Most CPUs are designed so that the contents of a single integer register can store the address (location) of any datum in the computer's virtual memory. Therefore, the total number of addresses in the virtual memory – the total amount of data the computer can keep in its working area – is determined by the width of these registers. 32 bit was convenient as a 32-bit register could reference 232 addresses, or 4 GB of RAM.
The range of integers values able to be stored in a 32 bit variable is 0 through 4,294,967,295 (if not using twos complement, where one bit is used to hold the sign) . If using it, the range is −2,147,483,648 to 2,147,483,647 . Therefore, a OS with 32-bit memory addressing can directly access 4 GB of RAM.
Some operating systems reserve portions of process address space for OS use, effectively reducing the total address space available for mapping memory for user programs. For instance, Windows XP DLLs and userland OS components are mapped into each process's address space, leaving only 2 to 3.8 GB (depending on the settings) address space available, even if the computer has 4 GB of RAM.
The main disadvantage of 64-bit architectures is that relative to 32-bit architectures the same data occupies more space in memory (due to swollen pointers and possibly other types and alignment padding). This increases the memory requirements of a given process and can have implications for efficient processor cache utilization. Maintaining a partial 32-bit model is one way to handle this and is in general reasonably effective. In fact, the highly performance-oriented z/OS operating system takes this approach currently, requiring program code to reside in any number of 32-bit address spaces while data objects can (optionally) reside in 64-bit regions.
The emergence of the 64-bit architecture effectively increases the memory ceiling to 264 addresses, equivalent to approximately 17.2 billion gigabytes, 16.8 million terabytes, or 16 exabytes of RAM. To put this in perspective, in the days when 4 MB of main memory was commonplace, the maximum memory ceiling of 232 addresses was about 1,000 times larger than typical memory configurations. Today, when 2 GB of main memory is common, the ceiling of 264 addresses is about ten billion times larger, i.e. ten million times more headroom than the 232 case.
