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Lab7: On-Chip Debugger + semihosting

預期目標

  • 學習 OpenOCD 的操作與擴充
  • ARM Semihosting 機制的應用

ARM Semihost

  • 使用 ARM 所開發的目標系統 (target system),不一定會提供所有的輸入/輸出設備。因此 ARM 設計了 semihost 這種機制,讓運行 ARM debugger 的主機可以與目標系統進行 I/O 溝通,以利產品開發
  • Semihost 的實作是透過使用定義好的軟體中斷 (SVCs),使程式在執行過程中產生中斷。一旦目標系統上的程式呼叫到對應的指令 (semihosting call),便產生軟體中斷,接著 Debug Agent 就會負責處理此中斷,進行與主機的溝通
  • FreeRTOS extensions
  • lib/semifs.c
  • mbed (mbed.org) target supports semihosted local filesystem, accessible via the mbed USB interface. Should also work with semihosting-compatible debuggers.

  • QEMU ARM semihosting

User-mode Emulation

  • 透過模擬器執行 printf() 一類需要系統呼叫的 ARM 程式

    sudo apt-get install qemu-user-static
    cd Labs/Lab7
    make factorial
    ./factorial

OpenOCD + STM32F4

  • OpenOCD 的安裝請見 Lab-6
  • ARM On-Chip Debugging demonstration
  • Beginner’s look at On-Chip Debugging
  • Architecture and Core Commands of OpenOCD
    • Command: arm semihosting
      • Display status of semihosting, after optionally changing that status.
      • “Semihosting allows for code executing on an ARM target to use the I/O facilities on the host computer i.e. the system where OpenOCD is running. The target application must be linked against a library implementing the ARM semihosting convention that forwards operation requests by using a special SVC instruction that is trapped at the Supervisor Call vector by OpenOCD.”

作業

  • 適度修改 Makefile 與 OpenOCD 設定檔,使 Host 端可以自動下載 ELF image 到 STM32F4-Discovery 環境中,並且執行程式,預期將輸出以下:

    factorial(0) = 1
    factorial(1) = 1
    factorial(2) = 2
    ...
    factorial(9) = 362880
  • factorial.c 的 printf() 呼叫,將改以 semihost 方式執行
  • 評分測試方法:
  • cd Labs/Lab7 && make all auto