64 Bit Address Space

With the release of zSeries mainframes in 2000, IBM extended the addressability

of the architecture to 64 bits. 

With 64-bit addressing, the potential size of a z/OS address

space expands to a size so vast that we need new terms to describe it.

Each address space,called a 64-bit address space, is 16 exabytes (EB) in size; an exabyte is slightly more than one billion gigabytes. 

The new address space has logically 264 addresses. It is 8 billion times the size of the former 2 GB address space. The number is 16 with 18 zeros after it:

16,000,000,000,000,000,000 bytes, or 16 EB (see the slide).

We say that the potential size is 16 exabytes because z/OS, by default, continues to create

address spaces with a size of 2 GB. The address space exceeds this limit only if a

program running in it allocates virtual storage above the 2 GB address. If so, the z/OS

operating system increases the storage available to the user from 2 GB to 16 EB.

The 16 MB address became the dividing point between the two previous architectures (the 24-bit addressability introduced with MVS/370 and the 31-bit addressing introduced in the operating system MVS Extended Architecture or MVS/XA), and is commonly called the line.  The area that separates the virtual storage area below the 2 GB address from the user private area is called the bar.