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Just reading some notes in a purdue lecture about OSs, and it says:
A program sees memory as an array of bytes that goes from address 0 to 2^32-1 (0 to 4GB-1)
Why 4 GB?
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4 GB is the theoretical maximum of memory you can use with a 32-bit OS. Practically you cannot use the full 4GB memory (maybe only 3,5 GB) because you also need some adress-space for other hardware components like: CPU, HDD, grafic card, etc.
A 32 bits can address up to 4gb so a 32 bit os can not use more than 4gb of ram. Past that, a 32 bit os must reserve some ram for fixed buffers below the 32 bit line so you effective ram available will be less than 4gb. Likely something in the 3.5gb range, depending on the hardware installed.
What are the RAM limits of a 32-bit operating system? All 32-bit operating systems have a 4GB RAM limit.
Because 32 bits are able to represent numbers up to 232 − 1 = 4294967295 = 4 GiB − 1 and therefore address up to 232 individual bytes which would be 4 GiB then.
There are ways to circumvent that, though. For example using PAE even a 32-bit operating system can support more memory. Historically this has most commonly been used on servers, though. Also, the non-server Windows SKUs don't support it. By now all that is moot, though, given that 64-bit CPUs, OSes and driver support are commonplace.
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