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Ok... settle the debate between my and my other ITS friend.
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Mac Enthusiast
Join Date: Jan 2003
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He says 64-bit chips can only access 8GB total... I know that it is MUCH more than that. He thinks that 32-64 bit chips are a linear process. But, I told him it's more exponential than linear. How much Ram can a 64-bit chip (theoretically) access?
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All as artificial as the Matrix itself, although only a human mind could invent something as insipid as Love.
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Moderator Emeritus  Join Date: Dec 2000
Location: College Park, MD
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32bit = 2^32
64bit = 2^64
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Forum Regular
Join Date: Apr 2001
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Addicted to MacNN
Join Date: Oct 2001
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Originally posted by THE MAC GOD:
He says 64-bit chips can only access 8GB total
What? RAM? Don't think so....
A 32 bit processor can theoretically address 4GB of RAM, although no 32 bit mac actually support it. I'm not aware of any PC that can actually address 4GB of RAM either.
The true answer to what can be accessed or addressed is based on the chipset design that must process the signals to get the RAM data back and forth to the processor, not the number of bits of the processor. The theoretical stuff is meaningless until implemented in reality (where most people live).
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I'm a bird. I am the 1% (of pets).
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Senior User
Join Date: Aug 2001
Location: NY
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The above posters are correct but in the real world it's more of a function of what the memory controller can address for any particular piece of hardware. The new G5's physically can address up to 16GB but it uses 42 bit memory addressing (I think AMD's Opteron does the same). This still gives you more memory capacity than the entire VT cluster of 1100 G5's RAM.
-Jerry C.
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Forum Regular
Join Date: May 2001
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Originally posted by Hydra:
The above posters are correct but in the real world it's more of a function of what the memory controller can address for any particular piece of hardware. The new G5's physically can address up to 16GB but it uses 42 bit memory addressing (I think AMD's Opteron does the same). This still gives you more memory capacity than the entire VT cluster of 1100 G5's RAM.
-Jerry C.
Yup. Internally the chip uses 64bit wide registers to hold addresses/instructions/data/etc... But they only bothered to poke 42 pins for memory addressing through the chip to the outside world ... cuz 64bits allow a really crazy amount of ram.
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If your computer stops responding for a long time, turn it off and then back on. - Microsoft
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Posting Junkie
Join Date: Jun 2003
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Yeah, the G5 can address INSANE amounts of RAM in theory. Up to 4 Terabytes to be exact, with its 42-bit addressing. And the G5s have true hardware support for 16 GB already. So do some Opteron machines.
To go from a 32-bit design to a 33-bit 8 GB design would be pointless. That would buy you a couple extra years at most. 32-bit to 42-bit memory addressing? Now we're talking. Even the Xeon (a 32-bit chip) can (sort of) handle 36-bit addressing, which is 64 GB.
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Senior User
Join Date: Dec 2002
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Depends on the instruction set of said 64-bit chip  The G5 can address 8GB of RAM because that's the most you can put on the motherboard (as of now). I believe 2^64 is slightly high, since you need to take out some bits for the op-code  Either way, it is crazy high!
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Travis Sanderson
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