The Frequently Asked Questions Page contains questions and answers about upgrading your Mac. The questions in this section are of general technical interest and not necessarily tied to a particular MicroMac Technology upgrade product.
Click here for detailed FAQs on a MicroMac product.

80/40MHz or 66/33MHz. Why are there sometimes two frequencies mentioned when talking about processor speed?

This dual frequency indication is most often used in conjunction with 68040 processors, for example in conjunction with a Carrera040 with a 80/40MHz 68040 processor. The higher frequency, in this case 80MHz, specifies the processor frequency, the lower frequency, in this case 40MHz, specifies the bus frequency. In this case the CPU processes instructions with a clock frequency of 80MHz, whereas the memory and input/output bus is clocked with a frequency of 40MHz.

CPU-chip	Processor Clock vs Bus Clock	Ratio	Processor used in these Macintosh Models
68000	identical	1:1	Mac Plus, SE, Classic, PowerBook 100, Portable
68020	identical	1:1	Mac II and the original LC
68030	identical	1:1	most of the Mac II series, first generation of PowerBooks and Duos, many LC-series and Performa series computers
68040	fixed at 2:1 ratio	2:1	all Centris and Quadra models, many LC and Performa Models, a number of PowerBooks and Duos
PowerPC	hardware programmable ratio	1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1	PowerMacs, high end Performas, high end PowerBooks

The 68000 processor, the 68020 processor, and the 68030 processor have a common processor and bus clock. Therefore these processors do not have a dual frequency designation. If we were to give them a dual frequency designation, both frequencies would be the same, such as a 33/33MHz 68030 processor.
    The 68040 processor operate at a processor clock frequency that is twice the bus clock frequency.
    The PowerPC processors operate a processor clock frequency that is 1 times, 1.5 times, 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, 5 times, 5.5 times, 6 times, 6.5 times or 7 times the bus frequency. A 250MHz 604 processor operates with a 250MHz processor clock frequency and, for example, a 50MHz bus clock frequency.

So far, so good. Why is this so confusing? There are two reasons, understatement marketing and incorrect benchmark programs.
    Back in the old days (when Apple and Motorola did not yet realize that they had to compete with Microsoft and Intel) Apple marketed the 68040 Quadra machines by mentioning the bus frequency and not the processor frequency, for example a 25MHz Quadra 610. (Intel, by contrast, marketed their 486 chip with a 25MHz bus frequency and a 100 MHz processor clock as 486DX100.) More recently Apple sold the 68040 Performa machines by dual marking the bus frequency and the processor frequency, for example the 33/66MHz Performa 630. (Apple still embarrassingly put the lower bus clock frequency as the first number rather than the more important processor clock).
    Popular benchmark programs, such as Norton utilities, incorrectly report the bus frequency rather than the processor clock frequency.

Bottom Line: A 80/40MHz Carrera040 processor processes instructions with an 80MHz clock, and does that about four times as fast as 20MHz 68030 in a Mac IIsi.

Note for the Nerds: For those who still don't believe it, take an oscilloscope and measure the frequency on the processor clock frequency pin of your 68040 (this is pin number R9for the 68040 PGA chip and pin number 23 on the 68040 CQFP chip).

What is an FPU? Why do I need an FPU?

The FPU is the floating point unit, or the math co-processor.

	CPU-chip	FPU-chip
	68000	no FPU available
	68020	68881 and sometimes 68882
	68030	68882
	68LC040	68040
	PowerPC	always integrated

For Macs with 68000 processors there is no FPU, unless you use an accelerator. For 68020-based Macs there is the 68881 or 68882 FPU. For 68030-based Macs there is the 68882 FPU. Both 68881 and 68882 FPU are chips separate from the CPU. The 68040 CPU chips without FPU are marked 68LC040. (LC stands for low cost) The FPU is not a separate chip, but integrated into the CPU chip itself. 68040 chips with the FPU are marked 68040 (without the "LC".)

What does the FPU do for me? The FPU makes your Mac or your accelerated Mac "somewhat fast and somewhat more compatible." This non-committal answer refers to the three different type of programs available:
    The first set of applications, such as typical word processing programs and database programs, do not care about the FPU. The application perform equally well with and without the FPU.
    The second set of programs, such as spreadsheets and image many graphic programs (Photoshop filters), use the FPU to perform mathematical and graphical calculations. These application will work without the FPU, but they work faster and smoother with the FPU.
    The third set of applications, such as high end graphic, 3D-rendering and multimedia programs require the FPU. These applications will not run without the FPU. These programs include Pixar Typestry and Real Audio.

MicroMac Technology offers a complete line of FPUs. For details see our FPU page.

Bottom Line: If you want to run a program that requires an FPU, you need to get an FPU, especially when you are upgrading with an accelerator or a logic board trade-up. An FPU makes your system run smoother and faster.

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This page was last modified on May 16, 1998.