A unique feature of the SuperMac S900 and S910 is their second processor slot. This slot accepts a proprietary CPU card and allows these machines to function as dual-processor computers without the need to remove the primary CPU and 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 PowerPC 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.
Also bear in mind that the Classic Mac OS cannot use more than a single CPU, although some applications can use the additional CPU. Mac OS X has built-in support for multiple CPUs.
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 CPU:bus speed ratio. This results in a bus speed of 45 MHz 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 180 MHz – a 4:1 bus ratio.
The following table lists the combinations of processor speeds that can be used together and the actual operating speeds of the secondary processor for each combination. Remember that secondary processors achieves optimal performance when paired with a primary processor of the same speed.
| Primary CPU | System Bus | Secondary CPU | 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 |
For more information, see Multi-Processing: Benefits of a Secondary Processor, a SuperMac Knowledgebase article.
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