[32bit-64bit] how much memory can be accessed by a 32 bit machine?

What is meant by 32bit or 64 bit machine?

It’s the processor architecture…a 32 bit machine can read and write 32bit data at a time same way with 64 bit machine….

whats the maximum memory that a 32 bit machine can access?

It is 2^32=4Gb (4Gigabit = 0.5 GigaByte)

That means 4Gb ram?

If I consider the same way for a 64 bit machine then I can have a ram of 16ExbiBytes ..is that possible?

Are my concepts right?

What's typically meant by 32-bit or 64-bit machine is the size of the externally visible ("architected") general-purpose integer registers.

This has very little to do with how the hardware is built though. For example, let's consider the (long obsolete) Intel Pentium Pro. It's normally considered a "32-bit" processor, even though it supports up to 36-bit physical addresses, has a 64-bit wide data bus, and internally computations on all supported operand types are carried out in a single set of registers (which are therefore 80 bits wide, to support the largest floating point type).

At least in the case of Intel processors, even though larger physical addressing has been available for a long time, the largest amount of memory directly visible within the address space of any one process on a 32-bit processor is also limited to 4 gigabytes (32-bit addressing). The 36-bit physical addressing allows addressing up to 64 gigabytes of RAM, but only 4 gigabytes of that can be directly visible at any given time.

The change to 64-bit machines mostly involved changing what was made visible to the user (or to code at the assembly language level). Again, what you see is rarely identical to what's real. For example, most 64-bit code sees pointers/addresses as being 64 bits, but actual processors don't support that large of addresses. Current CPUs support 48-bit virtual addresses, and (at least as far as I've noticed) a maximum of 40 bits of physical addressing. On the other hand, they're designed so in the future, when larger memory becomes practical, they can extend the physical addressing out to 48 bits without affecting software at all. Even when they increase the 48-bit virtual addressing, in a typical case it'll only affect a small amount of the operating system kernel (normal code is unaffected, because it already assumed addresses are 64 bits).

So, no: a 64-bit machine does not really support up to 64 bits of physical addressing, but most typical 64-bit software should remain compatible with a future processor that did support directly addressing that much RAM.

Yes, on a 32bit machine the maximum amount of memory usable is around 4GB. Actually, depending on the OS it might be less due to parts of the address space being reserved: On Windows you can only use 3.5GB for example.

On 64bit you can indeed address 2^64 bytes of memory. Not that you'll ever have those - but then again, a long time ago the same thing was said about ever needing more than 640kb of memory...

Going back to a really basic idea, we have 32 bits for our memory addresses. That works out to 2^32 unique combinations of addresses. By convention, each address points to 1 byte of data. Therefore, we can access up to a total 2^32 bytes of data.

In a 32 bit OS, each register stores 32 bits or 4 bytes. 32 bits (1 word) of information are processed per clock cycle. If you want to access a particular 1 byte, conceptually, we can "extract" the individual bytes (e.g. byte 0, byte 1, byte 2, byte 3 etc.) by doing bitwise logical operations.

E.g. to get "dddddddd", take "aaaaaaaabbbbbbbbccccccccdddddddd" and logical AND with "00000000000000000000000011111111".

Basically, the term "x-bit machine" does not depend on your machine. That is why we do not need to change our processors or other hardware in order to migrate from a 32bit system to a 64bit one (or vice versa).

32bit and 64bit stands for the addressing capability of the OS running on your machine.

However, it still does not mean that a x-bit operating system is capable to address 2^x GB memory. Because the 'B' in "GB" means "byte" and not "bit". 1 byte equals 8 bits.

Actually a 32bit system can not even address 2^32/8 = 2^29 GB memory space while there should some memory be reserved to the OS.

It is something just below 3 GB.

Yes, a 32-bit architecture is limited to addressing a maximum of 4 gigabytes of memory. Depending on the operating system, this number can be cut down even further due to reserved address space.

This limitation can be removed on certain 32-bit architectures via the use of PAE (Physical Address Extension), but it must be supported by the processor. PAE eanbles the processor to access more than 4 GB of memory, but it does not change the amount of virtual address space available to a single process—each process would still be limited to a maximum of 4 GB of address space.

And yes, theoretically a 64-bit architecture can address 16.8 million terabytes of memory, or 2^64 bytes. But I don't believe the current popular implementations fully support this; for example, the AMD64 architecture can only address up to 1 terabyte of memory. Additionally, your operating system will also place limitations on the amount of supported, addressable memory. Many versions of Windows (particularly versions designed for home or other non-server use) are arbitrarily limited.

No your concepts are not right. And to set it right you need the answer to the question that you incorrectly answered:

What is meant by 32bit or 64 bit machine?

The answer to the question is "something significant in the CPU is 32bit or 64 bit". So the question is what is that something significant? Lot of people say the width of data bus that determine whether the machine is 32bit or 64 bit. But none of the latest 32 bit processors have 32 bit or 64 bit wide data buses. most 32 bit systems will have 36 bit at least to support more RAM. Most 64 bit processors have no more than 48bit wide data bus because that is hell lot of memory already.

So according to me a 32 bit or 64 bit machine is determined by the size of its general purpose registers used in computation or "the natural word size" used by the computer.

Note that a 32 bit OS is a different thing. You can have a 32 bit OS running on 64 bit computer. Additionally, you can have 32 bit application running on 64 bit OS. If you do not understand the difference, post another question.

So the maximum amount of RAM a processor can address is 2^(width of data bus in bits), given that the proper addressing mode is switched on in the processor.

Further note, there is nothing stopping someone to introduce a multiplex between data Bus and memory banks, that will select a bank and then address the RAM (in two steps). This way you can address even more RAM. But that is impractical, and highly inefficient.

basically 32bit architecture can address 4GB as you expected. There are some techniques which allows processor to address more data like AWE or PAE.