前言 – What is semihosting?

Semihosting 是 ARM 中的一個機制，使得 ARM target 可以藉由運行 debugger 的 host computer 的 Input/Output 裝置來進行溝通。

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。[1]

Semihosting Interface

Semihosting 透過 SVC 指令來調用相關的命令，以下為三種不同的使用方式：

SVC 0x123456: In ARM state for all architectures.
SVC 0xAB: In ARM state and Thumb state, excluding ARMv6-M and ARMv7-M. This behavior is not guaranteed on all debug targets from ARM or from third parties.
BKPT 0xAB: For ARMv6-M and ARMv7-M, Thumb state only.

同時，Operations type 被存放於 R0 暫存器，parameters 之位置存放於 R1 暫存器

可用的 semihosting operation number 如下列表：

0x00-0x31: Used by ARM.
0x32-0xFF: Reserved for future use by ARM.
0x100-0x1FF: Reserved for user applications. These are not used by ARM.
If you are writing your own SVC operations, however, you are advised to use a different SVC number rather than using the semihosted SVC number and these operation type numbers.
0x200-0xFFFFFFFF: Undefined and currently unused. It is recommended that you do not use these.