FreeRTOS-Kernel/portable/GCC/ARM_CM3_MPU/port.c

/*
 * FreeRTOS Kernel <DEVELOPMENT BRANCH>
 * Copyright (C) 2021 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
 *
 * SPDX-License-Identifier: MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
 * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 * https://www.FreeRTOS.org
 * https://github.com/FreeRTOS
 *
 */

/*-----------------------------------------------------------
* Implementation of functions defined in portable.h for the ARM CM3 MPU port.
*----------------------------------------------------------*/

/* Defining MPU_WRAPPERS_INCLUDED_FROM_API_FILE prevents task.h from redefining
 * all the API functions to use the MPU wrappers.  That should only be done when
 * task.h is included from an application file. */
#define MPU_WRAPPERS_INCLUDED_FROM_API_FILE

/* Scheduler includes. */
#include "FreeRTOS.h"
#include "task.h"
#include "mpu_syscall_numbers.h"

#undef MPU_WRAPPERS_INCLUDED_FROM_API_FILE

#ifndef configSYSTICK_CLOCK_HZ
    #define configSYSTICK_CLOCK_HZ    configCPU_CLOCK_HZ
    /* Ensure the SysTick is clocked at the same frequency as the core. */
    #define portNVIC_SYSTICK_CLK      ( 1UL << 2UL )
#else

/* The way the SysTick is clocked is not modified in case it is not the same
 * as the core. */
    #define portNVIC_SYSTICK_CLK    ( 0 )
#endif

#ifndef configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS
    #warning "configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS is not defined. We recommend defining it to 0 in FreeRTOSConfig.h for better security."
    #define configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS    1
#endif

/* Constants required to access and manipulate the NVIC. */
#define portNVIC_SYSTICK_CTRL_REG                 ( *( ( volatile uint32_t * ) 0xe000e010 ) )
#define portNVIC_SYSTICK_LOAD_REG                 ( *( ( volatile uint32_t * ) 0xe000e014 ) )
#define portNVIC_SYSTICK_CURRENT_VALUE_REG        ( *( ( volatile uint32_t * ) 0xe000e018 ) )
#define portNVIC_SHPR3_REG                        ( *( ( volatile uint32_t * ) 0xe000ed20 ) )
#define portNVIC_SHPR2_REG                        ( *( ( volatile uint32_t * ) 0xe000ed1c ) )
#define portNVIC_SYS_CTRL_STATE_REG               ( *( ( volatile uint32_t * ) 0xe000ed24 ) )
#define portNVIC_MEM_FAULT_ENABLE                 ( 1UL << 16UL )

/* Constants required to access and manipulate the MPU. */
#define portMPU_TYPE_REG                          ( *( ( volatile uint32_t * ) 0xe000ed90 ) )
#define portMPU_REGION_BASE_ADDRESS_REG           ( *( ( volatile uint32_t * ) 0xe000ed9C ) )
#define portMPU_REGION_ATTRIBUTE_REG              ( *( ( volatile uint32_t * ) 0xe000edA0 ) )
#define portMPU_CTRL_REG                          ( *( ( volatile uint32_t * ) 0xe000ed94 ) )
#define portEXPECTED_MPU_TYPE_VALUE               ( 8UL << 8UL ) /* 8 regions, unified. */
#define portMPU_ENABLE                            ( 0x01UL )
#define portMPU_BACKGROUND_ENABLE                 ( 1UL << 2UL )
#define portPRIVILEGED_EXECUTION_START_ADDRESS    ( 0UL )
#define portMPU_REGION_VALID                      ( 0x10UL )
#define portMPU_REGION_ENABLE                     ( 0x01UL )
#define portPERIPHERALS_START_ADDRESS             0x40000000UL
#define portPERIPHERALS_END_ADDRESS               0x5FFFFFFFUL

/* Constants required to access and manipulate the SysTick. */
#define portNVIC_SYSTICK_INT                      ( 0x00000002UL )
#define portNVIC_SYSTICK_ENABLE                   ( 0x00000001UL )
#define portMIN_INTERRUPT_PRIORITY                ( 255UL )
#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
#define portNVIC_SVC_PRI                          ( ( ( uint32_t ) configMAX_SYSCALL_INTERRUPT_PRIORITY - 1UL ) << 24UL )

/* Constants required to set up the initial stack. */
#define portINITIAL_XPSR                          ( 0x01000000 )
#define portINITIAL_EXC_RETURN                    ( 0xfffffffdUL )
#define portINITIAL_CONTROL_IF_UNPRIVILEGED       ( 0x03 )
#define portINITIAL_CONTROL_IF_PRIVILEGED         ( 0x02 )

/* Constants required to check the validity of an interrupt priority. */
#define portFIRST_USER_INTERRUPT_NUMBER           ( 16 )
#define portNVIC_IP_REGISTERS_OFFSET_16           ( 0xE000E3F0 )
#define portAIRCR_REG                             ( *( ( volatile uint32_t * ) 0xE000ED0C ) )
#define portMAX_8_BIT_VALUE                       ( ( uint8_t ) 0xff )
#define portTOP_BIT_OF_BYTE                       ( ( uint8_t ) 0x80 )
#define portMAX_PRIGROUP_BITS                     ( ( uint8_t ) 7 )
#define portPRIORITY_GROUP_MASK                   ( 0x07UL << 8UL )
#define portPRIGROUP_SHIFT                        ( 8UL )

/* Constants used during system call enter and exit. */
#define portPSR_STACK_PADDING_MASK                ( 1UL << 9UL )

/* Offsets in the stack to the parameters when inside the SVC handler. */
#define portOFFSET_TO_LR                          ( 5 )
#define portOFFSET_TO_PC                          ( 6 )
#define portOFFSET_TO_PSR                         ( 7 )

/* For strict compliance with the Cortex-M spec the task start address should
 * have bit-0 clear, as it is loaded into the PC on exit from an ISR. */
#define portSTART_ADDRESS_MASK                    ( ( StackType_t ) 0xfffffffeUL )

/* Does addr lie within [start, end] address range? */
#define portIS_ADDRESS_WITHIN_RANGE( addr, start, end ) \
    ( ( ( addr ) >= ( start ) ) && ( ( addr ) <= ( end ) ) )

/* Is the access request satisfied by the available permissions? */
#define portIS_AUTHORIZED( accessRequest, permissions ) \
    ( ( ( permissions ) & ( accessRequest ) ) == accessRequest )

/* Max value that fits in a uint32_t type. */
#define portUINT32_MAX    ( ~( ( uint32_t ) 0 ) )

/* Check if adding a and b will result in overflow. */
#define portADD_UINT32_WILL_OVERFLOW( a, b )    ( ( a ) > ( portUINT32_MAX - ( b ) ) )
/*-----------------------------------------------------------*/

/*
 * Configure a number of standard MPU regions that are used by all tasks.
 */
static void prvSetupMPU( void ) PRIVILEGED_FUNCTION;

/*
 * Return the smallest MPU region size that a given number of bytes will fit
 * into.  The region size is returned as the value that should be programmed
 * into the region attribute register for that region.
 */
static uint32_t prvGetMPURegionSizeSetting( uint32_t ulActualSizeInBytes ) PRIVILEGED_FUNCTION;

/*
 * Setup the timer to generate the tick interrupts.  The implementation in this
 * file is weak to allow application writers to change the timer used to
 * generate the tick interrupt.
 */
void vPortSetupTimerInterrupt( void );

/*
 * Standard FreeRTOS exception handlers.
 */
void xPortPendSVHandler( void ) __attribute__( ( naked ) ) PRIVILEGED_FUNCTION;
void xPortSysTickHandler( void )  __attribute__( ( optimize( "3" ) ) ) PRIVILEGED_FUNCTION;
void vPortSVCHandler( void ) __attribute__( ( naked ) ) PRIVILEGED_FUNCTION;

/*
 * Starts the scheduler by restoring the context of the first task to run.
 */
static void prvRestoreContextOfFirstTask( void ) __attribute__( ( naked ) ) PRIVILEGED_FUNCTION;

/*
 * C portion of the SVC handler.  The SVC handler is split between an asm entry
 * and a C wrapper for simplicity of coding and maintenance.
 */
void vSVCHandler_C( uint32_t * pulRegisters ) __attribute__( ( noinline ) ) PRIVILEGED_FUNCTION;

/**
 * @brief Checks whether or not the processor is privileged.
 *
 * @return 1 if the processor is already privileged, 0 otherwise.
 */
BaseType_t xIsPrivileged( void ) __attribute__( ( naked ) );

/**
 * @brief Lowers the privilege level by setting the bit 0 of the CONTROL
 * register.
 *
 * Bit 0 of the CONTROL register defines the privilege level of Thread Mode.
 *  Bit[0] = 0 --> The processor is running privileged
 *  Bit[0] = 1 --> The processor is running unprivileged.
 */
void vResetPrivilege( void ) __attribute__( ( naked ) );

/**
 * @brief Make a task unprivileged.
 */
void vPortSwitchToUserMode( void );

/**
 * @brief Enter critical section.
 */
#if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 )
    void vPortEnterCritical( void ) FREERTOS_SYSTEM_CALL;
#else
    void vPortEnterCritical( void ) PRIVILEGED_FUNCTION;
#endif

/**
 * @brief Exit from critical section.
 */
#if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 )
    void vPortExitCritical( void ) FREERTOS_SYSTEM_CALL;
#else
    void vPortExitCritical( void ) PRIVILEGED_FUNCTION;
#endif

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

/**
 * @brief Sets up the system call stack so that upon returning from
 * SVC, the system call stack is used.
 *
 * @param pulTaskStack The current SP when the SVC was raised.
 * @param ucSystemCallNumber The system call number of the system call.
 */
    void vSystemCallEnter( uint32_t * pulTaskStack,
                           uint8_t ucSystemCallNumber ) PRIVILEGED_FUNCTION;

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

/**
 * @brief Raise SVC for exiting from a system call.
 */
    void vRequestSystemCallExit( void ) __attribute__( ( naked ) ) PRIVILEGED_FUNCTION;

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

/**
 * @brief Sets up the task stack so that upon returning from
 * SVC, the task stack is used again.
 *
 * @param pulSystemCallStack The current SP when the SVC was raised.
 */
    void vSystemCallExit( uint32_t * pulSystemCallStack ) PRIVILEGED_FUNCTION;

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */

/**
 * @brief Checks whether or not the calling task is privileged.
 *
 * @return pdTRUE if the calling task is privileged, pdFALSE otherwise.
 */
BaseType_t xPortIsTaskPrivileged( void ) PRIVILEGED_FUNCTION;
/*-----------------------------------------------------------*/

/* Each task maintains its own interrupt status in the critical nesting
 * variable.  Note this is not saved as part of the task context as context
 * switches can only occur when uxCriticalNesting is zero. */
static UBaseType_t uxCriticalNesting = 0xaaaaaaaa;

#if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) )

/*
 * This variable is set to pdTRUE when the scheduler is started.
 */
    PRIVILEGED_DATA static BaseType_t xSchedulerRunning = pdFALSE;

#endif

/*
 * Used by the portASSERT_IF_INTERRUPT_PRIORITY_INVALID() macro to ensure
 * FreeRTOS API functions are not called from interrupts that have been assigned
 * a priority above configMAX_SYSCALL_INTERRUPT_PRIORITY.
 */
#if ( configASSERT_DEFINED == 1 )
    static uint8_t ucMaxSysCallPriority = 0;
    static uint32_t ulMaxPRIGROUPValue = 0;
    static const volatile uint8_t * const pcInterruptPriorityRegisters = ( const volatile uint8_t * const ) portNVIC_IP_REGISTERS_OFFSET_16;
#endif /* configASSERT_DEFINED */
/*-----------------------------------------------------------*/

/*
 * See header file for description.
 */
StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
                                     TaskFunction_t pxCode,
                                     void * pvParameters,
                                     BaseType_t xRunPrivileged,
                                     xMPU_SETTINGS * xMPUSettings )
{
    if( xRunPrivileged == pdTRUE )
    {
        xMPUSettings->ulTaskFlags |= portTASK_IS_PRIVILEGED_FLAG;
        xMPUSettings->ulContext[ 0 ] = portINITIAL_CONTROL_IF_PRIVILEGED;
    }
    else
    {
        xMPUSettings->ulTaskFlags &= ( ~( portTASK_IS_PRIVILEGED_FLAG ) );
        xMPUSettings->ulContext[ 0 ] = portINITIAL_CONTROL_IF_UNPRIVILEGED;
    }

    xMPUSettings->ulContext[ 1 ] = 0x04040404;                                        /* r4. */
    xMPUSettings->ulContext[ 2 ] = 0x05050505;                                        /* r5. */
    xMPUSettings->ulContext[ 3 ] = 0x06060606;                                        /* r6. */
    xMPUSettings->ulContext[ 4 ] = 0x07070707;                                        /* r7. */
    xMPUSettings->ulContext[ 5 ] = 0x08080808;                                        /* r8. */
    xMPUSettings->ulContext[ 6 ] = 0x09090909;                                        /* r9. */
    xMPUSettings->ulContext[ 7 ] = 0x10101010;                                        /* r10. */
    xMPUSettings->ulContext[ 8 ] = 0x11111111;                                        /* r11. */
    xMPUSettings->ulContext[ 9 ] = portINITIAL_EXC_RETURN;                            /* EXC_RETURN. */

    xMPUSettings->ulContext[ 10 ] = ( uint32_t ) ( pxTopOfStack - 8 );                /* PSP with the hardware saved stack. */
    xMPUSettings->ulContext[ 11 ] = ( uint32_t ) pvParameters;                        /* r0. */
    xMPUSettings->ulContext[ 12 ] = 0x01010101;                                       /* r1. */
    xMPUSettings->ulContext[ 13 ] = 0x02020202;                                       /* r2. */
    xMPUSettings->ulContext[ 14 ] = 0x03030303;                                       /* r3. */
    xMPUSettings->ulContext[ 15 ] = 0x12121212;                                       /* r12. */
    xMPUSettings->ulContext[ 16 ] = 0;                                                /* LR. */
    xMPUSettings->ulContext[ 17 ] = ( ( uint32_t ) pxCode ) & portSTART_ADDRESS_MASK; /* PC. */
    xMPUSettings->ulContext[ 18 ] = portINITIAL_XPSR;                                 /* xPSR. */

    #if ( configUSE_MPU_WRAPPERS_V1 == 0 )
    {
        /* Ensure that the system call stack is double word aligned. */
        xMPUSettings->xSystemCallStackInfo.pulSystemCallStack = &( xMPUSettings->xSystemCallStackInfo.ulSystemCallStackBuffer[ configSYSTEM_CALL_STACK_SIZE - 1 ] );
        xMPUSettings->xSystemCallStackInfo.pulSystemCallStack = ( uint32_t * ) ( ( uint32_t ) ( xMPUSettings->xSystemCallStackInfo.pulSystemCallStack ) &
                                                                                 ( uint32_t ) ( ~( portBYTE_ALIGNMENT_MASK ) ) );

        /* This is not NULL only for the duration of a system call. */
        xMPUSettings->xSystemCallStackInfo.pulTaskStack = NULL;
    }
    #endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */

    return &( xMPUSettings->ulContext[ 19 ] );
}
/*-----------------------------------------------------------*/

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

    void vPortSVCHandler( void ) /* __attribute__( ( naked ) ) PRIVILEGED_FUNCTION */
    {
        __asm volatile
        (
            ".syntax unified                \n"
            ".extern vSVCHandler_C          \n"
            ".extern vSystemCallEnter       \n"
            ".extern vSystemCallExit        \n"
            "                               \n"
            "tst lr, #4                     \n"
            "ite eq                         \n"
            "mrseq r0, msp                  \n"
            "mrsne r0, psp                  \n"
            "                               \n"
            "ldr r2, [r0, #24]              \n"
            "ldrb r1, [r2, #-2]             \n"
            "cmp r1, %0                     \n"
            "blt vSystemCallEnter           \n"
            "cmp r1, %1                     \n"
            "beq vSystemCallExit            \n"
            "b vSVCHandler_C                \n"
            "                               \n"
            : /* No outputs. */
            : "i" ( NUM_SYSTEM_CALLS ), "i" ( portSVC_SYSTEM_CALL_EXIT )
            : "r0", "r1", "r2", "memory"
        );
    }

#else /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */

    void vPortSVCHandler( void ) /* __attribute__( ( naked ) ) PRIVILEGED_FUNCTION */
    {
        /* Assumes psp was in use. */
        __asm volatile
        (
            #ifndef USE_PROCESS_STACK /* Code should not be required if a main() is using the process stack. */
                "   tst lr, #4                      \n"
                "   ite eq                          \n"
                "   mrseq r0, msp                   \n"
                "   mrsne r0, psp                   \n"
            #else
                "   mrs r0, psp                     \n"
            #endif
            "   b %0                            \n"
            ::"i" ( vSVCHandler_C ) : "r0", "memory"
        );
    }

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */
/*-----------------------------------------------------------*/

void vSVCHandler_C( uint32_t * pulParam ) /* PRIVILEGED_FUNCTION */
{
    uint8_t ucSVCNumber;
    uint32_t ulPC;

    #if ( ( configUSE_MPU_WRAPPERS_V1 == 1 ) && ( configENFORCE_SYSTEM_CALLS_FROM_KERNEL_ONLY == 1 ) )
        #if defined( __ARMCC_VERSION )

            /* Declaration when these variable are defined in code instead of being
             * exported from linker scripts. */
            extern uint32_t * __syscalls_flash_start__;
            extern uint32_t * __syscalls_flash_end__;
        #else
            /* Declaration when these variable are exported from linker scripts. */
            extern uint32_t __syscalls_flash_start__[];
            extern uint32_t __syscalls_flash_end__[];
        #endif /* #if defined( __ARMCC_VERSION ) */
    #endif /* #if ( ( configUSE_MPU_WRAPPERS_V1 == 1 ) && ( configENFORCE_SYSTEM_CALLS_FROM_KERNEL_ONLY == 1 ) ) */

    /* The stack contains: r0, r1, r2, r3, r12, LR, PC and xPSR.  The first
     * argument (r0) is pulParam[ 0 ]. */
    ulPC = pulParam[ portOFFSET_TO_PC ];
    ucSVCNumber = ( ( uint8_t * ) ulPC )[ -2 ];

    switch( ucSVCNumber )
    {
        case portSVC_START_SCHEDULER:
            portNVIC_SHPR2_REG |= portNVIC_SVC_PRI;
            prvRestoreContextOfFirstTask();
            break;

        case portSVC_YIELD:
            portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;

            /* Barriers are normally not required
             * but do ensure the code is completely
             * within the specified behaviour for the
             * architecture. */
            __asm volatile ( "dsb" ::: "memory" );
            __asm volatile ( "isb" );

            break;

    #if ( configUSE_MPU_WRAPPERS_V1 == 1 )
        #if ( configENFORCE_SYSTEM_CALLS_FROM_KERNEL_ONLY == 1 )
            case portSVC_RAISE_PRIVILEGE: /* Only raise the privilege, if the
                                           * svc was raised from any of the
                                           * system calls. */

                if( ( ulPC >= ( uint32_t ) __syscalls_flash_start__ ) &&
                    ( ulPC <= ( uint32_t ) __syscalls_flash_end__ ) )
                {
                    __asm volatile
                    (
                        "   mrs r1, control     \n" /* Obtain current control value. */
                        "   bic r1, #1          \n" /* Set privilege bit. */
                        "   msr control, r1     \n" /* Write back new control value. */
                        ::: "r1", "memory"
                    );
                }

                break;
        #else /* if ( configENFORCE_SYSTEM_CALLS_FROM_KERNEL_ONLY == 1 ) */
            case portSVC_RAISE_PRIVILEGE:
                __asm volatile
                (
                    "   mrs r1, control     \n" /* Obtain current control value. */
                    "   bic r1, #1          \n" /* Set privilege bit. */
                    "   msr control, r1     \n" /* Write back new control value. */
                    ::: "r1", "memory"
                );
                break;
        #endif /* #if( configENFORCE_SYSTEM_CALLS_FROM_KERNEL_ONLY == 1 ) */
    #endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 1 ) */

        default: /* Unknown SVC call. */
            break;
    }
}
/*-----------------------------------------------------------*/

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

    void vSystemCallEnter( uint32_t * pulTaskStack,
                           uint8_t ucSystemCallNumber ) /* PRIVILEGED_FUNCTION */
    {
        extern TaskHandle_t pxCurrentTCB;
        extern UBaseType_t uxSystemCallImplementations[ NUM_SYSTEM_CALLS ];
        xMPU_SETTINGS * pxMpuSettings;
        uint32_t * pulSystemCallStack;
        uint32_t ulSystemCallLocation, i;
        const uint32_t ulStackFrameSize = 8;

        #if defined( __ARMCC_VERSION )
            /* Declaration when these variable are defined in code instead of being
             * exported from linker scripts. */
            extern uint32_t * __syscalls_flash_start__;
            extern uint32_t * __syscalls_flash_end__;
        #else
            /* Declaration when these variable are exported from linker scripts. */
            extern uint32_t __syscalls_flash_start__[];
            extern uint32_t __syscalls_flash_end__[];
        #endif /* #if defined( __ARMCC_VERSION ) */

        ulSystemCallLocation = pulTaskStack[ portOFFSET_TO_PC ];
        pxMpuSettings = xTaskGetMPUSettings( pxCurrentTCB );

        /* Checks:
         * 1. SVC is raised from the system call section (i.e. application is
         *    not raising SVC directly).
         * 2. pxMpuSettings->xSystemCallStackInfo.pulTaskStack must be NULL as
         *    it is non-NULL only during the execution of a system call (i.e.
         *    between system call enter and exit).
         * 3. System call is not for a kernel API disabled by the configuration
         *    in FreeRTOSConfig.h.
         * 4. We do not need to check that ucSystemCallNumber is within range
         *    because the assembly SVC handler checks that before calling
         *    this function.
         */
        if( ( ulSystemCallLocation >= ( uint32_t ) __syscalls_flash_start__ ) &&
            ( ulSystemCallLocation <= ( uint32_t ) __syscalls_flash_end__ ) &&
            ( pxMpuSettings->xSystemCallStackInfo.pulTaskStack == NULL ) &&
            ( uxSystemCallImplementations[ ucSystemCallNumber ] != ( UBaseType_t ) 0 ) )
        {
            pulSystemCallStack = pxMpuSettings->xSystemCallStackInfo.pulSystemCallStack;

            /* Make space on the system call stack for the stack frame. */
            pulSystemCallStack = pulSystemCallStack - ulStackFrameSize;

            /* Copy the stack frame. */
            for( i = 0; i < ulStackFrameSize; i++ )
            {
                pulSystemCallStack[ i ] = pulTaskStack[ i ];
            }

            /* Use the pulSystemCallStack in thread mode. */
            __asm volatile ( "msr psp, %0" : : "r" ( pulSystemCallStack ) );

            /* Raise the privilege for the duration of the system call. */
            __asm volatile (
                " mrs r1, control     \n" /* Obtain current control value. */
                " bic r1, #1          \n" /* Clear nPRIV bit. */
                " msr control, r1     \n" /* Write back new control value. */
                ::: "r1", "memory"
                );

            /* Remember the location where we should copy the stack frame when we exit from
             * the system call. */
            pxMpuSettings->xSystemCallStackInfo.pulTaskStack = pulTaskStack + ulStackFrameSize;

            /* Store the value of the Link Register before the SVC was raised.
             * It contains the address of the caller of the System Call entry
             * point (i.e. the caller of the MPU_<API>). We need to restore it
             * when we exit from the system call. */
            pxMpuSettings->xSystemCallStackInfo.ulLinkRegisterAtSystemCallEntry = pulTaskStack[ portOFFSET_TO_LR ];

            /* Start executing the system call upon returning from this handler. */
            pulSystemCallStack[ portOFFSET_TO_PC ] = uxSystemCallImplementations[ ucSystemCallNumber ];
            /* Raise a request to exit from the system call upon finishing the
             * system call. */
            pulSystemCallStack[ portOFFSET_TO_LR ] = ( uint32_t ) vRequestSystemCallExit;

            /* Record if the hardware used padding to force the stack pointer
             * to be double word aligned. */
            if( ( pulTaskStack[ portOFFSET_TO_PSR ] & portPSR_STACK_PADDING_MASK ) == portPSR_STACK_PADDING_MASK )
            {
                pxMpuSettings->ulTaskFlags |= portSTACK_FRAME_HAS_PADDING_FLAG;
            }
            else
            {
                pxMpuSettings->ulTaskFlags &= ( ~portSTACK_FRAME_HAS_PADDING_FLAG );
            }

            /* We ensure in pxPortInitialiseStack that the system call stack is
             * double word aligned and therefore, there is no need of padding.
             * Clear the bit[9] of stacked xPSR. */
            pulSystemCallStack[ portOFFSET_TO_PSR ] &= ( ~portPSR_STACK_PADDING_MASK );
        }
    }

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */
/*-----------------------------------------------------------*/

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

    void vRequestSystemCallExit( void ) /* __attribute__( ( naked ) ) PRIVILEGED_FUNCTION */
    {
        __asm volatile ( "svc %0 \n" ::"i" ( portSVC_SYSTEM_CALL_EXIT ) : "memory" );
    }

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */
/*-----------------------------------------------------------*/

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

    void vSystemCallExit( uint32_t * pulSystemCallStack ) /* PRIVILEGED_FUNCTION */
    {
        extern TaskHandle_t pxCurrentTCB;
        xMPU_SETTINGS * pxMpuSettings;
        uint32_t * pulTaskStack;
        uint32_t ulSystemCallLocation, i;
        const uint32_t ulStackFrameSize = 8;

        #if defined( __ARMCC_VERSION )
            /* Declaration when these variable are defined in code instead of being
             * exported from linker scripts. */
            extern uint32_t * __privileged_functions_start__;
            extern uint32_t * __privileged_functions_end__;
        #else
            /* Declaration when these variable are exported from linker scripts. */
            extern uint32_t __privileged_functions_start__[];
            extern uint32_t __privileged_functions_end__[];
        #endif /* #if defined( __ARMCC_VERSION ) */

        ulSystemCallLocation = pulSystemCallStack[ portOFFSET_TO_PC ];
        pxMpuSettings = xTaskGetMPUSettings( pxCurrentTCB );

        /* Checks:
         * 1. SVC is raised from the privileged code (i.e. application is not
         *    raising SVC directly). This SVC is only raised from
         *    vRequestSystemCallExit which is in the privileged code section.
         * 2. pxMpuSettings->xSystemCallStackInfo.pulTaskStack must not be NULL -
         *    this means that we previously entered a system call and the
         *    application is not attempting to exit without entering a system
         *    call.
         */
        if( ( ulSystemCallLocation >= ( uint32_t ) __privileged_functions_start__ ) &&
            ( ulSystemCallLocation <= ( uint32_t ) __privileged_functions_end__ ) &&
            ( pxMpuSettings->xSystemCallStackInfo.pulTaskStack != NULL ) )
        {
            pulTaskStack = pxMpuSettings->xSystemCallStackInfo.pulTaskStack;

            /* Make space on the task stack for the stack frame. */
            pulTaskStack = pulTaskStack - ulStackFrameSize;

            /* Copy the stack frame. */
            for( i = 0; i < ulStackFrameSize; i++ )
            {
                pulTaskStack[ i ] = pulSystemCallStack[ i ];
            }

            /* Use the pulTaskStack in thread mode. */
            __asm volatile ( "msr psp, %0" : : "r" ( pulTaskStack ) );

            /* Drop the privilege before returning to the thread mode. */
            __asm volatile (
                " mrs r1, control     \n" /* Obtain current control value. */
                " orr r1, #1          \n" /* Set nPRIV bit. */
                " msr control, r1     \n" /* Write back new control value. */
                ::: "r1", "memory"
                );

            /* Return to the caller of the System Call entry point (i.e. the
             * caller of the MPU_<API>). */
            pulTaskStack[ portOFFSET_TO_PC ] = pxMpuSettings->xSystemCallStackInfo.ulLinkRegisterAtSystemCallEntry;
            /* Ensure that LR has a valid value.*/
            pulTaskStack[ portOFFSET_TO_LR ] = pxMpuSettings->xSystemCallStackInfo.ulLinkRegisterAtSystemCallEntry;

            /* If the hardware used padding to force the stack pointer
             * to be double word aligned, set the stacked xPSR bit[9],
             * otherwise clear it. */
            if( ( pxMpuSettings->ulTaskFlags & portSTACK_FRAME_HAS_PADDING_FLAG ) == portSTACK_FRAME_HAS_PADDING_FLAG )
            {
                pulTaskStack[ portOFFSET_TO_PSR ] |= portPSR_STACK_PADDING_MASK;
            }
            else
            {
                pulTaskStack[ portOFFSET_TO_PSR ] &= ( ~portPSR_STACK_PADDING_MASK );
            }

            /* This is not NULL only for the duration of the system call. */
            pxMpuSettings->xSystemCallStackInfo.pulTaskStack = NULL;
        }
    }

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */
/*-----------------------------------------------------------*/

BaseType_t xPortIsTaskPrivileged( void ) /* PRIVILEGED_FUNCTION */
{
    BaseType_t xTaskIsPrivileged = pdFALSE;
    const xMPU_SETTINGS * xTaskMpuSettings = xTaskGetMPUSettings( NULL ); /* Calling task's MPU settings. */

    if( ( xTaskMpuSettings->ulTaskFlags & portTASK_IS_PRIVILEGED_FLAG ) == portTASK_IS_PRIVILEGED_FLAG )
    {
        xTaskIsPrivileged = pdTRUE;
    }

    return xTaskIsPrivileged;
}
/*-----------------------------------------------------------*/

static void prvRestoreContextOfFirstTask( void )
{
    __asm volatile
    (
        " ldr r0, =0xE000ED08                   \n" /* Use the NVIC offset register to locate the stack. */
        " ldr r0, [r0]                          \n"
        " ldr r0, [r0]                          \n"
        " msr msp, r0                           \n" /* Set the msp back to the start of the stack. */
        "                                       \n"
        /*------------ Program MPU. ------------ */
        " ldr r3, pxCurrentTCBConst2            \n" /* r3 = pxCurrentTCBConst2. */
        " ldr r2, [r3]                          \n" /* r2 = pxCurrentTCB. */
        " add r2, r2, #4                        \n" /* r2 = Second item in the TCB which is xMPUSettings. */
        "                                       \n"
        " dmb                                   \n" /* Complete outstanding transfers before disabling MPU. */
        " ldr r0, =0xe000ed94                   \n" /* MPU_CTRL register. */
        " ldr r3, [r0]                          \n" /* Read the value of MPU_CTRL. */
        " bic r3, #1                            \n" /* r3 = r3 & ~1 i.e. Clear the bit 0 in r3. */
        " str r3, [r0]                          \n" /* Disable MPU. */
        "                                       \n"
        " ldr r0, =0xe000ed9c                   \n" /* Region Base Address register. */
        " ldmia r2!, {r4-r11}                   \n" /* Read 4 sets of MPU registers [MPU Region # 0 - 3]. */
        " stmia r0, {r4-r11}                    \n" /* Write 4 sets of MPU registers [MPU Region # 0 - 3]. */
        "                                       \n"
        " ldr r0, =0xe000ed94                   \n" /* MPU_CTRL register. */
        " ldr r3, [r0]                          \n" /* Read the value of MPU_CTRL. */
        " orr r3, #1                            \n" /* r3 = r3 | 1 i.e. Set the bit 0 in r3. */
        " str r3, [r0]                          \n" /* Enable MPU. */
        " dsb                                   \n" /* Force memory writes before continuing. */
        "                                       \n"
        /*---------- Restore Context. ---------- */
        " ldr r3, pxCurrentTCBConst2            \n" /* r3 = pxCurrentTCBConst2. */
        " ldr r2, [r3]                          \n" /* r2 = pxCurrentTCB. */
        " ldr r1, [r2]                          \n" /* r1 = Location of saved context in TCB. */
        "                                       \n"
        " ldmdb r1!, {r0, r4-r11}               \n" /* r0 contains PSP after the hardware had saved context. r4-r11 contain hardware saved context. */
        " msr psp, r0                           \n"
        " stmia r0, {r4-r11}                    \n" /* Copy the hardware saved context on the task stack. */
        " ldmdb r1!, {r3-r11, lr}               \n" /* r3 contains CONTROL register. r4-r11 and LR restored. */
        " msr control, r3                       \n"
        " str r1, [r2]                          \n" /* Save the location where the context should be saved next as the first member of TCB. */
        "                                       \n"
        " mov r0, #0                            \n"
        " msr basepri, r0                       \n"
        " bx lr                                 \n"
        "                                       \n"
        " .ltorg                                \n" /* Assemble current literal pool to avoid offset-out-of-bound errors with lto. */
        " .align 4                              \n"
        "pxCurrentTCBConst2: .word pxCurrentTCB \n"
    );
}
/*-----------------------------------------------------------*/

/*
 * See header file for description.
 */
BaseType_t xPortStartScheduler( void )
{
    #if ( configASSERT_DEFINED == 1 )
    {
        volatile uint8_t ucOriginalPriority;
        volatile uint32_t ulImplementedPrioBits = 0;
        volatile uint8_t * const pucFirstUserPriorityRegister = ( volatile uint8_t * const ) ( portNVIC_IP_REGISTERS_OFFSET_16 + portFIRST_USER_INTERRUPT_NUMBER );
        volatile uint8_t ucMaxPriorityValue;

        /* Determine the maximum priority from which ISR safe FreeRTOS API
         * functions can be called.  ISR safe functions are those that end in
         * "FromISR".  FreeRTOS maintains separate thread and ISR API functions
         * to ensure interrupt entry is as fast and simple as possible.
         *
         * Save the interrupt priority value that is about to be clobbered. */
        ucOriginalPriority = *pucFirstUserPriorityRegister;

        /* Determine the number of priority bits available.  First write to all
         * possible bits. */
        *pucFirstUserPriorityRegister = portMAX_8_BIT_VALUE;

        /* Read the value back to see how many bits stuck. */
        ucMaxPriorityValue = *pucFirstUserPriorityRegister;

        /* Use the same mask on the maximum system call priority. */
        ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;

        /* Check that the maximum system call priority is nonzero after
         * accounting for the number of priority bits supported by the
         * hardware. A priority of 0 is invalid because setting the BASEPRI
         * register to 0 unmasks all interrupts, and interrupts with priority 0
         * cannot be masked using BASEPRI.
         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
        configASSERT( ucMaxSysCallPriority );

        /* Check that the bits not implemented in hardware are zero in
         * configMAX_SYSCALL_INTERRUPT_PRIORITY. */
        configASSERT( ( configMAX_SYSCALL_INTERRUPT_PRIORITY & ( ~ucMaxPriorityValue ) ) == 0U );

        /* Calculate the maximum acceptable priority group value for the number
         * of bits read back. */

        while( ( ucMaxPriorityValue & portTOP_BIT_OF_BYTE ) == portTOP_BIT_OF_BYTE )
        {
            ulImplementedPrioBits++;
            ucMaxPriorityValue <<= ( uint8_t ) 0x01;
        }

        if( ulImplementedPrioBits == 8 )
        {
            /* When the hardware implements 8 priority bits, there is no way for
             * the software to configure PRIGROUP to not have sub-priorities. As
             * a result, the least significant bit is always used for sub-priority
             * and there are 128 preemption priorities and 2 sub-priorities.
             *
             * This may cause some confusion in some cases - for example, if
             * configMAX_SYSCALL_INTERRUPT_PRIORITY is set to 5, both 5 and 4
             * priority interrupts will be masked in Critical Sections as those
             * are at the same preemption priority. This may appear confusing as
             * 4 is higher (numerically lower) priority than
             * configMAX_SYSCALL_INTERRUPT_PRIORITY and therefore, should not
             * have been masked. Instead, if we set configMAX_SYSCALL_INTERRUPT_PRIORITY
             * to 4, this confusion does not happen and the behaviour remains the same.
             *
             * The following assert ensures that the sub-priority bit in the
             * configMAX_SYSCALL_INTERRUPT_PRIORITY is clear to avoid the above mentioned
             * confusion. */
            configASSERT( ( configMAX_SYSCALL_INTERRUPT_PRIORITY & 0x1U ) == 0U );
            ulMaxPRIGROUPValue = 0;
        }
        else
        {
            ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS - ulImplementedPrioBits;
        }

        /* Shift the priority group value back to its position within the AIRCR
         * register. */
        ulMaxPRIGROUPValue <<= portPRIGROUP_SHIFT;
        ulMaxPRIGROUPValue &= portPRIORITY_GROUP_MASK;

        /* Restore the clobbered interrupt priority register to its original
         * value. */
        *pucFirstUserPriorityRegister = ucOriginalPriority;
    }
    #endif /* configASSERT_DEFINED */

    /* Make PendSV and SysTick the same priority as the kernel, and the SVC
     * handler higher priority so it can be used to exit a critical section (where
     * lower priorities are masked). */
    portNVIC_SHPR3_REG |= portNVIC_PENDSV_PRI;
    portNVIC_SHPR3_REG |= portNVIC_SYSTICK_PRI;

    /* Configure the regions in the MPU that are common to all tasks. */
    prvSetupMPU();

    /* Start the timer that generates the tick ISR.  Interrupts are disabled
     * here already. */
    vPortSetupTimerInterrupt();

    /* Initialise the critical nesting count ready for the first task. */
    uxCriticalNesting = 0;

    #if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) )
    {
        xSchedulerRunning = pdTRUE;
    }
    #endif

    /* Start the first task. */
    __asm volatile (
        " ldr r0, =0xE000ED08   \n" /* Use the NVIC offset register to locate the stack. */
        " ldr r0, [r0]          \n"
        " ldr r0, [r0]          \n"
        " msr msp, r0           \n" /* Set the msp back to the start of the stack. */
        " cpsie i               \n" /* Globally enable interrupts. */
        " cpsie f               \n"
        " dsb                   \n"
        " isb                   \n"
        " svc %0                \n" /* System call to start first task. */
        " nop                   \n"
        " .ltorg                \n"
        ::"i" ( portSVC_START_SCHEDULER ) : "memory" );

    /* Should not get here! */
    return 0;
}
/*-----------------------------------------------------------*/

void vPortEndScheduler( void )
{
    /* Not implemented in ports where there is nothing to return to.
     * Artificially force an assert. */
    configASSERT( uxCriticalNesting == 1000UL );
}
/*-----------------------------------------------------------*/

void vPortEnterCritical( void )
{
    #if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 )
        if( portIS_PRIVILEGED() == pdFALSE )
        {
            portRAISE_PRIVILEGE();
            portMEMORY_BARRIER();

            portDISABLE_INTERRUPTS();
            uxCriticalNesting++;
            portMEMORY_BARRIER();

            portRESET_PRIVILEGE();
            portMEMORY_BARRIER();
        }
        else
        {
            portDISABLE_INTERRUPTS();
            uxCriticalNesting++;
        }
    #else /* if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 ) */
        portDISABLE_INTERRUPTS();
        uxCriticalNesting++;
    #endif /* if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 ) */
}
/*-----------------------------------------------------------*/

void vPortExitCritical( void )
{
    #if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 )
        if( portIS_PRIVILEGED() == pdFALSE )
        {
            portRAISE_PRIVILEGE();
            portMEMORY_BARRIER();

            configASSERT( uxCriticalNesting );
            uxCriticalNesting--;

            if( uxCriticalNesting == 0 )
            {
                portENABLE_INTERRUPTS();
            }

            portMEMORY_BARRIER();

            portRESET_PRIVILEGE();
            portMEMORY_BARRIER();
        }
        else
        {
            configASSERT( uxCriticalNesting );
            uxCriticalNesting--;

            if( uxCriticalNesting == 0 )
            {
                portENABLE_INTERRUPTS();
            }
        }
    #else /* if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 ) */
        configASSERT( uxCriticalNesting );
        uxCriticalNesting--;

        if( uxCriticalNesting == 0 )
        {
            portENABLE_INTERRUPTS();
        }
    #endif /* if ( configALLOW_UNPRIVILEGED_CRITICAL_SECTIONS == 1 ) */
}
/*-----------------------------------------------------------*/

void xPortPendSVHandler( void )
{
    /* This is a naked function. */

    __asm volatile
    (
        " ldr r3, pxCurrentTCBConst             \n" /* r3 = pxCurrentTCBConst. */
        " ldr r2, [r3]                          \n" /* r2 = pxCurrentTCB. */
        " ldr r1, [r2]                          \n" /* r1 = Location where the context should be saved. */
        "                                       \n"
        /*------------ Save Context. ----------- */
        " mrs r3, control                       \n"
        " mrs r0, psp                           \n"
        " isb                                   \n"
        "                                       \n"
        " stmia r1!, {r3-r11, lr}               \n" /* Store CONTROL register, r4-r11 and LR. */
        " ldmia r0, {r4-r11}                    \n" /* Copy hardware saved context into r4-r11. */
        " stmia r1!, {r0, r4-r11}               \n" /* Store original PSP (after hardware has saved context) and the hardware saved context. */
        " str r1, [r2]                          \n" /* Save the location from where the context should be restored as the first member of TCB. */
        "                                       \n"
        /*---------- Select next task. --------- */
        " mov r0, %0                            \n"
        " msr basepri, r0                       \n"
        " dsb                                   \n"
        " isb                                   \n"
        " bl vTaskSwitchContext                 \n"
        " mov r0, #0                            \n"
        " msr basepri, r0                       \n"
        "                                       \n"
        /*------------ Program MPU. ------------ */
        " ldr r3, pxCurrentTCBConst             \n" /* r3 = pxCurrentTCBConst. */
        " ldr r2, [r3]                          \n" /* r2 = pxCurrentTCB. */
        " add r2, r2, #4                        \n" /* r2 = Second item in the TCB which is xMPUSettings. */
        "                                       \n"
        " dmb                                   \n" /* Complete outstanding transfers before disabling MPU. */
        " ldr r0, =0xe000ed94                   \n" /* MPU_CTRL register. */
        " ldr r3, [r0]                          \n" /* Read the value of MPU_CTRL. */
        " bic r3, #1                            \n" /* r3 = r3 & ~1 i.e. Clear the bit 0 in r3. */
        " str r3, [r0]                          \n" /* Disable MPU. */
        "                                       \n"
        " ldr r0, =0xe000ed9c                   \n" /* Region Base Address register. */
        " ldmia r2!, {r4-r11}                   \n" /* Read 4 sets of MPU registers [MPU Region # 0 - 3]. */
        " stmia r0, {r4-r11}                    \n" /* Write 4 sets of MPU registers [MPU Region # 0 - 3]. */
        "                                       \n"
        " ldr r0, =0xe000ed94                   \n" /* MPU_CTRL register. */
        " ldr r3, [r0]                          \n" /* Read the value of MPU_CTRL. */
        " orr r3, #1                            \n" /* r3 = r3 | 1 i.e. Set the bit 0 in r3. */
        " str r3, [r0]                          \n" /* Enable MPU. */
        " dsb                                   \n" /* Force memory writes before continuing. */
        "                                       \n"
        /*---------- Restore Context. ---------- */
        " ldr r3, pxCurrentTCBConst             \n" /* r3 = pxCurrentTCBConst. */
        " ldr r2, [r3]                          \n" /* r2 = pxCurrentTCB. */
        " ldr r1, [r2]                          \n" /* r1 = Location of saved context in TCB. */
        "                                       \n"
        " ldmdb r1!, {r0, r4-r11}               \n" /* r0 contains PSP after the hardware had saved context. r4-r11 contain hardware saved context. */
        " msr psp, r0                           \n"
        " stmia r0, {r4-r11}                    \n" /* Copy the hardware saved context on the task stack. */
        " ldmdb r1!, {r3-r11, lr}               \n" /* r3 contains CONTROL register. r4-r11 and LR restored. */
        " msr control, r3                       \n"
        "                                       \n"
        " str r1, [r2]                          \n" /* Save the location where the context should be saved next as the first member of TCB. */
        " bx lr                                 \n"
        "                                       \n"
        " .ltorg                                \n" /* Assemble current literal pool to avoid offset-out-of-bound errors with lto. */
        " .align 4                              \n"
        "pxCurrentTCBConst: .word pxCurrentTCB  \n"
        ::"i" ( configMAX_SYSCALL_INTERRUPT_PRIORITY )
    );
}
/*-----------------------------------------------------------*/

void xPortSysTickHandler( void )
{
    uint32_t ulDummy;

    ulDummy = portSET_INTERRUPT_MASK_FROM_ISR();
    traceISR_ENTER();
    {
        /* Increment the RTOS tick. */
        if( xTaskIncrementTick() != pdFALSE )
        {
            traceISR_EXIT_TO_SCHEDULER();
            /* Pend a context switch. */
            portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;
        }
        else
        {
            traceISR_EXIT();
        }
    }
    portCLEAR_INTERRUPT_MASK_FROM_ISR( ulDummy );
}
/*-----------------------------------------------------------*/

/*
 * Setup the systick timer to generate the tick interrupts at the required
 * frequency.
 */
__attribute__( ( weak ) ) void vPortSetupTimerInterrupt( void )
{
    /* Stop and clear the SysTick. */
    portNVIC_SYSTICK_CTRL_REG = 0UL;
    portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;

    /* Configure SysTick to interrupt at the requested rate. */
    portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;
    portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK | portNVIC_SYSTICK_INT | portNVIC_SYSTICK_ENABLE );
}
/*-----------------------------------------------------------*/

static void prvSetupMPU( void )
{
    extern uint32_t __privileged_functions_start__[];
    extern uint32_t __privileged_functions_end__[];
    extern uint32_t __FLASH_segment_start__[];
    extern uint32_t __FLASH_segment_end__[];
    extern uint32_t __privileged_data_start__[];
    extern uint32_t __privileged_data_end__[];

    /* Check the expected MPU is present. */
    if( portMPU_TYPE_REG == portEXPECTED_MPU_TYPE_VALUE )
    {
        /* First setup the unprivileged flash for unprivileged read only access. */
        portMPU_REGION_BASE_ADDRESS_REG = ( ( uint32_t ) __FLASH_segment_start__ ) | /* Base address. */
                                          ( portMPU_REGION_VALID ) |
                                          ( portUNPRIVILEGED_FLASH_REGION );

        portMPU_REGION_ATTRIBUTE_REG = ( portMPU_REGION_READ_ONLY ) |
                                       ( portMPU_REGION_CACHEABLE_BUFFERABLE ) |
                                       ( prvGetMPURegionSizeSetting( ( uint32_t ) __FLASH_segment_end__ - ( uint32_t ) __FLASH_segment_start__ ) ) |
                                       ( portMPU_REGION_ENABLE );

        /* Setup the privileged flash for privileged only access.  This is where
         * the kernel code is * placed. */
        portMPU_REGION_BASE_ADDRESS_REG = ( ( uint32_t ) __privileged_functions_start__ ) | /* Base address. */
                                          ( portMPU_REGION_VALID ) |
                                          ( portPRIVILEGED_FLASH_REGION );

        portMPU_REGION_ATTRIBUTE_REG = ( portMPU_REGION_PRIVILEGED_READ_ONLY ) |
                                       ( portMPU_REGION_CACHEABLE_BUFFERABLE ) |
                                       ( prvGetMPURegionSizeSetting( ( uint32_t ) __privileged_functions_end__ - ( uint32_t ) __privileged_functions_start__ ) ) |
                                       ( portMPU_REGION_ENABLE );

        /* Setup the privileged data RAM region.  This is where the kernel data
         * is placed. */
        portMPU_REGION_BASE_ADDRESS_REG = ( ( uint32_t ) __privileged_data_start__ ) | /* Base address. */
                                          ( portMPU_REGION_VALID ) |
                                          ( portPRIVILEGED_RAM_REGION );

        portMPU_REGION_ATTRIBUTE_REG = ( portMPU_REGION_PRIVILEGED_READ_WRITE ) |
                                       ( portMPU_REGION_CACHEABLE_BUFFERABLE ) |
                                       ( portMPU_REGION_EXECUTE_NEVER ) |
                                       prvGetMPURegionSizeSetting( ( uint32_t ) __privileged_data_end__ - ( uint32_t ) __privileged_data_start__ ) |
                                       ( portMPU_REGION_ENABLE );

        /* By default allow everything to access the general peripherals.  The
         * system peripherals and registers are protected. */
        portMPU_REGION_BASE_ADDRESS_REG = ( portPERIPHERALS_START_ADDRESS ) |
                                          ( portMPU_REGION_VALID ) |
                                          ( portGENERAL_PERIPHERALS_REGION );

        portMPU_REGION_ATTRIBUTE_REG = ( portMPU_REGION_READ_WRITE | portMPU_REGION_EXECUTE_NEVER ) |
                                       ( prvGetMPURegionSizeSetting( portPERIPHERALS_END_ADDRESS - portPERIPHERALS_START_ADDRESS ) ) |
                                       ( portMPU_REGION_ENABLE );

        /* Enable the memory fault exception. */
        portNVIC_SYS_CTRL_STATE_REG |= portNVIC_MEM_FAULT_ENABLE;

        /* Enable the MPU with the background region configured. */
        portMPU_CTRL_REG |= ( portMPU_ENABLE | portMPU_BACKGROUND_ENABLE );
    }
}
/*-----------------------------------------------------------*/

static uint32_t prvGetMPURegionSizeSetting( uint32_t ulActualSizeInBytes )
{
    uint32_t ulRegionSize, ulReturnValue = 4;

    /* 32 is the smallest region size, 31 is the largest valid value for
     * ulReturnValue. */
    for( ulRegionSize = 32UL; ulReturnValue < 31UL; ( ulRegionSize <<= 1UL ) )
    {
        if( ulActualSizeInBytes <= ulRegionSize )
        {
            break;
        }
        else
        {
            ulReturnValue++;
        }
    }

    /* Shift the code by one before returning so it can be written directly
     * into the the correct bit position of the attribute register. */
    return( ulReturnValue << 1UL );
}
/*-----------------------------------------------------------*/

BaseType_t xIsPrivileged( void ) /* __attribute__ (( naked )) */
{
    __asm volatile
    (
        "   mrs r0, control                         \n" /* r0 = CONTROL. */
        "   tst r0, #1                              \n" /* Perform r0 & 1 (bitwise AND) and update the conditions flag. */
        "   ite ne                                  \n"
        "   movne r0, #0                            \n" /* CONTROL[0]!=0. Return false to indicate that the processor is not privileged. */
        "   moveq r0, #1                            \n" /* CONTROL[0]==0. Return true to indicate that the processor is privileged. */
        "   bx lr                                   \n" /* Return. */
        "                                           \n"
        "   .align 4                                \n"
        ::: "r0", "memory"
    );
}
/*-----------------------------------------------------------*/

void vResetPrivilege( void ) /* __attribute__ (( naked )) */
{
    __asm volatile
    (
        "   mrs r0, control                         \n" /* r0 = CONTROL. */
        "   orr r0, #1                              \n" /* r0 = r0 | 1. */
        "   msr control, r0                         \n" /* CONTROL = r0. */
        "   bx lr                                   \n" /* Return to the caller. */
        ::: "r0", "memory"
    );
}
/*-----------------------------------------------------------*/

void vPortSwitchToUserMode( void )
{
    /* Load the current task's MPU settings from its TCB. */
    xMPU_SETTINGS * xTaskMpuSettings = xTaskGetMPUSettings( NULL );

    /* Mark the task as unprivileged. */
    xTaskMpuSettings->ulTaskFlags &= ( ~( portTASK_IS_PRIVILEGED_FLAG ) );

    /* Lower the processor's privilege level. */
    vResetPrivilege();
}
/*-----------------------------------------------------------*/

void vPortStoreTaskMPUSettings( xMPU_SETTINGS * xMPUSettings,
                                const struct xMEMORY_REGION * const xRegions,
                                StackType_t * pxBottomOfStack,
                                uint32_t ulStackDepth )
{
    extern uint32_t __SRAM_segment_start__[];
    extern uint32_t __SRAM_segment_end__[];
    extern uint32_t __privileged_data_start__[];
    extern uint32_t __privileged_data_end__[];
    int32_t lIndex;
    uint32_t ul;

    if( xRegions == NULL )
    {
        /* No MPU regions are specified so allow access to all RAM. */
        xMPUSettings->xRegion[ 0 ].ulRegionBaseAddress =
            ( ( uint32_t ) __SRAM_segment_start__ ) | /* Base address. */
            ( portMPU_REGION_VALID ) |
            ( portSTACK_REGION );                     /* Region number. */

        xMPUSettings->xRegion[ 0 ].ulRegionAttribute =
            ( portMPU_REGION_READ_WRITE ) |
            ( portMPU_REGION_CACHEABLE_BUFFERABLE ) |
            ( portMPU_REGION_EXECUTE_NEVER ) |
            ( prvGetMPURegionSizeSetting( ( uint32_t ) __SRAM_segment_end__ - ( uint32_t ) __SRAM_segment_start__ ) ) |
            ( portMPU_REGION_ENABLE );

        xMPUSettings->xRegionSettings[ 0 ].ulRegionStartAddress = ( uint32_t ) __SRAM_segment_start__;
        xMPUSettings->xRegionSettings[ 0 ].ulRegionEndAddress = ( uint32_t ) __SRAM_segment_end__;
        xMPUSettings->xRegionSettings[ 0 ].ulRegionPermissions = ( tskMPU_READ_PERMISSION |
                                                                   tskMPU_WRITE_PERMISSION );

        /* Invalidate user configurable regions. */
        for( ul = 1UL; ul <= portNUM_CONFIGURABLE_REGIONS; ul++ )
        {
            xMPUSettings->xRegion[ ul ].ulRegionBaseAddress = ( ( ul - 1UL ) | portMPU_REGION_VALID );
            xMPUSettings->xRegion[ ul ].ulRegionAttribute = 0UL;
            xMPUSettings->xRegionSettings[ ul ].ulRegionStartAddress = 0UL;
            xMPUSettings->xRegionSettings[ ul ].ulRegionEndAddress = 0UL;
            xMPUSettings->xRegionSettings[ ul ].ulRegionPermissions = 0UL;
        }
    }
    else
    {
        /* This function is called automatically when the task is created - in
         * which case the stack region parameters will be valid.  At all other
         * times the stack parameters will not be valid and it is assumed that the
         * stack region has already been configured. */
        if( ulStackDepth > 0 )
        {
            /* Define the region that allows access to the stack. */
            xMPUSettings->xRegion[ 0 ].ulRegionBaseAddress =
                ( ( uint32_t ) pxBottomOfStack ) |
                ( portMPU_REGION_VALID ) |
                ( portSTACK_REGION ); /* Region number. */

            xMPUSettings->xRegion[ 0 ].ulRegionAttribute =
                ( portMPU_REGION_READ_WRITE ) |
                ( portMPU_REGION_EXECUTE_NEVER ) |
                ( prvGetMPURegionSizeSetting( ulStackDepth * ( uint32_t ) sizeof( StackType_t ) ) ) |
                ( portMPU_REGION_CACHEABLE_BUFFERABLE ) |
                ( portMPU_REGION_ENABLE );
            xMPUSettings->xRegionSettings[ 0 ].ulRegionStartAddress = ( uint32_t ) pxBottomOfStack;
            xMPUSettings->xRegionSettings[ 0 ].ulRegionEndAddress = ( uint32_t ) ( ( uint32_t ) ( pxBottomOfStack ) +
                                                                                   ( ulStackDepth * ( uint32_t ) sizeof( StackType_t ) ) - 1UL );
            xMPUSettings->xRegionSettings[ 0 ].ulRegionPermissions = ( tskMPU_READ_PERMISSION |
                                                                       tskMPU_WRITE_PERMISSION );
        }

        lIndex = 0;

        for( ul = 1UL; ul <= portNUM_CONFIGURABLE_REGIONS; ul++ )
        {
            if( ( xRegions[ lIndex ] ).ulLengthInBytes > 0UL )
            {
                /* Translate the generic region definition contained in
                 * xRegions into the CM3 specific MPU settings that are then
                 * stored in xMPUSettings. */
                xMPUSettings->xRegion[ ul ].ulRegionBaseAddress =
                    ( ( uint32_t ) xRegions[ lIndex ].pvBaseAddress ) |
                    ( portMPU_REGION_VALID ) |
                    ( ul - 1UL ); /* Region number. */

                xMPUSettings->xRegion[ ul ].ulRegionAttribute =
                    ( prvGetMPURegionSizeSetting( xRegions[ lIndex ].ulLengthInBytes ) ) |
                    ( xRegions[ lIndex ].ulParameters ) |
                    ( portMPU_REGION_ENABLE );

                xMPUSettings->xRegionSettings[ ul ].ulRegionStartAddress = ( uint32_t ) xRegions[ lIndex ].pvBaseAddress;
                xMPUSettings->xRegionSettings[ ul ].ulRegionEndAddress = ( uint32_t ) ( ( uint32_t ) xRegions[ lIndex ].pvBaseAddress + xRegions[ lIndex ].ulLengthInBytes - 1UL );
                xMPUSettings->xRegionSettings[ ul ].ulRegionPermissions = 0UL;

                if( ( ( xRegions[ lIndex ].ulParameters & portMPU_REGION_READ_ONLY ) == portMPU_REGION_READ_ONLY ) ||
                    ( ( xRegions[ lIndex ].ulParameters & portMPU_REGION_PRIVILEGED_READ_WRITE_UNPRIV_READ_ONLY ) == portMPU_REGION_PRIVILEGED_READ_WRITE_UNPRIV_READ_ONLY ) )
                {
                    xMPUSettings->xRegionSettings[ ul ].ulRegionPermissions = tskMPU_READ_PERMISSION;
                }

                if( ( xRegions[ lIndex ].ulParameters & portMPU_REGION_READ_WRITE ) == portMPU_REGION_READ_WRITE )
                {
                    xMPUSettings->xRegionSettings[ ul ].ulRegionPermissions = ( tskMPU_READ_PERMISSION | tskMPU_WRITE_PERMISSION );
                }
            }
            else
            {
                /* Invalidate the region. */
                xMPUSettings->xRegion[ ul ].ulRegionBaseAddress = ( ( ul - 1UL ) | portMPU_REGION_VALID );
                xMPUSettings->xRegion[ ul ].ulRegionAttribute = 0UL;
                xMPUSettings->xRegionSettings[ ul ].ulRegionStartAddress = 0UL;
                xMPUSettings->xRegionSettings[ ul ].ulRegionEndAddress = 0UL;
                xMPUSettings->xRegionSettings[ ul ].ulRegionPermissions = 0UL;
            }

            lIndex++;
        }
    }
}
/*-----------------------------------------------------------*/

BaseType_t xPortIsAuthorizedToAccessBuffer( const void * pvBuffer,
                                            uint32_t ulBufferLength,
                                            uint32_t ulAccessRequested ) /* PRIVILEGED_FUNCTION */

{
    uint32_t i, ulBufferStartAddress, ulBufferEndAddress;
    BaseType_t xAccessGranted = pdFALSE;
    const xMPU_SETTINGS * xTaskMpuSettings = xTaskGetMPUSettings( NULL ); /* Calling task's MPU settings. */

    if( ( xTaskMpuSettings->ulTaskFlags & portTASK_IS_PRIVILEGED_FLAG ) == portTASK_IS_PRIVILEGED_FLAG )
    {
        xAccessGranted = pdTRUE;
    }
    else
    {
        if( portADD_UINT32_WILL_OVERFLOW( ( ( uint32_t ) pvBuffer ), ( ulBufferLength - 1UL ) ) == pdFALSE )
        {
            ulBufferStartAddress = ( uint32_t ) pvBuffer;
            ulBufferEndAddress = ( ( ( uint32_t ) pvBuffer ) + ulBufferLength - 1UL );

            for( i = 0; i < portTOTAL_NUM_REGIONS_IN_TCB; i++ )
            {
                if( portIS_ADDRESS_WITHIN_RANGE( ulBufferStartAddress,
                                                 xTaskMpuSettings->xRegionSettings[ i ].ulRegionStartAddress,
                                                 xTaskMpuSettings->xRegionSettings[ i ].ulRegionEndAddress ) &&
                    portIS_ADDRESS_WITHIN_RANGE( ulBufferEndAddress,
                                                 xTaskMpuSettings->xRegionSettings[ i ].ulRegionStartAddress,
                                                 xTaskMpuSettings->xRegionSettings[ i ].ulRegionEndAddress ) &&
                    portIS_AUTHORIZED( ulAccessRequested, xTaskMpuSettings->xRegionSettings[ i ].ulRegionPermissions ) )
                {
                    xAccessGranted = pdTRUE;
                    break;
                }
            }
        }
    }

    return xAccessGranted;
}
/*-----------------------------------------------------------*/

#if ( configASSERT_DEFINED == 1 )

    void vPortValidateInterruptPriority( void )
    {
        uint32_t ulCurrentInterrupt;
        uint8_t ucCurrentPriority;

        /* Obtain the number of the currently executing interrupt. */
        __asm volatile ( "mrs %0, ipsr" : "=r" ( ulCurrentInterrupt )::"memory" );

        /* Is the interrupt number a user defined interrupt? */
        if( ulCurrentInterrupt >= portFIRST_USER_INTERRUPT_NUMBER )
        {
            /* Look up the interrupt's priority. */
            ucCurrentPriority = pcInterruptPriorityRegisters[ ulCurrentInterrupt ];

            /* The following assertion will fail if a service routine (ISR) for
             * an interrupt that has been assigned a priority above
             * configMAX_SYSCALL_INTERRUPT_PRIORITY calls an ISR safe FreeRTOS API
             * function.  ISR safe FreeRTOS API functions must *only* be called
             * from interrupts that have been assigned a priority at or below
             * configMAX_SYSCALL_INTERRUPT_PRIORITY.
             *
             * Numerically low interrupt priority numbers represent logically high
             * interrupt priorities, therefore the priority of the interrupt must
             * be set to a value equal to or numerically *higher* than
             * configMAX_SYSCALL_INTERRUPT_PRIORITY.
             *
             * Interrupts that  use the FreeRTOS API must not be left at their
             * default priority of  zero as that is the highest possible priority,
             * which is guaranteed to be above configMAX_SYSCALL_INTERRUPT_PRIORITY,
             * and  therefore also guaranteed to be invalid.
             *
             * FreeRTOS maintains separate thread and ISR API functions to ensure
             * interrupt entry is as fast and simple as possible.
             *
             * The following links provide detailed information:
             * https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html
             * https://www.FreeRTOS.org/FAQHelp.html */
            configASSERT( ucCurrentPriority >= ucMaxSysCallPriority );
        }

        /* Priority grouping:  The interrupt controller (NVIC) allows the bits
         * that define each interrupt's priority to be split between bits that
         * define the interrupt's pre-emption priority bits and bits that define
         * the interrupt's sub-priority.  For simplicity all bits must be defined
         * to be pre-emption priority bits.  The following assertion will fail if
         * this is not the case (if some bits represent a sub-priority).
         *
         * If the application only uses CMSIS libraries for interrupt
         * configuration then the correct setting can be achieved on all Cortex-M
         * devices by calling NVIC_SetPriorityGrouping( 0 ); before starting the
         * scheduler.  Note however that some vendor specific peripheral libraries
         * assume a non-zero priority group setting, in which cases using a value
         * of zero will result in unpredictable behaviour. */
        configASSERT( ( portAIRCR_REG & portPRIORITY_GROUP_MASK ) <= ulMaxPRIGROUPValue );
    }

#endif /* configASSERT_DEFINED */
/*-----------------------------------------------------------*/

#if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) )

    void vPortGrantAccessToKernelObject( TaskHandle_t xInternalTaskHandle,
                                         int32_t lInternalIndexOfKernelObject ) /* PRIVILEGED_FUNCTION */
    {
        uint32_t ulAccessControlListEntryIndex, ulAccessControlListEntryBit;
        xMPU_SETTINGS * xTaskMpuSettings;

        ulAccessControlListEntryIndex = ( ( uint32_t ) lInternalIndexOfKernelObject / portACL_ENTRY_SIZE_BITS );
        ulAccessControlListEntryBit = ( ( uint32_t ) lInternalIndexOfKernelObject % portACL_ENTRY_SIZE_BITS );

        xTaskMpuSettings = xTaskGetMPUSettings( xInternalTaskHandle );

        xTaskMpuSettings->ulAccessControlList[ ulAccessControlListEntryIndex ] |= ( 1U << ulAccessControlListEntryBit );
    }

#endif /* #if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) ) */
/*-----------------------------------------------------------*/

#if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) )

    void vPortRevokeAccessToKernelObject( TaskHandle_t xInternalTaskHandle,
                                          int32_t lInternalIndexOfKernelObject ) /* PRIVILEGED_FUNCTION */
    {
        uint32_t ulAccessControlListEntryIndex, ulAccessControlListEntryBit;
        xMPU_SETTINGS * xTaskMpuSettings;

        ulAccessControlListEntryIndex = ( ( uint32_t ) lInternalIndexOfKernelObject / portACL_ENTRY_SIZE_BITS );
        ulAccessControlListEntryBit = ( ( uint32_t ) lInternalIndexOfKernelObject % portACL_ENTRY_SIZE_BITS );

        xTaskMpuSettings = xTaskGetMPUSettings( xInternalTaskHandle );

        xTaskMpuSettings->ulAccessControlList[ ulAccessControlListEntryIndex ] &= ~( 1U << ulAccessControlListEntryBit );
    }

#endif /* #if ( ( configUSE_MPU_WRAPPERS_V1 == 0 ) && ( configENABLE_ACCESS_CONTROL_LIST == 1 ) ) */
/*-----------------------------------------------------------*/

#if ( configUSE_MPU_WRAPPERS_V1 == 0 )

    #if ( configENABLE_ACCESS_CONTROL_LIST == 1 )

        BaseType_t xPortIsAuthorizedToAccessKernelObject( int32_t lInternalIndexOfKernelObject ) /* PRIVILEGED_FUNCTION */
        {
            uint32_t ulAccessControlListEntryIndex, ulAccessControlListEntryBit;
            BaseType_t xAccessGranted = pdFALSE;
            const xMPU_SETTINGS * xTaskMpuSettings;

            if( xSchedulerRunning == pdFALSE )
            {
                /* Grant access to all the kernel objects before the scheduler
                 * is started. It is necessary because there is no task running
                 * yet and therefore, we cannot use the permissions of any
                 * task. */
                xAccessGranted = pdTRUE;
            }
            else
            {
                xTaskMpuSettings = xTaskGetMPUSettings( NULL ); /* Calling task's MPU settings. */

                ulAccessControlListEntryIndex = ( ( uint32_t ) lInternalIndexOfKernelObject / portACL_ENTRY_SIZE_BITS );
                ulAccessControlListEntryBit = ( ( uint32_t ) lInternalIndexOfKernelObject % portACL_ENTRY_SIZE_BITS );

                if( ( xTaskMpuSettings->ulTaskFlags & portTASK_IS_PRIVILEGED_FLAG ) == portTASK_IS_PRIVILEGED_FLAG )
                {
                    xAccessGranted = pdTRUE;
                }
                else
                {
                    if( ( xTaskMpuSettings->ulAccessControlList[ ulAccessControlListEntryIndex ] & ( 1U << ulAccessControlListEntryBit ) ) != 0 )
                    {
                        xAccessGranted = pdTRUE;
                    }
                }
            }

            return xAccessGranted;
        }

    #else /* #if ( configENABLE_ACCESS_CONTROL_LIST == 1 ) */

        BaseType_t xPortIsAuthorizedToAccessKernelObject( int32_t lInternalIndexOfKernelObject ) /* PRIVILEGED_FUNCTION */
        {
            ( void ) lInternalIndexOfKernelObject;

            /* If Access Control List feature is not used, all the tasks have
             * access to all the kernel objects. */
            return pdTRUE;
        }

    #endif /* #if ( configENABLE_ACCESS_CONTROL_LIST == 1 ) */

#endif /* #if ( configUSE_MPU_WRAPPERS_V1 == 0 ) */
/*-----------------------------------------------------------*/