Add BIOS and tiny kernel roadmap phases

Tutorial/phase-18-privilege-sv32/phase-18.md

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+# 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
+