Advantages of a 64 bit system

Question

From a developer perspective i am trying to understand , what is the selling point of a 64-bit system ?

I understand that more registers are at your disposal , more

Answer 1

When you have 64-bits of address space to play with, you can adopt certain designs that would be very hard with less of an address space. For example, a friend recently pointed out to me that address space for thread stacks can get to be a problem with thousands of threads on a 32-bit system. But on a 64-bit system, this is no longer even remotely close to being a problem. This is the main direct benefit to developers that can affect how you write programs. And this is true regardless of how much actual memory the machine has.

Most programs I have seen converted to 64-bit have seen performance improvements because of the extra registers available.

Having 64-bit addresses can offset this performance improvement in some programs. The extra space pointers take up mean they take more cache, which leaves less space in your cache for other things. Also they take up more memory bus bandwidth when being transferred to and from main memory.

There is at least one project out there that proposes to recompile most programs in Linux in a sort of mixed-mode in which all the extra registers are used, but only 32-bit pointers are used. I'm interested in how this pans out because it removes the one performance disadvantage of 64-bit programs.

There is also a small (but important) subset of programs and algorithms that can make use of 64-bit registers. For example, most of the SHA-3 candidates are designed to take advantage of the ability to manipulate 64-bits of data at a time when doing bitwise operations.

Lastly, since the data paths inside the CPU are now 64-bits wide, this can also mean there is more bandwidth inside the CPU for moving things around. But I would expect this to be a benefit on 64-bit CPUs running in 32-bit mode as well.