The Maximum memory that a computer can utilize is fairly hazily documented in many cases. In practice, it tends to be as much as one can afford up to the point where something stops working. In principle, non-financial limits are determined by the smallest of five limits.

It is also not uncommon for cache addressing to be useful for only part of memory meaning that memory above the cache addressing limit will not be cached and will be slower to access.

  CPU memory limits for common PC CPUs are
  8086, 80186 and similar --            1MB
  80286,  386SX, 486SLC --             16MB
  386DX, most 486s,  Older Pentiums --  4GB
  Pentium pro, Pentium II --           64GB

Chipsets often limit available memory to less than the CPU can address and cacheable memory to even less. The following website has memory specifications for many chipsets:

Maximum Memory for various Workstation Operating Systems:

Even when everything else is satisfactory, it may not be physically possible to fill out memory to the maximum that is conceptually permissible. A chipset that supports 512MB of memory can only use 256MB on a motherboard with 4 memory slots if the largest available memory modules are 64MB.

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