> For the complete documentation index, see [llms.txt](https://xiayingp.gitbook.io/build_a_os/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://xiayingp.gitbook.io/build_a_os/virtual-memory/page-table/part-1-how-to-translate-address.md).

# Part 1: How to translate address

## How to translate address

a RISC-V page table is logically an array of 2^27 (134,217,728) page table entries (PTEs). The paging hardware *translates a virtual address by using the top 27 bits of the 39 bits to index into the page table to find a PTE*, and making a 56-bit physical address whose top 44 bits come from the PPN in the PTE and whose bottom 12 bits are copied from the original virtual address. Thus a page table gives the operating system control over virtual-to-physical address translations at the granularity of *aligned chunks of 4096 (2^12) bytes*. Such a chunk is called a page.

![](/files/-M4bud-m8TZ1GnujEIqb)

* The total supported number of pages are: 2^27
* Each size is 2^12.
* GB size is 2^30.
* MAXVA is actually one bit less than the max allowed by Sv39, to avoid having to sign-extend virtual addresses that have the high bit set.

  `#define MAXVA (1L << (9 + 9 + 9 + 12 - 1))`
* So the total virtual memory can support up to 2^(27+12-30 - 1) => 2^8.&#x20;
* The available virtual memory is 256 GB.

the actual translation happens in three steps.

![](/files/-M4bufr5-ZZ8anm7y3Ry)

kernel must write the physical address of the root page-table page into the `satp` register. Each CPU has its own `satp`.

### Open questions:

#### Why kernel maps stacks via high-memory mappings?

Read: Reference: “Understanding the Linux Kernel”, third edition - sections “8.1.3. Memory Zones” and “8.1.6. Kernel Mappings of High-Memory Page Frames”.

For 32 bit machines: Page frames above the 896 MB boundary are not generally mapped in the fourth gigabyte of the kernel linear address spaces, so the kernel is unable to directly access them.


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