Minor Page Fault:
If the page is loaded in memory at the time the fault is generated, but is not marked in the memory management unit as being loaded in memory, then it is called a minor or soft page fault. The page fault handler in the operating system merely needs to make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory; it does not need to read the page into memory. This could happen if the memory is shared by different programs and the page is already brought into memory for other programs. The page could also have been removed from a process's Working Set, but not yet written to disk or erased, such as in operating systems that use Secondary Page Caching.
Major Page Fault:
This is the mechanism used by an operating system to increase the amount of program memory available on demand. The operating system delays loading parts of the program from disk until the program attempts to use it and the page fault is generated. If the page is not loaded in memory at the time of the fault, then it is called a major or hard page fault. The page fault handler in the OS needs to find a free location: either a page in memory, or another non-free page in memory. This latter might be used by another process, in which case the OS needs to write out the data in that page (if it hasn't been written out since it was last modified) and mark that page as not being loaded in memory in its process page table.
Invalid Page Fault:
If a page fault occurs for a reference to an address that's not part of the virtual address space, so that there can't be a page in memory corresponding to it, then it is called an invalid page fault. The page fault handler in the operating system then needs to terminate the code that made the reference, or deliver an indication to that code that the reference was invalid. A null pointer is usually represented as a pointer to address 0 in the address space; many operating systems set up the memory management unit to indicate that the page that contains that address is not in memory, and do not include that page in the virtual address space, so that attempts to read or write the memory referenced by a null pointer get an invalid page fault.
If the page is loaded in memory at the time the fault is generated, but is not marked in the memory management unit as being loaded in memory, then it is called a minor or soft page fault. The page fault handler in the operating system merely needs to make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory; it does not need to read the page into memory. This could happen if the memory is shared by different programs and the page is already brought into memory for other programs. The page could also have been removed from a process's Working Set, but not yet written to disk or erased, such as in operating systems that use Secondary Page Caching.
Major Page Fault:
This is the mechanism used by an operating system to increase the amount of program memory available on demand. The operating system delays loading parts of the program from disk until the program attempts to use it and the page fault is generated. If the page is not loaded in memory at the time of the fault, then it is called a major or hard page fault. The page fault handler in the OS needs to find a free location: either a page in memory, or another non-free page in memory. This latter might be used by another process, in which case the OS needs to write out the data in that page (if it hasn't been written out since it was last modified) and mark that page as not being loaded in memory in its process page table.
Invalid Page Fault:
If a page fault occurs for a reference to an address that's not part of the virtual address space, so that there can't be a page in memory corresponding to it, then it is called an invalid page fault. The page fault handler in the operating system then needs to terminate the code that made the reference, or deliver an indication to that code that the reference was invalid. A null pointer is usually represented as a pointer to address 0 in the address space; many operating systems set up the memory management unit to indicate that the page that contains that address is not in memory, and do not include that page in the virtual address space, so that attempts to read or write the memory referenced by a null pointer get an invalid page fault.
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