The redirection of character I/O is carried out within the Boot Monitor platform library routines in retarget.c and boot.s. During startup, the platform library executes a SuperVisor Call (SVC). If the image is being executed without a debugger (or the debugger is not capturing semihosting calls) the value returned by this SVC is -1, otherwise the value returned is positive. The platform library uses the return value to determine the hardware device used for outputting from the C library I/O functions. (Redirection is through a SVC to the debugger console or directly to a hardware device)

Supported devices for character output are:

The STDIO calls are redirected within retarget.c. Redirection depends on the setting of switches S6-2 and S6-3, see Boot Monitor configuration switches for details.