Unlike the annoying memory limits of Windows XP Home Edition, Windows XP Professional x64 can address far more memory than previous versions of the operating system. Just how much RAM can be added to the 64-bit o/s? Since the introduction of Windows 95, most computer users both in the home and in the office have used some form of 32-bit operating system. Windows 95, 98, 98SE, ME, 2000, XP Home, and XP Professional are all 32-bit operating systems. In the early days of 32-bit operating systems, RAM (computer memory) increased over time in the average computer from 32 megabytes (MB) to about 512 MB at the release of Windows XP in 2002. As memory use increased, reaching the memory limits of Windows XP became a real possibility. With the introduction of Windows XP x64 in mid-2005, memory limits looked like a thing of the past. Unfortunately, slow support by computer hardware vendors ensured that the memory limits of 32-bit operating systems would haunt computer users starting in about 2006. Why are There Memory Limits in Operating Systems? Many people ask why there even are memory limits in operating systems. Why can’t we simply add as much RAM as our motherboards will physically allow? The answer lies in the nature of addressable information. As many people know, computers use a binary language to calculate, store, and retrieve information. A bit of information can convey two states, on and off, signified by zeroes and ones in binary language. A 32-bit operating system can address only 2^32 (i.e. 2 raised to the 32nd power) bits of total memory. This amounts to 4,294,967,296 bits of information. This is why Windows XP has a 4 gigabyte (GB) memory limit. To make matters worse, the operating system can only address about 4 GB of cached memory in total. This means that the total amount of RAM recognized by Windows XP is 4 gigabytes, which includes all cached memory in the computer such as RAM, graphics memory in a video card, cached storage in a CPU, and other sources. This is why Windows XP only reports about 2.8 to 3.25 GB of RAM in a computer with 4 GB of RAM. The missing RAM is equal to the total cacheable memory from other sources in the computer. To make matters worse, Windows XP can only dedicate 3.25 GB of memory to any single process. Theoretical Limits of a 64-bit Operating System Using the logic discussed above, the theoretical memory limit of a 64-bit operating system is 2^64 (i.e. 2 raised to the 64th power) which equals 18,446,744,073,709,551,616 bytes (16 exabytes). To make things easier, this is about 18,446,744,073 GB. Of course, today no computer could possibly use that much memory. This figure simply represents the theoretical memory limit of a 64-bit operating system. The Memory Limits of Windows XP x64 Windows XP 64 uses the same kernel as Windows Server 2003 (version 5.2.3790.1830). The impracticality of expecting that any computer or operating could take advantage of 16 exabytes of memory led Microsoft to cap the memory limit of Windows XP x64 to 128 GB of memory per process. This memory limit is far above the 4 GB limit of previous 32-bit operating systems and leaves enough breathing room for unforeseeable increases in memory requirements of computers and software in the near future. Few home computers are capable of using even 8 GB of memory. This limit is not attributable to the operating system but the limitations associated with the motherboard. It is impractical to think that any home user could take advantage of even 8 GB of memory, let alone 128 GB. Motherboard manufacturers rarely make their motherboards capable of using 8 GB of memory although some high-end gaming boards can use 16 GB of memory. Many servers can never have enough memory. They are much more capable of using and sometimes exceeding the use of 128 GB of memory. Conclusion Although home users often hit the 4 GB limit of 32-bit operating systems, it is unlikely the 128 GB memory limit of Windows XP x64 will be a problem in the near future. A 64-bit operating system can theoretically address 16 exabytes of information. Still, Microsoft capped the limit in its Windows XP x64 operating system to 128 GB, a limit that is likely to outlast the use of Windows XP x64 as a home operating system.