Date: Fri, 10 Dec 2004 11:48:13 -0500 From: Pat <[EMAIL PROTECTED]>
What is the maximum CPU speed that an OldWorldMac can support? If the maximum bus to CPU multiple is 8x, then a 50 MHz system can support up to 400 MHz, right? That's mean that anything faster than 400 MHz is not supported, such as G3/800, G4/500, etc. Any comment on this?
The maximum bus speed is dependent on your computer's motherboard and the quality of circuitry on the CPU card--both. Many of the 7500/8500 MBs seem to be limited to about 50 MHz. Most of the 9500/9600 and PTP and S900 boards I've seen will go up to 60 MHz with a good CPU card--i.e. a CPU card which also can tolerate 60 MHz. Many CPU cards, even ones with adjustments to higher bus speeds, just won't run at higher bus speeds, even if the underlying motherboard will.
The CPU Multiplier is entirely dependent on the CPU chip used on the CPU card. The CPU chip is built with certain multiplier options (as can be found by reading Motorola's or IBM's datasheets for the CPUs) and the CPU card brings those multiplier options out to the jumper or switch settings so that the user can select the multiplier he or she wants.
So the CPU speed limit on Old World Macs (really, we're talking the PowerSurge family of machines based on Hammerhead, Bandit and Grand Central) is 50 - 60 MHz bus speed times whatever CPU multiplier the CPU card supports (within the limits of how fast the CPU chip can actually run).
The Sonnet G4/700 and G4/800 use a newer model of G4 chip which supports up to 16X multipliers. So 50 MHz X 16 = 800 MHz or 50 MHz X 14 = 700 MHz.
The sort of recent PowerLogix G3 ZIFs use the PPC750FX chip which supports bus multipliers up to 20X at least. So 50 MHz X 20 = 1 GHz.
Or even 60 MHz X 20 = 1.2 GHz.
Some caveats....
Some of the older CPU cards won't even work at 50 MHz. These tend to be older cards that were originally rated as a G3/300 or less.
Some 7500s seem to not operate all the way up to 50 MHz, although they should. These seem to be very rare, and I suspect many of the reports of this issue were actually caused by CPU cards that couldn't go above 45 MHz and the user blamed the motherboard by mistake.
Even if a CPU chip supports a high multiplier, and your motherboard supports a very high bus speed, you are still limited by the total speed rating of the CPU chip. So a PPC750FX (recent G3 chip) with a speed rating of 800 MHz, is not likely to actually work at 1.2 GHZ, even though your motherboard might do 60 MHz and the chip supports 20X. The internal workings of the CPU chip just can't go that fast.
So, at this time, your fastest G4 is the Sonnet at 800 MHz and with a host of compatibility issues--at least with OSX, the last I heard. Or a ZIF Carrier with a PowerLogix PPC750FX chip (I'm not sure what is the top speed they're selling these days), which should be very compatible, but you won't get the G4 advantages.
On a historical note, the first G3 chips released (PPC750) supported multipliers up to 8X. So their maximum theoretical speed with a 50 MHz bus was 400 MHz. Note that the early chips only ran up to about 300 MHz in real life. Once IBM and Motorola were producing faster G3s which could actually run at 400 MHz, IBM created a version of the G3 which supported a 10X multiplier. So there are some CPU cards based on a late model IBM PPC750 which can go up to 500 MHz with a 50 MHz bus speed.
When the G4 (PPC7400) was released, its maximum multiplier was 9X. That's why the older model G4 cards top out at 450 MHz. When the new models of G4 chip (PPC7450) came out, Sonnet used one of them on its 700 and 800 cards and took advantage of the 16X multiplier which was now available. The 7450 (as compared to the 7400 and 7410) also moved the L2 cache onto the CPU chip and provided support for an L3 cache. On the 7400 and 7410, the L2 cache is separate from the CPU chip and there is no CPU support for an L3 cache.
Jeff Walther
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