Why does 64-bit address space equal 2^64 bytes of RAM?

[Uri]

I read on HowStuffWorks that 64-bit RAM address space equals 2^64 bytes of RAM.

http://computer.howstuffworks.com/microprocessor6.htm

Why is that?

[bob]

I read on HowStuffWorks that 64-bit RAM address space equals 2^64 bytes of RAM.

http://computer.howstuffworks.com/microprocessor6.htm

Why is that?

If you use a 64 bit address space (no processor today does this yet) then with 64 bits, you have numbers between 0 and 2^64 - 1, which is a total of 2^64 bytes. Not sure exactly what you are looking for here. Today's 64 bit processors are using a 40 bit physical address space, which is only 1 terabyte of ram.

[Terry McCracken]

I read on HowStuffWorks that 64-bit RAM address space equals 2^64 bytes of RAM.

http://computer.howstuffworks.com/microprocessor6.htm

Why is that?

If you use a 64 bit address space (no processor today does this yet) then with 64 bits, you have numbers between 0 and 2^64 - 1, which is a total of 2^64 bytes. Not sure exactly what you are looking for here. Today's 64 bit processors are using a 40 bit physical address space, which is only 1 terabyte of ram.

Why don't they create 64bit address space?

[Dirt]

Today's 64 bit processors are using a 40 bit physical address space, which is only 1 terabyte of ram. :)

Why don't they create 64bit address space?

It would cost more, and possibly be a little slower. If almost no one wants to use all the address lines then it's a waste.

A quick search with Google suggests that the AMD Phenom supports 48 bits of physical address. I didn't check the Intel chips.

[Yar]

Today's 64 bit processors are using a 40 bit physical address space, which is only 1 terabyte of ram.

Why don't they create 64bit address space?

It would cost more, and possibly be a little slower. If almost no one wants to use all the address lines then it's a waste.

A quick search with Google suggests that the AMD Phenom supports 48 bits of physical address. I didn't check the Intel chips.

AMD uses 48 bits only for virtual memory. For physical RAM it uses 40 bits and it could be extended up to 52 bits in the future.
Read section 'Architectural features':
http://en.wikipedia.org/wiki/X86-64

With best regards,
Yar

[Uri]

If you use a 64 bit address space (no processor today does this yet) then with 64 bits, you have numbers between 0 and 2^64 - 1, which is a total of 2^64 bytes.

But how did you arrive at 2^64 bytes?

[Teemu Pudas]

If you use a 64 bit address space (no processor today does this yet) then with 64 bits, you have numbers between 0 and 2^64 - 1, which is a total of 2^64 bytes.

But how did you arrive at 2^64 bytes?

You quoted the answer to your own question.

[Uri]

You quoted the answer to your own question.

What do you mean?

[Teemu Pudas]

2^64 addresses = 2^64 bytes can be addressed. 64 bits leads to 2^64 addresses.

[Uri]

2^64 addresses = 2^64 bytes can be addressed. 64 bits leads to 2^64 addresses.

But if 1 byte is 8 bits so shouldn't 2^64 bits equal 2.3 x 10^18 bytes?