How does Cpu generate the logical address for the disk

If you think question is not proper please edit or make it correct, i am asking what i google and extract from the internet.

Cpu generates the logical address which is converted into physical address but the question here is how does the cpu generates the logical address for the data that is stored on the disk.

> Cpu generates the logical address which is converted into physical address but the question here is how does the cpu generates the logical address for the data that is stored on the disk.

It doesn't, at least not the way you're thinking it does.

Normally a program tries to access memory at a virtual address, but the CPU sees "virtual address isn't present" and complains to the OS (kernel) via. a page fault. The page fault handler figures out what went wrong, loads the data from disk into RAM, maps the RAM into the virtual address space, then lets the program continue/retry as if nothing happened. The second time the CPU tries to execute the code the data is in RAM so it works fine.

Of course the OS has to know the reason why data at a virtual address wasn't present, which means that the OS has to keep track of extra information that the CPU doesn't have - if the virtual address actually isn't valid at all (e.g. NULL), or if the data is in swap space (and where), or if the data is part of a memory mapped file (and which offset of which file).

There is Virtual address space and a physical address space. virtual address space defines the address space of the program. say there is a program of 4GB. in that case we can represent the address space for that program as 32 bits. (2^32 = 4GB) from 0 to 0xFFFFFFFF.
this is the space the program thinks it has.

while compilation of the program, the program is given logical addresses based on the address space of the program.

after loading in to the memory. the program counter that is assign to this program will point to these addresses (logical/virtual addresses) and cpu will only want to fetch these addresses where the program instruction's are. cpu doesn't know where are the instructions located in the memory. that is up to MMU to translate the addresses.

the main thing is CPU doesn't actually generate these addresses, these are the addresses that where given to the program while compilation, using these, the instructions in the program reference each other. so cpu just see what program counter is pointing and generate / asks for these instruction. which are located in the physical memory.

when ever a address for fetching data or operand , instruction pointed by the PC, cpu call for these addresses.

The disk doesn't work with virtual addresses. It works with physical addresses directly reading/writing data in RAM. Disk transfers are started by writing memory mapped registers of the disk controller such as AHCI (https://www.intel.com/content/www/us/en/io/serial-ata/ahci.html), NVME or others.

Every memory mapped register and memory that's supposed to be used by the controller need to be uncached by changing some flags in the page tables. This is necessary because, if the data is cached, then it is hard to keep consistency with what is actually written. The CPU might cache the data before actually writing it in RAM which prevent proper operations of the controller.

The page tables are simply written to access the physical memory related to the controller such as the PCI-e configuration space (registers) and other memory in use. The OS can decide to relate any virtual address to the physical memory in use by the controller as long as the accesses are not cached.

The paging mechanism does make it look like there is contiguous memory which is bigger than available physical memory but the underlying mechanism isn't that simple. The OS is responsible to give that appearance by setting the present flag on individual pages and using the page fault exception mechanism to be notified in case a page was swapped to the hard-disk. For example, Linux saves a pointer to a structure in the page table entry itself which gives information to locate the swapped pages on the hard-disk (https://www.kernel.org/doc/gorman/html/understand/understand014.html).

Once a page fault occurs, the OS determines if the process has that memory region of the virtual address space allocated by looking in its PCB. If it is the case, then it must mean that the page was swapped on the hard-disk so it can look at the pointer stored in that corresponding entry of the page table to find the page. It must then find a page which is less likely to be used right now to swap that page and bring back the one that the process wants to access.