32 bit vs 64 bit Comparison

This is a very useful post. I would also like to add that it's usually reasonable to use 64 bit on desktops and 32 bit on laptops. The reason I say this is because it's generally cheaper to get RAM for a desktop (and also less of a hassle to install). Furthermore, I find that average use on a laptop does not require more than 4GB of RAM. At that point you are spending extra money to increase laptop performance that would be much cheaper to do on a desktop, that would have a faster processor (assuming they are equivalent). The last thing is that if you are only running 32 bit on a desktop, then you are simply not utilizing its full potential. With faster processors and the ability to get RAM cheap, it makes no sense not to use 64 bit. This is of course my opinion, but something that I strongly support.

Just to add some clarification from the OP, one should look at both HW and SW limitations regarding memory.

A lot of the latest Intel CPU's are limited to 64GB of RAM while the hardware may limit that further. For instance running a 64-bit capable CPU with a 945 chipset will have a HW limit of 4GB, this means using a 64-bit OS on this will result in having less than 4GB usable, limitation would be similar to using 32-bit.

A lot of HW can use a lot more than 4GB RAM using 32-bit protected mode but the Windows OS may be capped to use less than 4GB, similar to the way some 64-bit Windows versions are limited to 8GB of RAM.

A 32-bit program that is flagged large address aware can use almost up to 4GB on a 64-bit OS albeit in 2x ~2GB chunks.

There is one thing that was not made clear in the original post although it may have been addressed in a later post. I didn't read the entire thread.

By default a 32 bit OS has a private virtual address space of 2 GB. There are 2 really important things to understand about this:

1. This is a private virtual address space. Each process has it's own 2 GB address space that is completely independent of all others. There is provision for sharing a portion of this address space.

2. The size of this virtual address space is completely independent of RAM size. Windows 95 had a minimum ram requirement of 4 MB RAM, yet the private address space was also 2 GB. In theory ALL of this address space could be used. Performance would have been poor but that doesn't invalidate the principle. And it wouldn't even need a large pagefile.

It really isn't about how much RAM can be used by an application but how much space there is for code and data.

I won't try to explain how this is accomplished. It is far from simple.

When the Win 32 platform was first designed a 2 GB address space was enormous. But for many modern applications it is a tight squeeze.

That is why 64 bit systems were designed in the first place.

64 bit native applications have a 8 TB (8192 GB) private address space. Again this is a private address space and is independent of RAM size. This allows developers to design applications in a more natural way without the constraints of a limited address space. This means more rapid development time and better application capability and performance. For some types of application this is an enormous advantage.

In a 32 bit OS the 2 GB address space was more or less fixed by the architecture. It could be increased to as much as 3 GB but this had such negative implications that it was not suitable for general use. But the current 8 TB address space is not a firm limit but one imposed because of practical considerations. Later 64 bit operating system could increase this by orders of magnitude.

Much more could be said but this post is long enough.