semihost_hardfault.c

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// ****************************************************************************
// semihost_hardfault.c
//                - Provides hard fault handler to allow semihosting code not
//                  to hang application when debugger not connected.
//
// ****************************************************************************
// Copyright 2017-2020 NXP
// All rights reserved.
//
// NXP Confidential. This software is owned or controlled by NXP and may only be 
// used strictly in accordance with the applicable license terms.  
//
// By expressly accepting such terms or by downloading, installing, activating 
// and/or otherwise using the software, you are agreeing that you have read, and 
// that you agree to comply with and are bound by, such license terms.  
// 
// If you do not agree to be bound by the applicable license terms, then you may not 
// retain, install, activate or otherwise use the software.
 
// ****************************************************************************
//
//                       ===== DESCRIPTION =====
//
// One of the issues with applications that make use of semihosting operations
// (such as printf calls) is that the code will not execute correctly when the
// debugger is not connected. Generally this will show up with the application
// appearing to just hang. This may include the application running from reset
// or powering up the board (with the application already in FLASH), and also
// as the application failing to continue to execute after a debug session is
// terminated.
//
// The problem here is that the "bottom layer" of the semihosted variants of
// the C library, semihosting is implemented by a "BKPT 0xAB" instruction.
// When the debug tools are not connected, this instruction triggers a hard
// fault - and the default hard fault handler within an application will
// typically just contains an infinite loop - causing the application to
// appear to have hang when no debugger is connected.
//
// The below code provides an example hard fault handler which instead looks
// to see what the instruction that caused the hard fault was - and if it
// was a "BKPT 0xAB", then it instead returns back to the user application.
//
// In most cases this will allow applications containing semihosting
// operations to execute (to some degree) when the debugger is not connected.
//
// == NOTE ==
//
// Correct execution of the application containing semihosted operations
// which are vectored onto this hard fault handler cannot be guaranteed. This
// is because the handler may not return data or return codes that the higher
// level C library code or application code expects. This hard fault handler
// is meant as a development aid, and it is not recommended to leave
// semihosted code in a production build of your application!
//
// ****************************************************************************
 
// Allow handler to be removed by setting a define (via command line)
#if !defined (__SEMIHOST_HARDFAULT_DISABLE)
 
__attribute__((naked))
void HardFault_Handler(void){
    __asm(  ".syntax unified\n"
        // Check which stack is in use
            "MOVS   R0, #4  \n"
            "MOV    R1, LR  \n"
            "TST    R0, R1  \n"
            "BEQ    _MSP    \n"
            "MRS    R0, PSP \n"
            "B  _process      \n"
            "_MSP:  \n"
            "MRS    R0, MSP \n"
        // Load the instruction that triggered hard fault
        "_process:     \n"
            "LDR    R1,[R0,#24] \n"
            "LDRH    R2,[r1] \n"
        // Semihosting instruction is "BKPT 0xAB" (0xBEAB)
            "LDR    R3,=0xBEAB \n"
            "CMP     R2,R3 \n"
            "BEQ    _semihost_return \n"
        // Wasn't semihosting instruction so enter infinite loop
            "B . \n"
        // Was semihosting instruction, so adjust location to
        // return to by 1 instruction (2 bytes), then exit function
        "_semihost_return: \n"
            "ADDS    R1,#2 \n"
            "STR    R1,[R0,#24] \n"
        // Set a return value from semihosting operation.
        // 32 is slightly arbitrary, but appears to allow most
        // C Library IO functions sitting on top of semihosting to
        // continue to operate to some degree
                    "MOVS   R1,#32 \n"
                    "STR R1,[ R0,#0 ] \n" // R0 is at location 0 on stack
        // Return from hard fault handler to application
            "BX LR \n"
        ".syntax divided\n") ;
}
 
#endif