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Chronos: RISC-V Interrupt & Scheduler Playground

Chronos là một project nhỏ để học interrupts và multitasking trên RISC-V. Hỗ trợ:

  • CLINT (Core-Local Interruptor) - Timer interrupts
  • PLIC (Platform-Level Interrupt Controller) - External interrupts
  • Preemptive Scheduler - Context switching và round-robin scheduling

Cấu trúc thư mục

chronos/
├── Makefile
├── linker.ld
├── src/
│   ├── asm/
│   │   ├── _start.S
│   │   └── trap.S
│   ├── kernel.c
│   ├── modules/
│   │   ├── clint/
│   │   │   ├── clint.c
│   │   │   └── clint.h
│   │   ├── plic/
│   │   │   ├── plic.c
│   │   │   └── plic.h
│   │   ├── uart/
│   │   │   ├── uart.c
│   │   │   └── uart.h
│   │   └── sched/
│   │       ├── sched.c
│   │       ├── sched.h
│   │       ├── context.h
│   │       └── switch.S
│   └── include/
│       └── defs.h
└── docs/
    ├── README.md
    ├── clint_vs_plic.md
    └── scheduler.md

Features

Modular Architecture

  • Clean separation of kernel subsystems
  • Each module is self-contained with its own header
  • Easy to add new modules

Scheduler Module

  • Context switching using RISC-V callee-saved registers
  • Cooperative scheduling - tasks can yield voluntarily
  • Preemptive scheduling - timer-driven task switches
  • Round-robin between 2 tasks (easily extensible)
  • See docs/scheduler.md for details

Interrupt Support

  • CLINT for timer interrupts (M-mode)
  • PLIC for external interrupts (UART, etc.)
  • Trap handler with interrupt/exception routing

Build & Run

make
make run

Expected output:

[CLINT] timer tick, task=0
[CLINT] timer tick, task=1
[CLINT] timer tick, task=0
[CLINT] timer tick, task=1
...

The task ID alternates showing preemptive multitasking is working!

Diagram

graph TD
    subgraph CPU["CPU (Hart 0)"]
        TRAP[Trap Handler]
    end

    CLINT["CLINT (Timer/Software)"]
    PLIC["PLIC (External IRQs)"]
    UART["UART (16550)"]

    CLINT -->|MTIP/MSIP| TRAP
    PLIC -->|MEIP| TRAP
    UART -->|IRQ 10| PLIC
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