RISC-V/Paging: Difference between revisions
(Created page with "Paging in RISC-V can be enabled by first setting up a page table, then pointing the CPU to that page table via the <code>satp</code> CSR. This article will describe these steps. == RISC-V Page Table Layout == There are various paging strategies available in the RISC-V ISA, differing by the number of virtual address bits: {| class="wikitable" |- ! Name !! Memory Size !! SXLEN |- | Sv32 || 4GiB || 32-bit only |- | Sv39 || 512GiB || 64-bit only |- |...") |
(Describe PPNs and VPNs) |
||
| Line 1: | Line 1: | ||
Paging in RISC-V can be enabled by first setting up a page table, then pointing the CPU to that page table via the [[RISC-V/CSR#satp | <code>satp</code> CSR]]. This article will describe these steps. | Paging in RISC-V can be enabled by first setting up a page table, then pointing the CPU to that page table via the [[RISC-V/CSR#satp | <code>satp</code> CSR]]. This article will describe these steps. | ||
== Terminology == | |||
There are many acronyms and other terms that are used in the RISC-V specification which, for the purpose of brevity and consistency, will be used in this article as well. I will assume a baseline knowledge of the purpose and basics of paging and virtual addressing. | |||
=== PPNs and VPNs === | |||
A physical page number (or PPN) is a unique, page-aligned identifier for a page in physical memory. The number of PPN segments for an address will depend on the paging mode. | |||
To construct a PPN from a physical address, you can simply (logically, not arithmetically) shift the value right by 12 bits (as 2**12 = 4096). Note that this operation will truncate the offset into the page, if the address was not page-aligned. | |||
A virtual page number (or VPN) is very similar to a PPN, but in the virtual address space. Again, the number of VPN segments for a virtual address will depend on the paging mode. | |||
== RISC-V Page Table Layout == | == RISC-V Page Table Layout == | ||
There are various paging | There are various paging modes available in the RISC-V ISA, differing by the number of virtual address bits: | ||
{| class="wikitable" | {| class="wikitable" | ||
|- | |- | ||
| Line 17: | Line 29: | ||
|} | |} | ||
Feel free to choose between these | Feel free to choose between these modes as you see fit; this page will describe all of them. Note that in 64-bit systems, Sv39 is more than enough for most hobby operating systems (though it is not much more difficult to go further). | ||
=== Sv32 === | === Sv32 === | ||
Revision as of 02:53, 24 November 2022
Paging in RISC-V can be enabled by first setting up a page table, then pointing the CPU to that page table via the satp CSR. This article will describe these steps.
Terminology
There are many acronyms and other terms that are used in the RISC-V specification which, for the purpose of brevity and consistency, will be used in this article as well. I will assume a baseline knowledge of the purpose and basics of paging and virtual addressing.
PPNs and VPNs
A physical page number (or PPN) is a unique, page-aligned identifier for a page in physical memory. The number of PPN segments for an address will depend on the paging mode.
To construct a PPN from a physical address, you can simply (logically, not arithmetically) shift the value right by 12 bits (as 2**12 = 4096). Note that this operation will truncate the offset into the page, if the address was not page-aligned.
A virtual page number (or VPN) is very similar to a PPN, but in the virtual address space. Again, the number of VPN segments for a virtual address will depend on the paging mode.
RISC-V Page Table Layout
There are various paging modes available in the RISC-V ISA, differing by the number of virtual address bits:
| Name | Memory Size | SXLEN |
|---|---|---|
| Sv32 | 4GiB | 32-bit only |
| Sv39 | 512GiB | 64-bit only |
| Sv48 | 256TiB | 64-bit only |
| Sv57 | 128PiB | 64-bit only |
Feel free to choose between these modes as you see fit; this page will describe all of them. Note that in 64-bit systems, Sv39 is more than enough for most hobby operating systems (though it is not much more difficult to go further).
Sv32
TODO
Sv39
TODO
Sv48
TODO
Sv57
TODO
Enabling Paging
TODO