table, I believe it should work. If so, I'll add it. If not, I'll note that here.
For more information, see SuperMac Knowledgebase article KB00261.
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