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Maximum System Memory

Every system has a maximum amount of memory that it will support. There are in fact several limiting factors that dictate how much memory can be used in any system:

• Addressability Limits: Each processor has a limit on the number of bytes of memory that it can access. This is dictated by the width of the system address bus; the more bits the bus has, the more memory the processor can use. See here for a table showing the maximum addressability for different CPUs. Every processor since the 80486DX has been able to access at least 4 GB of physical memory. In practice, this is not a limiting factor that comes into play very often because other system limitations (such as the chipset) have stricter constraints. There are very few PCs running even as much as 1 GB of system RAM, never mind 4 GB.
• Chipset Limits: The memory controller is part of the system chipset, and each chipset has a limit of the amount of memory that it can handle. For most desktop PCs, this ranges from 128 to 512 MB; higher-end chipsets like those for Pentium-Pro-based servers, can access much more. In addition, many chipsets have a cacheable memory limit below the total memory limit. This means that the system will access more memory than can be covered by the secondary cache, which can lead to significant performance decreases.
• Physical Packaging Limits: Modern PCs use either SIMMs or DIMMs for memory. The motherboard and chipset between them determine how many SIMMs or DIMMs can be used at once. Since there is a limit on how large each of these packages can be, this effectively puts another limit on maximum memory. For example, motherboards made with Intel's Triton II 430HX chipset have a chipset limit of 512 MB. However, most of these boards have only 4 SIMM sockets. Since the largest practically available SIMMs are 64 MB (larger ones are available, but at tremendous cost), this means that a motherboard of this type will normally max out at 256 MB for most practical purposes.
• Financial Limits: Even with today's lower memory prices, 4 GB of system memory would cost more than the rest of a typical PC combined. :^)

Since most PCs use far less memory than they can handle, maximum memory is not normally a concern. It can be for high-end workstations or servers however.

Note: In addition to the above limitations, the operating system itself can impose memory restrictions. For example, regardless of the amount of physical memory in the machine, older 16-bit operating systems like DOS and Windows 3.x cannot access more than 64 MB of system memory due to their design (from the days when 64 MB of system memory in a PC was virtually unheard of!)

Next: Maximum Cacheable Memory

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