Sv39

Sv39 is a 39-bit virtual memory addressing mode defined for the 64-bit RISC-V architecture. It provides a 39-bit virtual address space and uses a three-level page table with 4 KiB pages. Sv39 is the common address translation scheme for RV64 implementations and is widely used by Linux on RISC-V.

Virtual address format in Sv39 partitions a 39-bit virtual address into three 9-bit VPN fields and a 12-bit page offset. Specifically, the top-level VPN selects entries in a root page table, the middle VPN selects entries in the next level, and the bottom VPN selects entries in the final level, with the 12-bit offset addressing the page’s bytes. The root page table base address and the mode are supplied by the satp register; for Sv39, the mode field designates Sv39, and satp holds the physical address of the root page table.

Page table entries (PTEs) are 64 bits wide. Each PTE contains standard permission and status bits (valid, readable, writable, executable, user-accessible, global, accessed, dirty) along with a set of bits that form the physical page number (PPN). The PPN, combined with the PTE level, determines whether the entry points to the next-level page table or to the final physical page. Sv39 supports 4 KiB pages and can use larger pages (2 MiB and 1 GiB) via appropriate leaf PTEs at higher levels.

Translation proceeds by walking the three levels of page tables using the VPN components; if a required PTE is invalid or lacks proper permissions, a page fault is raised. On successful translation, the TLB may be populated with the resulting mapping and access/dirty bits are updated.

Sv39 also defines canonical, sign-extended virtual addresses and interacts with the processor’s privilege model and memory protection features. It is one of several RV64 paging schemes, alongside Sv39x and Sv48/ Sv57 variants.