[The Godfather]

First generation iMacs have 66MHz buses.
Second generation iMacs have 100MHz buses (that's 50% faster).

At least in iMacs, what does this exactly mean? Is this the speed at which the CPU talks to peripherals (ATI chip, ethernet, modem, sound chip...) or does this speed regulate also how fast the CPU talks to the RAM?

In my personal view, I have felt that 66MHz is NOT a bottleneck for my iMac/A performance. I use IE, Photoshop, VideoScript and Linux mainly, and all these apps could benefit better from a new CPU rather than a faster bus.

For example, the 466MHz iMac upgrades have proven to outperform those gray iMacs. So, where is the benefit of a 50% faster bus. I want to understand that.

You are starting to suspect where I am going... I want a !@#$% review of Newer's G4 upgrade!!!!

[wlonh]

fifty percentum? huh? new math?

[The Godfather]

Yeah, 100 is 50% higher than 66. Where am I wrong?

[Evangellydonut]

even the admin double posts! ^_^

To my understanding, general bus speed is the speed which your CPU talks to the RAM. the PCI and AGP slots have their own bus running at their own speeds. AGP is 66Mhz 32-bit bus, I guess 2xAGP makes 133Mhz...PCI on Macs are 33Mhz 64-bit while Intel has managed to push the PCI bus to 66Mhz on their latest Mobo to support the Itanium.

The bus speed is important because the CPU has to fetch instructions from the RAM, thus if RAM is much slower than CPU, then it'd have to sit idle to wait for instruction fetches. As cache gets bigger and faster, bus speed becomes a little less relevant, but the majority of the program you are running are still in the RAM. (anyone saw the specs on the Itanium? 256k L1, 1M L2, 4M L3 cache all at CPU speed, with 400Mhz DDR FSB!) As the multiplier of CPU to bus speed gets bigger, say 6+, the bigger the bottleneck problem, thus the less effect extra Mhz will have, and the more likely you'll crash due to conflicting instruction fetches or something. Of course Intel didn't care on those Celeries, which uses an 10.5x multiplier on their Celery 700...

Now, something I've been pondering about...ATA/66 is rated 66Mhz, while ATA/100 is rated 100Mhz. How is that pushed through the PCI bus of 33Mhz? similar idea of CPU pushing through bus to the RAM? Also, I've came across something saying add-on ATA/66 controller works better than the built-in ones on the G4s...anyone?

[wlonh]

33 + 33 + 33 = 99
33 + 33 = 66

gee i dunno, looks like 100 is approx. 33 percentum more than 66...


double-post? me? never!

[This message has been edited by wlonh (edited 07-26-2000).]

[peepska]

50% more means this:

x'=1.5x (1.5, since it's 50% more than 100%). Therefore: x'=1.5*66 and x'=100

Don't worry, most people don't understand this anyway.

[Evangellydonut]

33+33=66
33+33+(1/2)*(33+33)=99
thus the 50% increase. The calculation is done from the 66Mhz Mobo point of view, not the 100Mhz one

btw, just out of curiosity, if a post is deleted, you still keep the "post #" count? I think it should decrease! ^_^

[This message has been edited by Evangellydonut (edited 07-26-2000).]

[wlonh]

ok i am just plain stupid.

explain to me again how 100MHz is 50 percentum more than 66MHz

now if you mean it is 50% faster... well...

good lord, i pay no attention to such trivialities as 'post numbers' unless i check it to see if someone is relatively new around these here parts

[This message has been edited by wlonh (edited 07-26-2000).]

If I may:

33/66 = 1/2
so
33/66 = 50/100
so
100 is about 50% MORE than 66,
since it is 34 more than 66.
BUT, 66 is about 33% LESS than 100.
It depends on your viewing angle...
I went to college for this, or did I?
(so did Godfather it seems)
And I suspect that 66 MHZ is really 66.66MHZ.

And to answer Evangellydonut's reflection, the specs for ATA controllers are maximal data throughput, stated in MB/s. Maybe someone has a better explanation, as in why use MHZ for PCI and AGP buses for example, and can we convert those ratings in data throughput?

Sincerely, Flipper

[The Godfather]

I am not sure of this, but the product of bus width (in bytes per cycle) and the frequency (in Hz) should be equal to the bandwidth. So:

64 bit bus = 8 byte bus

8 byte * 66MHz = 528 Megabytes per second, not bad.

[Ca$h68]

yes, it is 66.66 mhz. And the 100mhz buses are actually 99.99mhz. But who cares!?

The real question is, can we change the bus speed in our 1st gen iMacs.

Ca$h

[Evangellydonut]

actually, in theory, it's 66.666.....repeat indefinitely, while 100Mhz buses should be exactly 100Mhz.
Don't ask me why they use 100/3 multiples 'cuz I have no clue...

and you can probably change the bus speed since those things went on market after beige G3s, which can be OC'd to 83Mhz. It's just a matter of how to achieve such a task...probably involving some resistor/capacitor solder/desolder skills...

[TimmyDee51]

Does the bus OC have to be done with soldering or is there some sort of utility that can solve the problem for you, perhaps something similar to the Powerlogix backside cache utility. If so, this would be a lot simpler. Also, If you OC your bus to 83 MHz or some other speed, would it really make a difference since the RAM in a rev A-D iMac is still PC66, signifying its rating of 66 MHz? This would mean that the RAM speed now, not the bus speed, would be the bottleneck, essentially solving nothing. This dilemma could be solved, however, if PC100 RAM fit into a PC66 slot. My guess is no, but I have only looked at my PC66 chips and have not seen a PC100.

[Evinyatar]

I have PC-100 SODIMMs in my iMac Rev D (now rev B ). Could that explain the frequent freezes which only appeared after I installed the RAM? Is it possible the bus can't catch up on the RAM?

[Evangellydonut]

Timmy, as far as I'm aware of, to OC the iMac mobo, you'll have to do it through hardware, and it's not just simple jumpers either...I haven't read anything on it from Xlr8yourmac, but chances are, they'll have something that describes how to do such. You are right about the RAM bottleneck, but there should be some speed increase...Also, I haven't read anything on it, but I'd imagine there would be some minor stability difference between having a 5x CPU on 66 Mhz bus and a 4x CPU on 83 Mhz bus...

As for Evinyatar's question...in theory, PC100 RAMs should be backward compatible, and work fine with the iMac. Because all it has to do to compensate is for the RAM to sit idle more than it should...I've read something that said it's fine...however I've never tested it out myself. The only suggestion I can offer is get the Apple RAM guide, and see if the PC100 RAM matches all other requirements of RAM modules that can be used on the iMac...goodluck

[Gregory]

PC100 SDRAM has a clock speed of 125 mhz in order to insure that the bus isn't waiting it has to be faster, and a speed of 6-8 ns.

The G4 uses Maxbus as I understand which takes data over a wider memory bus path. In the b&w I see 98 MB/sec while G4 you see 240MB/sec - even though both use a "100 mhz bus." you can see that the G4 is going to perform operations in Photoshop even better yet.

Take a look at Radeon to ring more performance for high-end rendering. AGP 2x = 133, AGP = 266. Getting memory bus up to levels via RamBus or DDR will help even out performance on the high end. AMD uses a 200 mhz bus on its 700 mhz cpu.

The ONLY way you can see and appreciate and "understand" is to sit down in front of a G4 or check out the "rate your system" over on http://www.xlr8yourmac.com

IBM 75GXP IDE ATA/100 compliant hard drives offer 444 Mbps with 7200 rpm and 2048k cache (their SCSI Deskstar has 4096k cache) which are able to feed large files at higher burst and sustained read/write levels.

Putting a faster disk drive into even an iMac will boost performance significantly.

My 7100/80 40 mhz bus had 240 mhz G3, 60 ns RAM, but the faster bus, faster RAM, on a 100 mhz G3 was... day and night difference. And ATi uses 6 ns. 16MB SDRAM on its video card. Having 32 or 64MB (video) is going to help rendering large (100MB) PhotoShop files much faster.

You're doing a lot on an iMac - and Linux doesn't require a lot of cpu - cpu alone - even putting a 500 mhz into my G3 wasn't that much difference than the original 350 - both had 1MB L2 running at 6 ns and the difference between 175mhz and 250 mhz on the L2 doesn't make appreciable difference. Having 512k or 1MB L2 does.

You can pick up a lot of info from ChipGeek at http://www.ugeek.com/procspec/procmain.htm

Greg