Identity Paging

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Identity Paging, Identity Mapped Paging and 1:1 Paging are terms often used on the forum to describe a design choice where a portion of virtual addresses are mapped to physical addresses that have the same value. This means that if paging is enabled with identity paging, 0xb8000 is 0xb8000, as long as that area is identity mapped.

Advantages

When switching to paged protected mode, your 1:1 mapping region doesn't care of whether paging is enabled or disabled. Placing your switching code and important data such as the core page directory and a few system page tables in this region gives you an easier way to set up paging without headaches.

Example

Let's say you decide to use Identity Paging in the lowest 1MB. In this case vaddr 00000000..00000fff are mapped to frame #00000, vaddr 00001000..00001fff are mapped to frame #00001, and so on. (vaddr 000ff000..000fffff are mapped to frame #000ff)

You can easily do this with a loop filling the page table:

 void idpaging(uint32_t *first_pte, vaddr from, int size){
    from = from & 0xfffff000; // discard bits we don't want
    for(;size>0;from+=4096,size-=4096,first_pte++){
       *first_pte=from|1;     // mark page present.
    }
 }
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