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E.3.4. Redirecting character output to hardware devices

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 Software Interrupt instruction (SWI). If the image is being executed without a debugger (or the debugger is not capturing semihosting calls) the value returned by this SWI 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 SWI to the debugger console or directly to a hardware device).

Supported devices for character output are:

  • :UART-0 (default destination if debugger is not capturing semihosting calls)

  • :UART-1

  • :UART-2

  • :UART-3.

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

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