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3GB memory limit

The maximum amount of memory a 32-bit Windows operating system can use for applications. The memory range from 3GB to 4GB is assigned to memory on peripheral cards. See MMIO.



MMIO

(Memory Mapped Input Output) Using the same address register in the computer to access main memory as well as memory on peripheral controllers. For example, in a 32-bit Windows operating system, main memory up to 3GB is used for programs and data. The memory range from 3GB to 4GB is assigned to the memory on the controller cards, and that memory is used for data transfer and peripheral processing. If RAM chips actually exist on the motherboard for this 3GB-4GB region, they are not used.

Upper Memory Area
Memory mapped I/O (MMIO) existed long before PCs had multiple gigabytes of memory. For example, the upper memory area (UMA) region above 640KB was used for the graphics cards when screen resolutions were considerably smaller than they are today. See UMA.



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