The real question is, from a functional standpoint, what is the difference? But basically, yeah, the latter. All of the x86-64 implementations (all two of them, AMD64 and EM64T, both based on the same spec from AMD) are.
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>Funny, the key thing holding it back from being a true 64 bit
> chip, with 32 bit backward compatability, is that it can not
> address all of 64bit memory. haha.
a 64 bit cpu is a cpu with integer (or general purpose) registers that are 64 bits wide (architecturally visible). Period.
So both EM64T/AMD64 are fully capable 64 bits cpus.
The fact they can *also* work in 32 bit mode and execute 32 binaries doesnt change a thing about that, and doesnt make them any less 64 bit. BTW, these chips can also execute 16 bit code, so would one claim they are not even true 32 bits cpus ?
The fact neither cpu currently implements the full adress space 64 bit can provide, virtual or physical, doesnt change that eiter. Did anyone call the pentium a 36 bit processor because it could adress up to 36 bits of RAM ? And who needs 16 million terrabyte 64 bits addressing could provide ? Not even Google.
For anything but >$1.000.000 large smp boxes, there is no point in implementing more than the current 40/52(?) bits, as even for virtual memory youd need a pile of harddisks that reaches 3x higher than the ozonlayer to make "full use" of 64 bit addressing. Pointless.
Anyway, current limitations are implementation choices and hardly bare more relevance to the "64 bitness" of the cpu as the limited number of DIMM slots on your motherboard.
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