I am ready following on this website:
"Another big difference between 32-bit processors and 64-bit processors is the maximum amount of memory (RAM) that is supported. 32-bit computers support a maximum of 4 GB (2^32 bytes) of memory, whereas 64-bit CPUs can address a theoretical maximum of 18 EB (2^64 bytes). However, the practical limit of 64-bit CPUs (as of 2018) is 8 TB of addressable RAM."
It confuses me little that how in the above example of 32-bit system 2^32 is used to calculate the addressable RAM? I thought 32-bits means that the ALU registers length is 32-bits so data bus size is 32 bit instead of address bus. Same for 64-bit architecture. Please correct me if my understanding of architecture bits vs bus-widths is wrong above.
Best Answer
As with many texts written for beginners the situation has been somewhat simplified.
The assumption is that a 32-bit processor does all address operations and calculations also in 32 bits. From that it assumes a 32-bit address bus. Therefore the maximum address range is 4GBytes.
That same reasoning is then extended to a 64-bit processor.
In real life it can be a lot more complex. A good example would be the MC68000 processor, which was sold as a 16-bit processor, had 32-bit wide registers and a 24 bit address bus.
If a processor has an MMU that can also extend a 32-bit address bus to more bits. There are some 32-bit processor systems out there with a 40-bit address bus.
In the early days of computing I added a sort of MMU* to my MC6809 processor which extended the 16 bit address bus to 20 bits giving me a whopping 1Mbyte address range with an 8-bit processor.
*A 16x8 SRAM.