imple
b008b37d49
Add a Tutorial tree that mirrors the roadmap from Phase 0 through Linux bring-up. Each phase and subphase gets a short learning note with consistent sections for context, goals, new concepts, mental model, learning tasks, pitfalls, tooling, testing, and references. The tutorial material is intentionally explanatory rather than implementation code. It gives a systems-oriented learner enough FPGA, SystemVerilog, RISC-V, firmware, and Linux bring-up context to approach each roadmap phase without turning the notes into copy-paste RTL.
1.8 KiB
1.8 KiB
Phase 9 - CSRs + M-Mode Trap Handling
Context
This phase adds architectural control registers and the mechanism for exceptions and interrupts. The CPU stops merely halting on faults and starts transferring control to software handlers.
Goals
- Implement key machine-mode CSRs.
- Implement CSR instructions and
mret. - Add trap entry for illegal instructions, breakpoints, ecalls, and access faults.
New Concepts
- CSR: control and status register, special architectural register accessed by CSR instructions.
- Trap: synchronous exception or asynchronous interrupt that redirects execution.
mepc: CSR holding PC to return to after a trap.mcause: CSR describing why the trap occurred.mtvec: CSR holding trap handler address.mret: instruction returning from machine-mode trap.
How To Think About It
Traps are controlled context switches within the CPU. Hardware records enough state for software to inspect the cause, handle it, and resume or terminate.
Learning Tasks
- Draw the exact state updates on trap entry.
- Write a table of each exception source and its
mcause. - Understand which PC is saved for each exception type.
Pitfalls
- Saving the wrong PC in
mepc. - Updating architectural state for a faulting instruction that should not retire.
- Treating all exceptions as illegal instructions.
Tooling And Testing
- Create tiny programs that intentionally trigger one exception at a time.
- Inspect CSR values after trap entry in simulation.
- Test
mretreturning to a known instruction address.
References
- RISC-V privileged architecture spec: https://riscv.org/technical/specifications/
- RISC-V CSR instruction semantics: https://riscv.org/technical/specifications/
- RISC-V educational trap handling notes: https://osblog.stephenmarz.com/