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iMac G5 FSB and i845G/GL

aafuss1

aafuss1

macrumors 68000
Original poster
I was reading the iMac G5 developer note and noticed the Education and 17" configurations have a 533 MHz FSB.

Conincidently, this is similare to a PC I'm using right as I type this, that has a P4 2.4GHz with the Intel 82845G IO/Controller and ICH4 southbridge (similar to the Apple U3L and Shasta chipset used in the iMac G5) which has the same FSB as the iMac G5 Education and 17" configurations-533 MHz.
 
Actually the P4 has a quad pumped bus. It's 133.33MHz x4. Like the 800MHz P4, which is a quad 200MHz ... or the top Athlon with it's 400MHz, which is actually 200MHz DDR. The original P4 was a quad-pumped 100MHz bus, and seemed to be built for RAMBUS RAM.

The 1.6 in the iMac is a full 533MHz bus, using 400MHz DDR RAM with a little overhead. 1/3 the CPU speed. The Tower 1.6 was 800MHz, 1/2 CPU speed, with 333MHz DDR, but had to be installed in pairs so it could run dual channel at 666.67MHz.
 
solvs said:
Actually the P4 has a quad pumped bus. It's 133.33MHz x4. Like the 800MHz P4, which is a quad 200MHz ... or the top Athlon with it's 400MHz, which is actually 200MHz DDR. The original P4 was a quad-pumped 100MHz bus, and seemed to be built for RAMBUS RAM.

The 1.6 in the iMac is a full 533MHz bus, using 400MHz DDR RAM with a little overhead. 1/3 the CPU speed. The Tower 1.6 was 800MHz, 1/2 CPU speed, with 333MHz DDR, but had to be installed in pairs so it could run dual channel at 666.67MHz.
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Not quite, solvs - the PowerPC G5's system bus is also DDR ("double-pumped") like the Athlon - meaning that the 533 MHz is actually 266.67 x2...still better than the best base P4 bus, at 200 MHz x4. The Dual 2.5 GHz PowerMac G5 (as an example - applies to the other models too) tops the P4 on both counts with a 1,250 MHz system bus (625 x2) - WAY faster than the P4.
 
wrldwzrd89 said:
Not quite, solvs - the PowerPC G5's system bus is also DDR ("double-pumped") like the Athlon - meaning that the 533 MHz is actually 266.67 x2...still better than the best base P4 bus, at 200 MHz x4. The Dual 2.5 GHz PowerMac G5 (as an example - applies to the other models too) tops the P4 on both counts with a 1,250 MHz system bus (625 x2) - WAY faster than the P4.
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Are you sure about that? Because it doesn't say that in anything I've read. It actually very clearly says on Apple's G5 pages that it's up to 1.25GHz FSB. No * or anything to denote DDR 625. The memory clearly says 400MHz DDR, but everything I've read about the G5 says that the FSB is half the clock speed. Not 1/4 double pumped. There is a big difference.
 
solvs said:
Are you sure about that? Because it doesn't say that in anything I've read. It actually very clearly says on Apple's G5 pages that it's up to 1.25GHz FSB. No * or anything to denote DDR 625. The memory clearly says 400MHz DDR, but everything I've read about the G5 says that the FSB is half the clock speed. Not 1/4 double pumped. There is a big difference.
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Look what I found, solvs (emphasis mine):
From http://arstechnica.com/cpu/02q2/ppc970/ppc970-2.html
When you combine the 970's 32KB d-cache with its sizable 512KB L2, its 900MHz DDR frontside bus and its support for up to 8 data prefetch streams, then you can see that this chip was made for floating-point- and SIMD-intensive media applications.
Click to expand...
 
yeah, that 900mhz is half the 1.8Ghz speed, which makes the FSB half of the processor speed. On that page I saw nothing about double pumping or anything to suggest that its less than a 1/2 processor speed.

But it could be double since you might have to install in pairs.
 
varmit said:
yeah, that 900mhz is half the 1.8Ghz speed, which makes the FSB half of the processor speed. On that page I saw nothing about double pumping or anything to suggest that its less than a 1/2 processor speed.

But it could be double since you might have to install in pairs.
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Nope - the paired install of RAM is only there because without it, the speed of the RAM (400 MHz) wouldn't even come close to the speed of the system bus. With pairing, the RAM can effectively operate at 800 MHz - much closer to the (effective) system bus speed.
 
wrldwzrd89 said:
Nope - the paired install of RAM is only there because without it, the speed of the RAM (400 MHz) wouldn't even come close to the speed of the system bus. With pairing, the RAM can effectively operate at 800 MHz - much closer to the (effective) system bus speed.
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Isn't that why the RAM is double-pumped (DDR)? 😕
 
MacinDoc said:
Isn't that why the RAM is double-pumped (DDR)? 😕
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Actually, MacinDoc - the RAM in the PowerMac G5 is effectively quad-pumped (DDR + Dual Channel). Its core runs at 200 MHz, but the combination of DDR and Dual Channel allows it to transmit data at 800 MHz - 4x core speed. EDIT: Apparently the iMac G5 has DDR memory only - no dual channel. This is due to the reduced FSB (frontside bus) speed compared with the equivalent PowerMac G5 (rev A).
 
some clarifications

ddr: double data rate. a word gets transferred on both the rising and the falling edge of the clock signal.

fsb: frontside bus. connects the processor(s) to the northbridge.

'quad pumped': intel uses this to refer to a ddr memory bus which accesses two memory banks at the same time, thus transferring two words in half a cycle. it's quite similar to the design that apple uses in the g5. the fsb is quite different though. it's hypertransport, which only transfers 8 bits per cycle, but scales much better (thus the 1 ghz+ fsb speed).

apple used memory interleaving some time ago, which also effectively doubles data rates by accessing two identical banks in sequence, so that one could refresh while the other transmitted data.
 
Not to hijack the thread, but it seems you guys know your stuff. Would a G5 1.6 GHz on a 800 MHz fsb w/ 333 MHz DDR RAM be about equivalent in terms of speed to an iMac G5 1.8 GHz, 600 MHz fsb, 400 MHz DDR RAM? I posted this elsewhere but have gotten no solid opinions.
 
noht* said:
ddr: double data rate. a word gets transferred on both the rising and the falling edge of the clock signal.

fsb: frontside bus. connects the processor(s) to the northbridge.

'quad pumped': intel uses this to refer to a ddr memory bus which accesses two memory banks at the same time, thus transferring two words in half a cycle. it's quite similar to the design that apple uses in the g5. the fsb is quite different though. it's hypertransport, which only transfers 8 bits per cycle, but scales much better (thus the 1 ghz+ fsb speed).

Thanks-never knew that HyperTransport was similar to quad pumped buses in some ways.
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