版本 b4b50d88d06cc2dbfbd3143a8b8cd65fe0989d6b

Lab7: On-Chip Debugger + semihosting

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
- http://albert-oma.blogspot.tw/2012/04/semihosting.html
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
- http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0205g/Bgbjhiea.html
FreeRTOS extensions: https://github.com/hugovincent/mbed-freertos
- 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
- http://balau82.wordpress.com/2010/11/04/qemu-arm-semihosting/

.. code-block:: prettyprint

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: http://www.youtube.com/watch?v=NSVOuHToJ6c
Beginner’s look at On-Chip Debugging: http://hackaday.com/2012/09/27/beginners-look-at-on-chip-debugging/
Architecture and Core Commands of OpenOCD
- http://openocd.sourceforge.net/doc/html/Architecture-and-Core-Commands.html
- 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：

.. code-block:: prettyprint

factorial(0) = 1
factorial(1) = 1
factorial(2) = 2
...
factorial(9) = 362880