What is the gain from 64-bit architecture?

Question

I played with 8-bit machines (C64), I used 16-bit machines (Win 3.1), I enjoyed flat 32-bit address space (Linux).

Every time as a user (which was also means being a developer for fun or profit) I felt the need for more bits and welcomed the next wider bit architecture. Accessing to bigger memory space got easier, graphics got better and sound was a bliss after 8-bit.

However when marketing drums started to kick in for 64-bit some years ago I thought, "ok, this is Internet age, they can use this kind of power to do more calculations easier and commodity hardware manufacturers want to spread their portfolio for this kind of market".

Professionally I became a "system programmer" working for mobile devices which some what means being an "embedded programmer" as well. Working mostly on 32-bit from the start ARM architecture based products, it was a while I forgot about different architectures other than being 32-bit.

Also when ARM announced a new 64-bit architecture towards end of 2011, I read it as ARM wants to go into server market, increasing their portfolio. Which makes sense.

Now with new iPhone 5S claiming to be the first 64-bit processor ever used on a smartphone, thoughts became a little bit unsettled in my head again. Mobile devices, being the ultimate personal devices are becoming 64-bit.

So I wonder, is there something I'm missing? What does being 64-bit offers to the users including programmers?

Answer 1

It's important to distinguish between 64-bit architectures in general and the 64-bit architectures we commonly see. In an abstract sense, a 64-bit architecture just gives you wider registers (bigger numbers and more addressable memory). Looking at concrete examples of architectures, you see that the 32 to 64-bit jump was used as an opportunity to make significant, incompatible, improvements in processor design.

The first one that comes to mind, looking at both x86_64 and ARMv8, you see a significant increase in the number of registers available. Both architectures have doubled the number of general-purpose registers on the processor. This greatly increases the opportunities for optimization of software. Similarly, they have improved vector processing capabilities. Moving to a new, incompatible, architecture gives designers to opportunity to remove little used features that were dragged along through the years for the sake of backwards compatibility.

Answer 2

One major benefit of having a huge address space is that you can represent all storage simply as an address. For example, my laptop has 16 GiByte of RAM and 1 TiByte HDD. With 64 bit addresses, you can easily address all that storage using a unified API. 32 bit wouldn't be enough. I could even individually address all of my USB key fobs, SD cards, microSD cards, external HDDs, NAS, DropBox, Google Drive etc.

Even my phone has 1 GiByte RAM, 4 GiByte internal flash and a 32 GiByte microSD card.

Unfortunately, no operating system does this yet (with the exception of OS/400, which has been doing exactly this since 1989, although it actually has 128 bit addressing), but that would be one benefit.

Even without such futuristic changes, there is a rather simple reason: at the rate RAM capacity in smartphones is growing, we're going to hit the "4 GiByte wall" real soon now.

Answer 3

The fundamental advantage is larger register size ( and ALU capability ).

Basially, I can now multiply 10 trillion million billion by 7 in one step. Before with 32 bit I had to split the operands into two pieces that could fit in 32 bit registers and use off die memory to store intermediate results.

This doesn't seem like a big deal, until you look at how many time code written for a 32 bit processor has to split a math operand or split a four word hash key ( or high precision floating point ) or something into two pieces to sort or branch on it.