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operating-systemram32-bit
fernando1979
fernando1979

Reputation: 1947

Why the Operative System of 32 bits has the limitation of 4 GB of RAM?

Why the Operative System of 32 bits has the limitation of 4 GB of RAM?. I know the processor has a register, the PC(Program Counter), where the adress of an instruction is located and if this register is of 32 bits, its a hardware limitation because of size of the register. But why the Operative System of 32 bits has this limitation? Its harcoded in its kernel that the maximum ram avalaible can be 2 exponent 32 bits?

Thanks

Upvotes: 0

Views: 258

Answers (2)

Brendan
Brendan

Reputation: 37252

Why the Operative System of 32 bits has the limitation of 4 GB of RAM?

It probably doesn't.

"32-bit" typically refers to the size of general purpose registers and may have nothing to do with the size of any address. For example, for modern 64-bit operating systems addresses are often only 48 bits.

Also; most operating systems use some form of paging where virtual address size may have nothing to do with physical address size. For example; for 32-bit 80x86 using PAE (Physical Address Extensions); virtual addresses are limited to 32-bit (causing each process to be limited to 4 GiB of "virtual space", minus whatever kernel reserves for itself); but physical addresses are/were 36 bits (giving a limit of "up to 64 GiB of RAM, minus space used for devices, ROMs, etc").

Even when physical address size is 32-bit, there are other hardware restrictions - e.g. some of those bits may be ignored by the hardware and/or used for other purposes (e.g. one bit used as an "encrypt the RAM or not" flag), and/or not supported by the RAM controller; and some of the physical address space must be used by things that are not RAM (ROM, devices, etc); so it's extremely unlikely that "32-bit physical addresses" will mean "max. of 4 GiB of RAM".

Finally; it is possible for hardware to support bank switching, where RAM is split into banks and some banks are mapped into the physical address space while others are not; and where "which bank/s are selected" is controlled by OS using special hardware. This was very common for 8-bit and 16-bit CPUs (e.g. "expanded memory" cards plugged into an ISA slot in PCs in the early 1980s); but has become significantly less common as physical address sizes have increased.

Upvotes: 2

TheCoolDrop
TheCoolDrop

Reputation: 1076

The limit is because in 32-bit architectures you are referencing the addresses in memory using 32-bit addresses. So this means that in 32-bit architecture you can only reference 2^32 addresses. Next we need to take into consideration that each address means that we are referencing a single byte which is 8 bits. This means that in effect we can reference 2^32 * 8 bits

Now lets get to the mathy part of the answer. If you can reference 2^32 * 8 bits then you can reference 2^35 bits and 2^35 = 34359738368 bits = 4294967296 bytes = 4194304 kilobytes = 4096 megabytes

And that is why you can only reference 4GiB of memory in 32-bit computers.

Upvotes: 1

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