Per preferred Microsoft naming conventions, Windows Vista 圆4 Edition refers to the 64-bit version of Windows Vista in this article. Microsoft refers to the instruction set as 圆4, which parallels the widely accepted x86 nomenclature used for the instructions that run on most 32-bit processors. 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. Modern computing systems increasingly confront the 4 GB ceiling thanks to memory-intensive applications and the need to store multiple processes in memory simultaneously.
Half of that allocated memory is user-accessible memory, while the other half is for kernel processes such as drivers. 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. 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. A 32-bit processor can reference 2^32 bytes, or 4 GB of memory. The bit size of a processor refers to the size of the address space it can reference. Until recently, almost all consumer PCs used 32-bit processors. 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. When the user needs these processes, the page table remaps them into physical memory, where the user can access them at high speeds. 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. A paging table is responsible for moving segments of virtual memory into physical memory as necessary.
This solution is often referred to as “virtual memory,” in which the computer simulates having large amounts of contiguous physical memory.
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 is especially true when multiple processes or applications execute simultaneously. Often the maximum size of the physical memory on a computer is less than the amount needed for all running programs. To understand why 64-bit operating systems are a logical step in the evolution of the PC, consider this background information.
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