To understand why 64-bit operating systems are a logical step in the evolution of the PC, consider this background information. More often than not, the maximum size of the physical memory on a computer is less than the amount needed for all running programs. This is especially true when multiple processes or applications execute simultaneously. The solution for this is that programs store some of their data on the hard drive and copy it back and forth to physical memory as needed.
This solution is often referred to as “virtual memory,” in which the computer simulates having large amounts of contiguous physical memory. A paging table is responsible for moving segments of virtual memory into physical memory as necessary. If the amount of memory demanded by all running processes exceeds the available physical memory (RAM), the paging table stores low-priority processes on the hard drive in the page file, which is much slower than RAM. When the user needs these processes, the page table re-maps them into physical memory, where the user can access them at high speeds (see Figure 1). The total number of addresses available in the virtual memory – the total amount of data the computer can keep in its working area for applications – is determined by the width of the registers on the computer processor.
Figure : The paging table moves segments of virtual memory into physical memory as needed to provide more memory to running processes.
Until recently, almost all consumer PCs used 32-bit processors. The bit size of a processor refers to the size of the address space it can reference. A 32-bit processor can reference 2^32 bytes, or 4 GB of memory. These 32-bit processors were standard at a time when 4 GB was thought to be more than enough memory space for software applications on Windows. When a process, such as running a program, is created on an x86 Windows computer with a 32-bit processor, the operating system allocates its 4 GB of virtual memory, irrespective of the actual physical memory installed on a system. Half of that allocated memory is user-accessible memory, while the other half is for kernel processes such as drivers. Modern computing systems increasingly confront the 4 GB ceiling thanks to memory-intensive applications and the need to store multiple processes in memory simultaneously.
In 2003, AMD released the first widely accepted 64-bit processor aimed at consumers, the Athlon 64, and coined AMD64 as the name for the new instruction set. Microsoft refers to the instruction set as x64, which parallels the widely accepted x86 nomenclature used for the instructions that run on most 32-bit processors.
Processors capable of referencing larger address spaces provide the opportunity to use more physical memory than ever before, potentially reducing the overhead spent moving processes in and out of physical memory. The 64-bit processors are theoretically capable of referencing 2^64 locations in memory, or 16 exabytes, which is more than 4 billion times the number of memory locations 32-bit processors can reference. However, all 64-bit versions of Microsoft operating systems currently impose a 16 TB limit on address space and allow no more than 128 GB of physical memory due to the impracticality of having 16 TB of RAM. Processes created on 64-bit versions of Windows are allotted 8 TB in virtual memory for user processes and 8 TB for kernel processes to create a virtual memory of 16 TB.
To summarize, the ability of Windows Vista and Windows 7 x64 Editions to add address more memory space than previous versions of Windows helps minimize the time spent swapping processes in and out of memory by storing more of them in RAM.