Menu
In a book I read the following:
32-bit processors have 2^32 possible addresses, while current 64-bit processors have a 48-bit address space
My expectation was that if it's a 64-bit processor, the address space should also be 2^64.
So I was wondering what is the reason for this limitation?
913
The AMD64 architecture defines a 64-bit virtual address format, of which the low-order 48 bits are used in current implementations. This allows up to 256 TiB (248 bytes) of virtual address space.
A 48-bit memory address can directly address every byte of 256 terabytes of storage. 48-bit can refer to any other data unit that consumes 48 bits in width. Examples include 48-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.
x86-64 address spaces The x86-64 architecture is 64-bit: registers (and addresses) are 64 bits wide. However, virtual addresses on current x86-64 processors only have 48 meaningful bits. This means that not all 64-bit patterns correspond to meaningful virtual addresses.
In 64-bit Windows, the theoretical amount of virtual address space is 2^64 bytes (16 exabytes), but only a small portion of the 16-exabyte range is actually used.
Because that's all that's needed. 48 bits give you an address space of 256 terabyte. That's a lot. You're not going to see a system which needs more than that any time soon.
So CPU manufacturers took a shortcut. They use an instruction set which allows a full 64-bit address space, but current CPUs just only use the lower 48 bits. The alternative was wasting transistors on handling a bigger address space which wasn't going to be needed for many years.
So once we get near the 48-bit limit, it's just a matter of releasing CPUs that handle the full address space, but it won't require any changes to the instruction set, and it won't break compatibility.
180
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
