What is 64-bit ?
64-bit can be defined as a computer architecture generally having integer registers that are 64 bits wide and thus directly supports dealing both internally and externally with 64-bit “chunks” of integer data. 64-bit has a larger address space and can hence support more physical memory.

What’s the difference between 64-bit and 32-bit?
The biggest difference between 64-bit and 32-bit processors is that 64-bit processors have a larger address space. That is, they have the ability to communicate directly with more memory. A 32-bit processor can directly address a maximum of 4 gigabytes (GB) of memory. This seemed like a lot when the first 32-bit mainstream CPU, the Intel 80386, was introduced in 1986 but has now become a serious limitation for some users and applications. For many of the applications that we use everyday, such as word processors and spreadsheets, this is more than enough memory. However, if you work with large datasets, such as the human genome or geospatial data, 4GB suddenly becomes very limiting.

64-bit computing shatters the 4GB limit giving a virtual address space in excess of 16 exabytes. That’s more than 18 billion billion bytes. A 64-bit processor supports 128 GB of physical memory, and 16 terabytes (TB) of virtual memory. This enables scenarios and applications that would be difficult or even impossible in 32-bit Windows. Table 1 shows the overall differences in memory between 32-bit and 64-bit Windows XP. This table includes several kinds of memory that only the operating system actually has to worry about, but I’ve included them for completeness. Most of us can happily ignore them and focus on two types: physical memory and total virtual address space (virtual memory).

So, are all 64-bit processors x64 processors?
No, hardly. There are other 64-bit processors on the market today, running other operating systems. But both AMD and Intel manufacture processors that support the x64 architecture: AMD calls its processors AMD64; Intel calls its processors EM64T. Both are completely compatible — the same programs run on either company’s processors.
16-bit to 32-bit transition
We’ve been here before. The transition from 32-bit to 64-bit is a major leap, but it’s something that computer users have been through before. When the original version of Windows NT was released, it was a 32-bit operating system introduced into a 16-bit processing world. As such, Windows NT faced many of the same challenges that 64-bit editions of Operating Systems face today:

• Compatibility with existing applications
• Drivers
• New applications that take advantage of the new architecture

Bear in mind that just as the move from 32 bits to 64 bits today, that earlier transition wasn’t without pain and difficulties, mostly in the same areas.

64-bits in Real Terms
The idea of a 4 terabytes of physical memory, much less 16 exabytes of address space, is a bit mind-blowing. To help you wrap your head around the scale of data that we are talking about, consider the following:

• A DVD can hold 4.7GB of data storing over 2 hours of high quality MPEG-2 video.
• 250 DVDs can contain about a terabyte of data.

• A fully loaded Xserve RAID can currently hold 5.6 terabytes of data.

• The largest physical library in the world, the U.S. Library of Congress, contains about 20 terabytes of text.

• The Internet Archive, dedicated to maintaining an archive of the Internet, holds over a petabyte (1000 terabytes) of data and is growing at over 20 terabytes a month. It would take 175 Xserve RAIDs, 4000 250GB hard drives, or 4.4 million DVDs to store that much data.

• An exabyte can contain 1000 Internet Archives—at least right now.

Soon
The wave of the move to 64-bit computing will be users who move to 64-bit in the upcoming year. Many of these users will choose Windows Vista or Mac OS X Tiger as the vehicle to make the transition; others will still be on XP x64 or 64-bit Linux distros. This wave of users will be the mainstream of 64-bit adoption, including consumers and business users who are looking for the security and performance benefits of 64-bit Windows but who demand that early-adopter issues such as driver and software compatibilities be mostly resolved.

Businesses and consumers who demand the highest levels of security will have an additional reason to choose 64-bit editions of Windows Vista: All kernel-mode drivers running in 64-bit Windows Vista must be signed by the vendor.

While a signed driver is not a guarantee of safety, it will certainly help identify and prevent many malicious attacks, while giving Microsoft the ability to help developers improve the overall quality of drivers and reduce the number of driver related crashes. Combined with the built in hardware support for Data Execution Prevention (DEP), this will help make 64-bit Windows Vista the most secure and reliable Windows ever, and will make it an attractive choice for security-conscious businesses and consumers.

* Modified exerpts from Microsoft’s Whitepaper and Apple’s Developer base.