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Tyler McAdams · Jul 12, 2003, 02:23 PM

Somebody posted somewhere that the G5 only addresses 32bits worth of memory...huh? I thought a 64bit chip should address 64bit's worth of memory at a time? Somebody please clear this up please. The memory configuration is a 128bit interface is it not? This has got me a bit confused. How exactly does this interface work with the cpu(s)? Thanks!

hmurchison2001 · Jul 12, 2003, 03:03 PM

The G5 has two 32bit bidirectional Busses per processor.

The Ram bank is Dual Channel(128bits total)

so the Memory Controller talks to the memory at 128bits while the FSB connectons to the controller at 32bit Bidirectionaly.

Someone please correct me if I'm wrong without jumping down my throat. I'm not an Engineer...yet

Scotttheking · Jul 12, 2003, 03:07 PM

It has 36 bit memory addressing. (or is it 42bit?)
I've heard that the northbridge can only do 34bit, which is 16GB, but I'd have to check on that.

BTW, the amount of addressable bits has nothing to do with the FSB width or the memory width.

Angus_D · Jul 12, 2003, 05:29 PM

Whatever it is, it's more than you'll need for the forseeable future.

kupan787 · Jul 12, 2003, 06:59 PM

Originally posted by Scotttheking:
It has 36 bit memory addressing. (or is it 42bit?)

I want to say 48. Or is that the G4?

Commodus · Jul 12, 2003, 07:01 PM

Last I heard, it was 42-bit memory addressing. That's well beyond what you're likely to need with a first-generation G5.

Tyler McAdams · Jul 12, 2003, 07:14 PM

Originally posted by Commodus:
Last I heard, it was 42-bit memory addressing. That's well beyond what you're likely to need with a first-generation G5.

I hate to be the one to say this... but can somebody give me a compairison between say the G5 here and a x86 northbridge? (are they all the same?) Also, would this be different because it's a 64bit chip vs. a 32bit chip? (the northbridge)

normyzo · Jul 13, 2003, 09:35 PM

Regardless of what the PPC 970 chip actually can address internally, you CAN stick 8GB of RAM in the machines. That is in the official spec.

Guess what that means? The machines can address at LEAST 33-bit, cause that's what 33-bit is. 8GB.

Dan

Tom N · Jul 14, 2003, 08:46 AM

the PowerPC 970 physically has a 42 bit memory address bus external to the chip, this would let you access 2^42 or roughly 4368 GB of RAM. The processor itself can accesses 2^64 or roughly 18 exabytes (18 billion billion bytes) of virtual memory.

The question becomes what does the G5 motherboard and apples memory controller allow for? My guess would be the Apple memory controller allows for addressing the full 42 bits of physical memory, however, I would guess the motherboard does not and stops at some point between 33 bits and 42 bits. 36 bit memory bus would allow for 64 GB of RAM, otherwise I think they would implement the full 42 bits.

Just guessing... You can find the specs at IBMs web site for the PowerPC 970

Tom N.

OreoCookie · Jul 14, 2003, 01:11 PM

Originally posted by Scotttheking:
It has 36 bit memory addressing. (or is it 42bit?)
I've heard that the northbridge can only do 34bit, which is 16GB, but I'd have to check on that.

BTW, the amount of addressable bits has nothing to do with the FSB width or the memory width.

The CPU is able to handle 42 bit.

I am not sure about the northbridge though, it's the first time I hear that. But I think that is irrelevant for most of us

The OS is also able to address more than that. If the specific app is written to take advantage of 64 bit mode, it will be able to access all RAM. If it's an `old' 32 bit it can't access more than 4 gigs.

itai195 · Jul 14, 2003, 02:44 PM

The CPU has 42 bit addressing. ApplePI only supports 36 bit memory addressing, which would theoretically give you up to 64GB of memory in a PowerMac G5.

deboerjo · Jul 17, 2003, 09:30 AM

Originally posted by Tyler McAdams:
I hate to be the one to say this... but can somebody give me a compairison between say the G5 here and a x86 northbridge? (are they all the same?) Also, would this be different because it's a 64bit chip vs. a 32bit chip? (the northbridge)

If you're comparing memory/processor bus bandwidth, the bandwidth of the frontside bus and memory is the same for the latest P4s and the 1.6GHz G5. In both cases you have a 128-bit wide (dual-channel), 400MHz memory bus (6400MB/s), and a 64-bit wide, 800MHz processor bus (also 6400MB/s). The 1.8 and 2.0GHz G5s have a faster processor bus (aka Frontside Bus or FSB) than the P4; it's the same width, but 900MHz and 1GHz respectively. That's the first time Macs have had a faster FSB than PCs. However, in these machines the memory remains at 400MHz (6400MB/s) so the increase in FSB speed on the 1.8 and 2.0 probably does little or no good compared with the 1.6.

Now if you're comparing the memory addressing, the P4/Athlon use 32-bit addressing internally and externally. The AMD Opteron and the Intel IA64 chips are much like the G5; they use 64 bit addressing internally and something like 42 bit addressing externally, I don't know the exact number.

And for the comptuer hardware newbies out there, what's the difference? A processor has a bunch of pins that are used to tell the computer what memory address it wants to talk to next. The more pins, the more memory physical the computer can have and still have the processor able to talk to it. The G5 has 42 such pins, the G4 and P4 have 32. There is a separate set of pins that is used to transfer the data contained at the memory address indicated by the 42 pins. The more pins in this group, the faster things get transferred because you're doing more at once. The G4, G5 and P4 have 64 such pins.

-Jon

CIA · Jul 17, 2003, 03:41 PM

However, in these machines the memory remains at 400MHz (6400MB/s) so the increase in FSB speed on the 1.8 and 2.0 probably does little or no good compared with the 1.6.

But later down the line when Apple ups the specs on these machines, this is one less thing on the motherboard apple needs to update, saving them money later in the product cycle. Apple just updates whatever it needs to in order to use 500+Mhz DDR, and the FSB can still handle it (granted only to a certain processor speed dependant point) without becoming saturated for the near future.

Also, reagardless of what the memory specs are, unless you are wearing a white lab coat and carry a clipboard everywhere, or run a server farm, the fact is the G5 will be able to house more memory then you will EVER need. By the time you do need more, you can buy a G6 or 7 or whatever. 6100's from back in the day were great, but imagine if they couldn't be upgraded (like the G5 aparently can't). What good is all that memory if you can't upgrade the proc to take advantage of new apps?

Big Mac · Jul 17, 2003, 10:59 PM

Originally posted by CIA:
[SNIP]

Also, reagardless of what the memory specs are, unless you are wearing a white lab coat and carry a clipboard everywhere, or run a server farm, the fact is the G5 will be able to house more memory then you will EVER need. By the time you do need more, you can buy a G6 or 7 or whatever. 6100's from back in the day were great, but imagine if they couldn't be upgraded (like the G5 aparently can't). What good is all that memory if you can't upgrade the proc to take advantage of new apps?

6100s couldn't receive processor upgrades, which is the point I believe you're trying to make, CIA. Additionally, though there was speculation earlier on that the G5 chips are soldered on the board, I recall seeing a MacRumors article suggesting they are on cards. I could be wrong, though. In any event, by the time a machine is old enough that it's really time to contemplate processor upgrades, they often don't make much financial sense.