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It is useful to be aware of the default behavior of the compilation tools if you do not yet know the full technical specifications of the target hardware.
For example, when you start work on software for an embedded application, you might not know the details of target peripheral devices, the memory map, or even the processor itself.
To enable you to proceed with software development before such details are known, the compilation tools have a default behavior that enables you to start building and debugging application code immediately.
In the ARM C library, support for some ISO C functionality, for example program I/O, is provided by the host debugging environment. The mechanism that provides this functionality is known as semihosting. When semihosting is executed, the debug agent suspends program execution. The debug agent then uses the debug capabilities of the host (for example printf output to debugger console) to service the semihosting operation before code execution is resumed on the target. The task performed by the host is transparent to the program running on the target.