So, till now we are done with the introduction part of the Paging Library. We will use the map_to function of the Mapper trait for our implementation, so let's take a look at that function first. virt = physical_memory_offset + frame.start_address().as_u64(); // read the page table entry and update `frame`, frame, When we try to map a page for that no level 1 table exists yet, the map_to function fails because it tries to allocate frames from the EmptyFrameAllocator for creating new page tables. Information may need to be copied to the swap area first. For example, the offset could be 10 TiB: By using the virtual memory in the range 10TiB..(10TiB + physical memory size) exclusively for page table mappings, we avoid the collision problems of the identity mapping. will ignore it. By writing to the identity-mapped level 1 table, our kernel can create up to 511 temporary mappings (512 minus the entry required for the identity mapping). In the second step, we will create a translation function that returns the physical address that a given virtual address is mapped to. Accessing other address spaces is still possible by changing the recursive entry, but a temporary mapping is required for switching back. At the end of the previous post, we tried to take a look at the page tables our kernel runs on, but failed since we couldn't access the physical frame that the CR3 register points to. Remember, the page table indexes are derived from the address in the following way: Let's assume that we want to access the level 1 page table that maps a specific page. The frame_allocator parameter uses the impl Trait syntax to be generic over all types that implement the FrameAllocator trait. Memory-mapped files, including running executables and shared This way, mapping 32 GiB of physical memory only requires 132 KiB for page tables since only one level 3 table and 32 level 2 tables are needed. addr.p4_index(), addr.p3_index(), addr.p2_index(), addr.p1_index() /// `physical_memory_offset`. need to consider priorities, and keep enough processes running Then it follows the recursive entry again and thinks that it reaches a level 2 table. This approach still has the disadvantage that we need to create a new mapping whenever we create a new page table. Allows process to share information as threads do. usable_regions = regions When we want to access the physical address 4 KiB, we can only do so through some virtual address that maps to it. virtual space. We then invoke the OffsetPageTable::new function with this reference. We don't need to use an unsafe block here because Rust treats the complete body of an unsafe fn like a large unsafe block. In this technique physical memory is broken into fixed-sized blocks called frames and logical memory is divided into blocks of the same size called pages. }; /// Returns a mutable reference to the active level 4 table. } Here, all the processes are divided into pages of 1 KB each so that operating system can store one page in one frame. Please can you help me with solutions to the below questions 1. At this point we no longer need our memory::translate_addr and memory::translate_addr_inner functions, so we can delete them. A read-only region can be mapped into multiple page tables. Reset the MMU to clear out information from the previous It heavily relies on the page table format of x86 and might not work on other architectures. The unused space (waste) in the last page will average. The best way to support me is to sponsor me on GitHub, since they don't charge any fees. Most common size is 4K; systems have used 512 bytes to 64K. It's worth noting that the last 12 bits always stay the same after translation, which makes sense because these bits are the page offset and not part of the translation. The unused space (waste) in the last page will average p/2. Implementing Basic Paging The Basics. On some architectures, partial results may need to Newer architectures are simpler to deal with than this. This will be essential for allocating memory or implementing multithreading in future posts. println! When the CPU now follows a different entry, it lands on a level 3 table but thinks it is already on a level 1 table. The page tables are stored in physical memory frames, indicated by the dashed lines. Before returning that frame, we increase by one so that we return the following frame on the next call. // the virtual address whose corresponding page tables you want to access, // sign extension By doing this, we effectively reserve a part of the virtual address space and map all current and future page table frames to that space. The virtual address space contains a single mapped page at address 0x803fe00000, marked in blue. To construct such addresses in Rust code, you can use bitwise operations: The above code assumes that the last level 4 entry with index 0o777 (511) is recursively mapped. But how do we know which frames are unused and how much physical memory is available? be reversed. information (including the PC), returns to user space, and the present. For our implementation, we first manually traversed the page tables to implement a translation function, and then used the MappedPageTable type of the x86_64 crate. First, we will take a look at the currently active page tables that our kernel runs on. Paging with Example In Operating Systems, Paging is a storage mechanism used to retrieve processes from the secondary storage into the main memory in the form of pages. If that page is needed again But the usage of register for the page table is satisfactory only if page table is small.