93 lines
4.7 KiB
Markdown
93 lines
4.7 KiB
Markdown
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---
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title: "page_map.h"
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summary: "virtual memory management"
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---
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Virtual memory spaces are a bit more tricky than the physical memory space.
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To understand them, we have to understand first, that the physical memory space is divided into so-called **pages** / **page frames**.
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These pages have a size of 4KB.
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A virtual memory space is a table of page mappings.
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Per default there are no pages mapped to such a table.
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When the OS maps a page to a **page table**, it says:
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"This page is now accessible from this virtual space, at this address".
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When the Computer is in paging mode, only mapped pages are accessible.
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Now every Process gets its own page table and tada: we have successfully isolated the processes from each other,
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because every process can only access the data that it needs to access.
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# `VIRTUAL_ADDRESS_MAX` - macro
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The highest mappable virtual address.
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4 level page maps have a maximum address space of 256TB.
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# `page_map_flag_E` - enum
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- **Present** - This indicates if the entry is used or should be ignored. Automatically set when mapping a page.
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- **Read & Write** - A mapped Page is always readable. This flag allows writing to that page.
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- **User Super** - If set, user mode access to the page is allowed.
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- **Write Through** - Enables _Write Through Caching_ for this page.
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- **Cache Disabled** - If this bit is set, the page won't be cached.
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- **Accessed** - Set by the CPU, when this PDE or PTE was read. Won't be reset by the CPU.
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- **Dirty** - Set when the page has been modified.
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- **Larger Pages** - When this bit is set in a PDE or PTE, the entry points to a 1GB or 2MB page.
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- **Custom 1 - 3** - Not used in NoxOS.
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- **No Execute** - When this bit is set, the CPU won't execute code that lies in that page.
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# `page_map_T` - struct [page aligned]
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This struct contains 512 entries.
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These entries contain an address and flags.
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The addresses link like this:
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- **PML4** --> **Page Directory** or _1GB Page_
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- **Page Directory** --> **Page Table** or _2MB Page_
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- **Page Table** --> _4KB Page_
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A pointer to a [page_map_T](https://nerdcult.net/projects/noxos/docs/codebase/mm/page_map.h/#page_map_t---struct-page-aligned) can be loaded into `cr3` to load this pagemap.
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# `page_map_create()` - function (page_map_T*)
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Allocates a [page_map_T](https://nerdcult.net/projects/noxos/docs/codebase/mm/page_map.h/#page_map_t---struct-page-aligned) and returns a pointer to it.
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The page map is populated with the mappings for the kernel executable and the loaded IDT.
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# `page_map_fetch_current()` - function (page_map_T*) [ASM implementation]
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This function will return the page map, that is currently loaded.
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To achieve this, it just reads the `cr3` register.
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# `page_map_load(page_map*)` - function (void) [ASM implementation]
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Loads the given page map.
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To achieve this, it writes the `cr3` register.
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# `page_map_map_memory(page_map*, virtual_address, physical_address, flags)` - function (void)
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This maps **_physical_address_** to **_virtual_address_** in **_page_map_**.
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The **_flags_** will be applied to the page mapping / page table entry.
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It always applies the _Present_ flag automatically.
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# `page_map_unmap_memory(page_map*, virtual_address)` - function (void)
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Removes a page mapping from the **_page_map_**.
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Page map structure intern pages won't be checked if they're still needed or not.
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# `page_map_get_physical_address(page_map*, virtual_address)` - function (void*)
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Returns the physical address of the page, that is mapped to **_virtual_address_**.
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# `page_map_destruct(page_map*)` - function (void)
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Clears a page map and frees all page map structure intern pages.
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# `page_map_dump_info(page_map)` - function (void)
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Logs information about **_page_map_**, including a map of all mapped regions.
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A region is a block of continuously mapped pages.
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# `page_map_entry_set_flags(entry, uint64_t flags)` - function (void)
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This will set the provided flags to a page map entry.
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# `page_map_entry_get_flag(entry, page_map_flag_E flag)` - function (bool)
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Returns if the given flag is set in the page map entry, or not.
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# `page_map_entry_set_address(entry, void* address)` - function (void)
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This will set the provided address to a page map entry.
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# `page_map_entry_get_address(entry)` - function (void*)
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This will read and return the address set in the page map entry.
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# `paging_init()` - function (void)
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Initializes paging.
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This reads the current page map set by the kernel and writes it to [g_kernel_page_map](https://nerdcult.net/projects/noxos/docs/codebase/mm/page_map.h/#g_kernel_page_map---global-variable).
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# `g_kernel_page_map` - global variable
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The kernels page map. This page map is provided by the bootloader and read from `cr3` at [paging_init](https://nerdcult.net/projects/noxos/docs/codebase/mm/page_map.h/#paging_init---function-void).
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