Lab7: On-Chip Debugger + semihosting

預期目標

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
- Semihosting (半主機)
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
- The semihosting interface
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

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

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.”

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