# Phase 18 - S-Mode, U-Mode, Sv32 Virtual Memory

## Context

Linux normally runs with privilege separation and virtual memory. This phase adds
supervisor/user modes and Sv32 address translation.

## Goals

- Add supervisor and user privilege modes.
- Implement Sv32 page-table walking.
- Add S-mode CSRs and page-fault traps.

## New Concepts

- Privilege mode: execution level controlling access rights.
- S-mode: supervisor mode, where the OS kernel normally runs.
- U-mode: user mode, where applications normally run.
- MMU: memory management unit performing address translation and protection.
- Sv32: RISC-V 32-bit virtual-memory scheme with two-level page tables.
- TLB: translation lookaside buffer, cache of recent address translations.

## How To Think About It

Virtual memory is both translation and permission checking. A correct page walker that
ignores permissions is not enough for an OS.

## Learning Tasks

- Draw Sv32 virtual address fields, page table levels, and PTE format.
- Trace one virtual load through translation to physical memory.
- Learn which traps are page faults versus access faults.

## Pitfalls

- Confusing physical memory protection with virtual page permissions.
- Mishandling `satp` updates and `sfence.vma`.
- Letting user mode access supervisor-only pages.

## Tooling And Testing

- Start with hand-built page tables and one mapped page.
- Test instruction, load, and store page faults separately.
- Add trace visibility for virtual address, PTEs, physical address, and cause.

## References

- RISC-V privileged architecture spec, Sv32: https://riscv.org/technical/specifications/
- Linux memory-management docs: https://docs.kernel.org/mm/
- OSDev paging overview: https://wiki.osdev.org/Paging