Some of the people who ask are Linux users who haven't been able to set up 32-bit compatibility libraries for their 64-bit Linux distribution. To me, this is a portability request, e.g. can you make X-Plane work on yet another operating system. I don't view this as a high priority because you can run X-Plane on a 64-bit operating system using 32-bit compatibility libraries for a number of distros. Linux comes in a million flavors and we can't support them all.
Other users ask about 64-bit because they have 64-bit CPUs (and possibly paid more for a 64-bit operating system) and want to know when they get some benefit for their money. But...is 64-bits actually useful?
A 64-bit CPU is one that can deal with larger integral numbers - as a result it can access more memory than 4 GB, and it can do math with very large integer numbers more efficiently. 64-bit CPUs also have some architectural differences that are theoretically beneficial.
Is It Faster?
Joshua tried building an experimental 64-bit version of X-Plane for Linux when he first got his 64-bit AMD box and he found...
...no performance benefit at all.
Doh! This actually isn't surprising.
- X-Plane doesn't use very large integral number math. X-Plane uses a lot of floating point math.
- The biggest time-sinks for the CPU in X-Plane are talking to the driver and sorting through piles of scenery. The later task requires a lot of logic and memory access -- neither of which work any better on a 64-bit CPU.
Memory, Memory, Memory
Now 64-bits is useful because a 64-bit app (on a 64-bit OS and 64-bit CPU) can access more than 3 GB of RAM. We're reaching the point where a lot of users have 4 GB of RAM and a machine that could be using all of it. While I've been dragging my feat on the move, someday we will support 64-bits and X-Plane will be able to use more memory.
But in the meantime, I've been doing everything I can to reduce the amount of memory X-Plane uses. This is because optimizing memory usage benefits all users, while 64-bits only helps users with a 64-bit OS and a 64-bit CPU.
I believe that we'll reach a point where 64-bits is useful for users who want to use a ton of add-ons simultaneously. But the out-of-box X-Plane configuration needs to work with much less memory than 4 GB.